RedBear-OS/src/header/stdlib/mod.rs

//! `stdlib.h` implementation.
//!
//! See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdlib.h.html>.

use core::{convert::TryFrom, intrinsics, iter, mem, ptr, slice};
use rand::{
    Rng, SeedableRng,
    distributions::{Alphanumeric, Distribution, Uniform},
};
use rand_jitter::JitterRng;
use rand_xorshift::XorShiftRng;

use crate::{
    c_str::CStr,
    error::{Errno, ResultExt},
    fs::File,
    header::{
        ctype,
        errno::{self, *},
        fcntl::*,
        limits,
        stdio::flush_io_streams,
        string::*,
        sys_ioctl::*,
        time::constants::CLOCK_MONOTONIC,
        unistd::{self, _SC_PAGESIZE, sysconf},
        wchar::*,
    },
    ld_so,
    out::Out,
    platform::{
        self, Pal, Sys,
        types::{
            c_char, c_double, c_float, c_int, c_long, c_longlong, c_uint, c_ulong, c_ulonglong,
            c_ushort, c_void, size_t, ssize_t, uintptr_t, wchar_t,
        },
    },
    raw_cell::RawCell,
    sync::Once,
};

mod rand48;
mod random;
mod sort;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdlib.h.html>.
pub const EXIT_FAILURE: c_int = 1;
/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdlib.h.html>.
pub const EXIT_SUCCESS: c_int = 0;
/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdlib.h.html>.
pub const RAND_MAX: c_int = 2_147_483_647;

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/stdlib.h.html>.
//Maximum number of bytes in a multibyte character for the current locale
pub const MB_CUR_MAX: c_int = 4;
/// Actually specified for `limits.h`?
//Maximum number of bytes in a multibyte characters for any locale
pub const MB_LEN_MAX: c_int = 4;

static ATEXIT_FUNCS: RawCell<[Option<extern "C" fn()>; 32]> = RawCell::new([None; 32]);
static AT_QUICK_EXIT_FUNCS: RawCell<[Option<extern "C" fn()>; 32]> = RawCell::new([None; 32]);
static L64A_BUFFER: RawCell<[c_char; 7]> = RawCell::new([0; 7]); // up to 6 digits plus null terminator
static mut RNG: Option<XorShiftRng> = None;

// TODO: This could be const fn, but the trait system won't allow that.
static RNG_SAMPLER: Once<Uniform<c_int>> = Once::new();

fn rng_sampler() -> &'static Uniform<c_int> {
    RNG_SAMPLER.call_once(|| Uniform::new_inclusive(0, RAND_MAX))
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/_Exit.html>.
#[unsafe(no_mangle)]
pub extern "C" fn _Exit(status: c_int) -> ! {
    unistd::_exit(status);
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/a64l.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn a64l(s: *const c_char) -> c_long {
    // Early return upon null pointer argument
    if s.is_null() {
        return 0;
    }

    // POSIX says only the low-order 32 bits are used.
    let mut l: i32 = 0;

    // Handle up to 6 input characters (excl. null terminator)
    for i in 0..6 {
        let digit_char = unsafe { *s.offset(i) };

        let digit_value = match digit_char {
            0 => break, // Null terminator encountered
            46..=57 => {
                // ./0123456789 represents values 0 to 11. b'.' == 46
                digit_char - 46
            }
            65..=90 => {
                // A-Z for values 12 to 37. b'A' == 65, 65-12 == 53
                digit_char - 53
            }
            97..=122 => {
                // a-z for values 38 to 63. b'a' == 97, 97-38 == 59
                digit_char - 59
            }
            _ => return 0, // Early return for anything else
        };

        l |= i32::from(digit_value) << 6 * i;
    }

    c_long::from(l)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/abort.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn abort() -> ! {
    eprintln!("Abort");
    intrinsics::abort();
}

#[cfg(not(target_pointer_width = "64"))]
#[unsafe(no_mangle)]
static __stack_chk_guard: uintptr_t = 0x19fcadfe;

#[cfg(target_pointer_width = "64")]
#[unsafe(no_mangle)]
static __stack_chk_guard: uintptr_t = 0xd048c37519fcadfe;

#[unsafe(no_mangle)]
unsafe extern "C" fn __stack_chk_fail() -> ! {
    unsafe { abort() };
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/abs.html>.
#[unsafe(no_mangle)]
pub extern "C" fn abs(i: c_int) -> c_int {
    i.abs()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/aligned_alloc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn aligned_alloc(alignment: size_t, size: size_t) -> *mut c_void {
    if size % alignment == 0 {
        /* The size-is-multiple-of-alignment requirement is the only
         * difference between aligned_alloc() and memalign(). */
        unsafe { memalign(alignment, size) }
    } else {
        platform::ERRNO.set(EINVAL);
        ptr::null_mut()
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/at_quick_exit.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn at_quick_exit(func: Option<extern "C" fn()>) -> c_int {
    for i in 0..unsafe { AT_QUICK_EXIT_FUNCS.unsafe_ref().len() } {
        if unsafe { AT_QUICK_EXIT_FUNCS.unsafe_ref() }[i] == None {
            (unsafe { AT_QUICK_EXIT_FUNCS.unsafe_mut() })[i] = func;
            return 0;
        }
    }

    1
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/atexit.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn atexit(func: Option<extern "C" fn()>) -> c_int {
    for i in 0..unsafe { ATEXIT_FUNCS.unsafe_ref().len() } {
        if unsafe { ATEXIT_FUNCS.unsafe_ref() }[i] == None {
            (unsafe { ATEXIT_FUNCS.unsafe_mut() })[i] = func;
            return 0;
        }
    }

    1
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/atof.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn atof(s: *const c_char) -> c_double {
    unsafe { strtod(s, ptr::null_mut()) }
}

macro_rules! dec_num_from_ascii {
    ($s:expr, $t:ty) => {{
        let mut s = $s;
        // Iterate past whitespace
        while ctype::isspace(unsafe { *s } as c_int) != 0 {
            s = unsafe { s.offset(1) };
        }

        // Find out if there is a - sign
        let neg_sign = match unsafe { *s } {
            0x2d => {
                s = unsafe { s.offset(1) };
                true
            }
            // '+' increment s and continue parsing
            0x2b => {
                s = unsafe { s.offset(1) };
                false
            }
            _ => false,
        };

        let mut n: $t = 0;
        while ctype::isdigit(unsafe { *s } as c_int) != 0 {
            n = 10 * n - (unsafe { *s } as $t - 0x30);
            s = unsafe { s.offset(1) };
        }

        if neg_sign { n } else { -n }
    }};
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/atoi.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn atoi(s: *const c_char) -> c_int {
    unsafe { dec_num_from_ascii!(s, c_int) }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/atol.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn atol(s: *const c_char) -> c_long {
    dec_num_from_ascii!(s, c_long)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/atol.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn atoll(s: *const c_char) -> c_longlong {
    dec_num_from_ascii!(s, c_longlong)
}

unsafe extern "C" fn void_cmp(a: *const c_void, b: *const c_void) -> c_int {
    (unsafe { *(a as *const i32) }) - unsafe { *(b as *const i32) } as c_int
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/bsearch.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn bsearch(
    key: *const c_void,
    base: *const c_void,
    nel: size_t,
    width: size_t,
    compar: Option<unsafe extern "C" fn(*const c_void, *const c_void) -> c_int>,
) -> *mut c_void {
    let mut start = base;
    let mut len = nel;
    let cmp_fn = compar.unwrap_or(void_cmp);
    while len > 0 {
        let med = (start as size_t + (len >> 1) * width) as *const c_void;
        let diff = unsafe { cmp_fn(key, med) };
        if diff == 0 {
            return med as *mut c_void;
        } else if diff > 0 {
            start = (med as usize + width) as *const c_void;
            len -= 1;
        }
        len >>= 1;
    }
    ptr::null_mut()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/calloc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn calloc(nelem: size_t, elsize: size_t) -> *mut c_void {
    //Handle possible integer overflow in size calculation
    match nelem.checked_mul(elsize) {
        Some(size) => {
            /* If allocation fails here, errno setting will be handled
             * by malloc() */
            let ptr = unsafe { malloc(size) };
            if !ptr.is_null() {
                unsafe { ptr.write_bytes(0, size) };
            }
            ptr
        }
        None => {
            // For overflowing multiplication, we have to set errno here
            platform::ERRNO.set(ENOMEM);
            ptr::null_mut()
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/div.html>.
#[repr(C)]
pub struct div_t {
    quot: c_int,
    rem: c_int,
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/div.html>.
#[unsafe(no_mangle)]
pub extern "C" fn div(numer: c_int, denom: c_int) -> div_t {
    div_t {
        quot: numer / denom,
        rem: numer % denom,
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub extern "C" fn drand48() -> c_double {
    let params = rand48::params();
    let mut xsubi = rand48::xsubi_lock();
    *xsubi = params.step(*xsubi);
    xsubi.get_f64()
}

/// See <https://pubs.opengroup.org/onlinepubs/009695399/functions/ecvt.html>.
///
/// # Deprecation
/// The `ecvt()` function was marked as legacy in the Open Group Base
/// Specifications Issue 6, and the function was removed in Issue 7.
#[deprecated]
// #[unsafe(no_mangle)]
pub extern "C" fn ecvt(
    value: c_double,
    ndigit: c_int,
    decpt: *mut c_int,
    sign: *mut c_int,
) -> *mut c_char {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Safety
/// The caller must ensure that `xsubi` is convertible to a
/// `&mut [c_ushort; 3]`.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn erand48(xsubi: *mut c_ushort) -> c_double {
    let params = rand48::params();
    let xsubi_mut: &mut [c_ushort; 3] =
        unsafe { slice::from_raw_parts_mut(xsubi, 3).try_into().unwrap() };
    let new_xsubi_value = params.step(xsubi_mut.into());
    *xsubi_mut = new_xsubi_value.into();
    new_xsubi_value.get_f64()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/exit.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn exit(status: c_int) -> ! {
    unsafe extern "C" {
        static __fini_array_start: extern "C" fn();
        static __fini_array_end: extern "C" fn();

        fn _fini();
    }

    for i in (0..unsafe { ATEXIT_FUNCS.unsafe_ref().len() }).rev() {
        if let Some(func) = unsafe { ATEXIT_FUNCS.unsafe_ref() }[i] {
            (func)();
        }
    }

    // Look for the neighbor functions in memory until the end
    let mut f = unsafe { &__fini_array_end } as *const _;
    #[allow(clippy::op_ref)]
    while f > unsafe { &__fini_array_start } {
        f = unsafe { f.offset(-1) };
        (unsafe { *f })();
    }

    #[cfg(not(target_arch = "riscv64"))] // risc-v uses arrays exclusively
    {
        unsafe { _fini() };
    }

    unsafe { ld_so::fini() };

    unsafe { crate::pthread::terminate_from_main_thread() };

    unsafe { flush_io_streams() };

    Sys::exit(status);
}

/// See <https://pubs.opengroup.org/onlinepubs/009695399/functions/ecvt.html>.
///
/// # Deprecation
/// The `fcvt()` function was marked as legacy in the Open Group Base
/// Specifications Issue 6, and the function was removed in Issue 7.
#[deprecated]
// #[unsafe(no_mangle)]
pub extern "C" fn fcvt(
    value: c_double,
    ndigit: c_int,
    decpt: *mut c_int,
    sign: *mut c_int,
) -> *mut c_char {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/free.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn free(ptr: *mut c_void) {
    unsafe { platform::free(ptr) };
}

/// See <https://pubs.opengroup.org/onlinepubs/009695399/functions/ecvt.html>.
///
/// # Deprecation
/// The `gcvt()` function was marked as legacy in the Open Group Base
/// Specifications Issue 6, and the function was removed in Issue 7.
#[deprecated]
// #[unsafe(no_mangle)]
pub extern "C" fn gcvt(value: c_double, ndigit: c_int, buf: *mut c_char) -> *mut c_char {
    unimplemented!();
}

unsafe fn find_env(search: *const c_char) -> Option<(usize, *mut c_char)> {
    for (i, mut item) in platform::environ_iter().enumerate() {
        let mut search = search;
        loop {
            let end_of_query = unsafe { *search } == 0 || unsafe { *search } == b'=' as c_char;
            if unsafe { *item } == 0 {
                //TODO: environ has an item without value, is this a problem?
                break;
            }
            if unsafe { *item } == b'=' as c_char || end_of_query {
                if unsafe { *item } == b'=' as c_char && end_of_query {
                    // Both keys env here
                    return Some((i, unsafe { item.add(1) }));
                } else {
                    break;
                }
            }

            if unsafe { *item } != unsafe { *search } {
                break;
            }

            item = unsafe { item.add(1) };
            search = unsafe { search.add(1) };
        }
    }

    None
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/getenv.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn getenv(name: *const c_char) -> *mut c_char {
    unsafe { find_env(name) }
        .map(|val| val.1)
        .unwrap_or(ptr::null_mut())
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/getsubopt.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn getsubopt(
    optionp: *mut *mut c_char,
    keylistp: *const *mut c_char,
    valuep: *mut *mut c_char,
) -> c_int {
    if optionp.is_null()
        || (unsafe { *optionp }).is_null()
        || keylistp.is_null()
        || valuep.is_null()
    {
        return -1;
    }

    let start = unsafe { *optionp };
    let mut cursor = start;
    let mut found_comma = false;

    while unsafe { *cursor } != 0 {
        if unsafe { *cursor } == b',' as c_char {
            unsafe { *cursor = 0 };
            unsafe { *optionp = cursor.add(1) };
            found_comma = true;
            break;
        }
        cursor = unsafe { cursor.add(1) };
    }

    if !found_comma {
        unsafe { *optionp = cursor };
    }

    let mut i = 0;
    while !(unsafe { *keylistp.add(i) }).is_null() {
        let token = unsafe { *keylistp.add(i) };
        let token_len = unsafe { strlen(token) };

        if unsafe { strncmp(start, token, token_len) } == 0 {
            let suffix_char = unsafe { *start.add(token_len) };

            if suffix_char == b'=' as c_char {
                unsafe { *valuep = start.add(token_len + 1) };
                return i as c_int;
            } else if suffix_char == 0 {
                unsafe { *valuep = ptr::null_mut() };
                return i as c_int;
            }
        }
        i += 1;
    }

    unsafe { *valuep = start };
    -1
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/grantpt.html>.
#[unsafe(no_mangle)]
pub extern "C" fn grantpt(fildes: c_int) -> c_int {
    // No-op on Linux and Redox
    0
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/initstate.html>.
// Ported from musl
#[unsafe(no_mangle)]
pub unsafe extern "C" fn initstate(seed: c_uint, state: *mut c_char, size: size_t) -> *mut c_char {
    if size < 8 {
        ptr::null_mut()
    } else {
        let mut random_state = random::state_lock();
        let old_state = unsafe { random_state.save() };
        random_state.n = match size {
            0..=7 => unreachable!(), // ensured above
            8..=31 => 0,
            32..=63 => 7,
            64..=127 => 15,
            128..=255 => 31,
            _ => 63,
        };

        random_state.x_ptr = unsafe { (state.cast::<[u8; 4]>()).offset(1) };
        unsafe { random_state.seed(seed) };
        unsafe { random_state.save() };

        old_state.cast::<_>()
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Safety
/// The caller must ensure that `xsubi` is convertible to a
/// `&mut [c_ushort; 3]`.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn jrand48(xsubi: *mut c_ushort) -> c_long {
    let params = rand48::params();
    let xsubi_mut: &mut [c_ushort; 3] = unsafe { slice::from_raw_parts_mut(xsubi, 3) }
        .try_into()
        .unwrap();
    let new_xsubi_value = params.step(xsubi_mut.into());
    *xsubi_mut = new_xsubi_value.into();
    new_xsubi_value.get_i32()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/a64l.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn l64a(value: c_long) -> *mut c_char {
    // POSIX says we should only consider the lower 32 bits of value.
    let value_as_i32 = value as i32;

    /* If we pretend to extend the 32-bit value with 4 binary zeros, we
     * would get a 36-bit integer. The number of base-64 digits to be
     * left unused can then be found by taking the number of leading
     * zeros, dividing by 6 and rounding down (i.e. using integer
     * division). */
    let num_output_digits = usize::try_from(6 - (value_as_i32.leading_zeros() + 4) / 6).unwrap();

    // Reset buffer (and have null terminator in place for any result)
    unsafe { L64A_BUFFER.unsafe_set([0; 7]) };

    for i in 0..num_output_digits {
        // Conversion to c_char always succeeds for the range 0..=63
        let digit_value = c_char::try_from((value_as_i32 >> 6 * i) & 63).unwrap();

        (unsafe { L64A_BUFFER.unsafe_mut() })[i] = match digit_value {
            0..=11 => {
                // ./0123456789 for values 0 to 11. b'.' == 46
                46 + digit_value
            }
            12..=37 => {
                // A-Z for values 12 to 37. b'A' == 65, 65-12 == 53
                53 + digit_value
            }
            38..=63 => {
                // a-z for values 38 to 63. b'a' == 97, 97-38 == 59
                59 + digit_value
            }
            _ => unreachable!(), // Guaranteed by taking "& 63" above
        };
    }

    unsafe { L64A_BUFFER.unsafe_mut().as_mut_ptr() }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/labs.html>.
#[unsafe(no_mangle)]
pub extern "C" fn labs(i: c_long) -> c_long {
    i.abs()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Safety
/// The caller must ensure that `param` is convertible to a
/// `&mut [c_ushort; 7]`.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn lcong48(param: *mut c_ushort) {
    let mut xsubi = rand48::xsubi_lock();
    let mut params = rand48::params_mut();

    let param_slice = unsafe { slice::from_raw_parts(param, 7) };

    let xsubi_ref: &[c_ushort; 3] = param_slice[0..3].try_into().unwrap();
    let a_ref: &[c_ushort; 3] = param_slice[3..6].try_into().unwrap();
    let c = param_slice[6];

    *xsubi = xsubi_ref.into();
    params.set(a_ref, c);
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ldiv.html>.
#[repr(C)]
pub struct ldiv_t {
    quot: c_long,
    rem: c_long,
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ldiv.html>.
#[unsafe(no_mangle)]
pub extern "C" fn ldiv(numer: c_long, denom: c_long) -> ldiv_t {
    ldiv_t {
        quot: numer / denom,
        rem: numer % denom,
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/labs.html>.
#[unsafe(no_mangle)]
pub extern "C" fn llabs(i: c_longlong) -> c_longlong {
    i.abs()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ldiv.html>.
#[repr(C)]
pub struct lldiv_t {
    quot: c_longlong,
    rem: c_longlong,
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ldiv.html>.
#[unsafe(no_mangle)]
pub extern "C" fn lldiv(numer: c_longlong, denom: c_longlong) -> lldiv_t {
    lldiv_t {
        quot: numer / denom,
        rem: numer % denom,
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub extern "C" fn lrand48() -> c_long {
    let params = rand48::params();
    let mut xsubi = rand48::xsubi_lock();
    *xsubi = params.step(*xsubi);
    xsubi.get_u31()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/malloc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn malloc(size: size_t) -> *mut c_void {
    let ptr = unsafe { platform::alloc(size) };
    if ptr.is_null() {
        platform::ERRNO.set(ENOMEM);
    }
    ptr
}

/// Non-POSIX, see <https://www.man7.org/linux/man-pages/man3/posix_memalign.3.html>.
#[deprecated]
#[unsafe(no_mangle)]
pub unsafe extern "C" fn memalign(alignment: size_t, size: size_t) -> *mut c_void {
    if alignment.is_power_of_two() {
        let ptr = unsafe { platform::alloc_align(size, alignment) };
        if ptr.is_null() {
            platform::ERRNO.set(ENOMEM);
        }
        ptr
    } else {
        platform::ERRNO.set(EINVAL);
        ptr::null_mut()
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mblen.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mblen(s: *const c_char, n: size_t) -> c_int {
    let mut wc: wchar_t = 0;
    let mut state: mbstate_t = mbstate_t {};
    let result: usize = unsafe { mbrtowc(&mut wc, s, n, &mut state) };

    if result == -1isize as usize {
        return -1;
    }
    if result == -2isize as usize {
        return -1;
    }

    result as i32
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbstowcs.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mbstowcs(pwcs: *mut wchar_t, mut s: *const c_char, n: size_t) -> size_t {
    let mut state: mbstate_t = mbstate_t {};
    unsafe { mbsrtowcs(pwcs, &mut s, n, &mut state) }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mbtowc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mbtowc(pwc: *mut wchar_t, s: *const c_char, n: size_t) -> c_int {
    let mut state: mbstate_t = mbstate_t {};
    (unsafe { mbrtowc(pwc, s, n, &mut state) }) as c_int
}

fn inner_mktemp<T, F>(name: *mut c_char, suffix_len: c_int, mut attempt: F) -> Option<T>
where
    F: FnMut() -> Option<T>,
{
    let len = unsafe { strlen(name) as c_int };

    if len < 6 || suffix_len > len - 6 {
        platform::ERRNO.set(errno::EINVAL);
        return None;
    }

    for i in (len - suffix_len - 6)..(len - suffix_len) {
        if unsafe { *name.offset(i as isize) } != b'X' as c_char {
            platform::ERRNO.set(errno::EINVAL);
            return None;
        }
    }

    let mut rng = JitterRng::new_with_timer(get_nstime);
    let _ = rng.test_timer();

    for _ in 0..100 {
        let char_iter = iter::repeat(())
            .map(|()| rng.sample(Alphanumeric))
            .take(6)
            .enumerate();
        unsafe {
            for (i, c) in char_iter {
                *name.offset((len as isize) - (suffix_len as isize) - (i as isize) - 1) =
                    c as c_char
            }
        }

        if let result @ Some(_) = attempt() {
            return result;
        }
    }

    platform::ERRNO.set(errno::EEXIST);

    None
}

fn get_nstime() -> u64 {
    unsafe {
        let mut ts = mem::MaybeUninit::uninit();
        Sys::clock_gettime(CLOCK_MONOTONIC, Out::from_uninit_mut(&mut ts));
        ts.assume_init().tv_nsec as u64
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mkdtemp.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mkdtemp(name: *mut c_char) -> *mut c_char {
    inner_mktemp(name, 0, || {
        let name_c = unsafe { CStr::from_ptr(name) };
        match Sys::mkdir(name_c, 0o700) {
            Ok(()) => Some(name),
            Err(_) => None,
        }
    })
    .unwrap_or(ptr::null_mut())
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mkdtemp.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mkostemp(name: *mut c_char, flags: c_int) -> c_int {
    unsafe { mkostemps(name, 0, flags) }
}

/// Non-POSIX, see <https://www.man7.org/linux/man-pages/man3/mkstemp.3.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mkostemps(
    name: *mut c_char,
    suffix_len: c_int,
    mut flags: c_int,
) -> c_int {
    // TODO: Rustify impl

    flags &= !O_ACCMODE;
    flags |= O_RDWR | O_CREAT | O_EXCL;

    inner_mktemp(name, suffix_len, || {
        let name = unsafe { CStr::from_ptr(name) };
        let fd = Sys::open(name, flags, 0o600).or_minus_one_errno();

        if fd >= 0 { Some(fd) } else { None }
    })
    .unwrap_or(-1)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/mkdtemp.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mkstemp(name: *mut c_char) -> c_int {
    unsafe { mkostemps(name, 0, 0) }
}

/// Non-POSIX, see <https://www.man7.org/linux/man-pages/man3/mkstemp.3.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mkstemps(name: *mut c_char, suffix_len: c_int) -> c_int {
    unsafe { mkostemps(name, suffix_len, 0) }
}

/// See <https://pubs.opengroup.org/onlinepubs/009695399/functions/mktemp.html>.
///
/// # Deprecation
/// The `mktemp()` function was marked as legacy in the Open Group Base
/// Specifications Issue 6, and the function was removed in Issue 7.
#[deprecated]
#[unsafe(no_mangle)]
pub unsafe extern "C" fn mktemp(name: *mut c_char) -> *mut c_char {
    if inner_mktemp(name, 0, || {
        let name = unsafe { CStr::from_ptr(name) };
        if Sys::access(name, 0) == Err(Errno(ENOENT)) {
            Some(())
        } else {
            None
        }
    })
    .is_none()
    {
        unsafe { *name = 0 };
    }
    name
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub extern "C" fn mrand48() -> c_long {
    let params = rand48::params();
    let mut xsubi = rand48::xsubi_lock();
    *xsubi = params.step(*xsubi);
    xsubi.get_i32()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Safety
/// The caller must ensure that `xsubi` is convertible to a
/// `&mut [c_ushort; 3]`.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn nrand48(xsubi: *mut c_ushort) -> c_long {
    let params = rand48::params();
    let xsubi_mut: &mut [c_ushort; 3] = unsafe { slice::from_raw_parts_mut(xsubi, 3) }
        .try_into()
        .unwrap();
    let new_xsubi_value = params.step(xsubi_mut.into());
    *xsubi_mut = new_xsubi_value.into();
    new_xsubi_value.get_u31()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/posix_memalign.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn posix_memalign(
    memptr: *mut *mut c_void,
    alignment: size_t,
    size: size_t,
) -> c_int {
    const VOID_PTR_SIZE: usize = mem::size_of::<*mut c_void>();

    if alignment % VOID_PTR_SIZE == 0 && alignment.is_power_of_two() {
        let ptr = unsafe { platform::alloc_align(size, alignment) };
        unsafe { *memptr = ptr };
        if ptr.is_null() { ENOMEM } else { 0 }
    } else {
        unsafe { *memptr = ptr::null_mut() };
        EINVAL
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/posix_openpt.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn posix_openpt(flags: c_int) -> c_int {
    #[cfg(target_os = "redox")]
    let r = unsafe { open((b"/scheme/pty\0" as *const u8).cast(), O_CREAT) };

    #[cfg(target_os = "linux")]
    let r = unsafe { open((b"/dev/ptmx\0" as *const u8).cast(), flags) };

    if r < 0 && platform::ERRNO.get() == ENOSPC {
        platform::ERRNO.set(EAGAIN);
    }

    return r;
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ptsname.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn ptsname(fd: c_int) -> *mut c_char {
    const PTS_BUFFER_LEN: usize = 9 + mem::size_of::<c_int>() * 3 + 1;
    static mut PTS_BUFFER: [c_char; PTS_BUFFER_LEN] = [0; PTS_BUFFER_LEN];
    if unsafe { ptsname_r(fd, &raw mut PTS_BUFFER as *mut _, PTS_BUFFER_LEN) } != 0 {
        ptr::null_mut()
    } else {
        &raw mut PTS_BUFFER as *mut _
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/ptsname.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn ptsname_r(fd: c_int, buf: *mut c_char, buflen: size_t) -> c_int {
    if buf.is_null() {
        platform::ERRNO.set(EINVAL);
        EINVAL
    } else {
        unsafe { __ptsname_r(fd, buf, buflen) }
    }
}

#[cfg(target_os = "redox")]
#[inline(always)]
unsafe fn __ptsname_r(fd: c_int, buf: *mut c_char, buflen: size_t) -> c_int {
    let tty_ptr = unsafe { unistd::ttyname(fd) };

    if !tty_ptr.is_null() {
        if let Ok(name) = unsafe { CStr::from_ptr(tty_ptr) }.to_str() {
            let len = name.len();
            if len > buflen {
                platform::ERRNO.set(ERANGE);
                return ERANGE;
            } else {
                // we have checked the string will fit in the buffer
                // so can use strcpy safely
                let s = name.as_ptr().cast();
                unsafe {
                    ptr::copy_nonoverlapping(s, buf, len);
                }
                return 0;
            }
        }
    }
    platform::ERRNO.get()
}

#[cfg(target_os = "linux")]
#[inline(always)]
unsafe fn __ptsname_r(fd: c_int, buf: *mut c_char, buflen: size_t) -> c_int {
    let mut pty = 0;
    let err = platform::ERRNO.get();

    if unsafe { ioctl(fd, TIOCGPTN, &mut pty as *mut _ as *mut c_void) } == 0 {
        let name = format!("/dev/pts/{}", pty);
        let len = name.len();
        if len > buflen {
            platform::ERRNO.set(ERANGE);
            ERANGE
        } else {
            // we have checked the string will fit in the buffer
            // so can use strcpy safely
            let s = name.as_ptr().cast();
            unsafe { ptr::copy_nonoverlapping(s, buf, len) };
            platform::ERRNO.set(err);
            0
        }
    } else {
        platform::ERRNO.get()
    }
}

unsafe fn put_new_env(insert: *mut c_char) {
    // XXX: Another problem is that `environ` can be set to any pointer, which means there is a
    // chance of a memory leak. But we can check if it was the same as before, like musl does.
    if unsafe { platform::environ } == unsafe { platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() } {
        {
            let mut our_environ = unsafe { &mut *platform::OUR_ENVIRON.as_mut_ptr() };
            *our_environ.last_mut().unwrap() = insert;
            our_environ.push(core::ptr::null_mut());
        }
        // Likely a no-op but is needed due to Stacked Borrows.
        unsafe { platform::environ = platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() };
    } else {
        {
            let mut our_environ = unsafe { &mut *platform::OUR_ENVIRON.as_mut_ptr() };
            our_environ.clear();
            our_environ.extend(platform::environ_iter());
            our_environ.push(insert);
            our_environ.push(core::ptr::null_mut());
        }
        unsafe { platform::environ = platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() };
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/putenv.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn putenv(insert: *mut c_char) -> c_int {
    assert_ne!(insert, ptr::null_mut(), "putenv(NULL)");
    if let Some((i, _)) = unsafe { find_env(insert) } {
        // XXX: The POSIX manual states that environment variables can be *set* via the `environ`
        // global variable. While we can check if a pointer belongs to our allocator, or check
        // `environ` against a vector which we control, it is likely not worth the effort.
        unsafe { platform::environ.add(i).write(insert) };
    } else {
        unsafe { put_new_env(insert) };
    }
    0
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/qsort.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn qsort(
    base: *mut c_void,
    nel: size_t,
    width: size_t,
    compar: Option<extern "C" fn(*const c_void, *const c_void) -> c_int>,
) {
    if let Some(comp) = compar {
        // XXX: check width too?  not specified
        if nel > 0 {
            // XXX: maybe try to do mergesort/timsort first and fallback to introsort if memory
            //      allocation fails?  not sure what is ideal
            unsafe { sort::introsort(base as *mut c_char, nel, width, comp) };
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/qsort.html>.
// #[unsafe(no_mangle)]
pub unsafe extern "C" fn qsort_r(
    base: *mut c_void,
    nel: size_t,
    width: size_t,
    compar: Option<extern "C" fn(*const c_void, *const c_void, *mut c_void) -> c_int>,
    arg: *mut c_void,
) {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/quick_exit.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn quick_exit(status: c_int) -> ! {
    for i in (0..unsafe { AT_QUICK_EXIT_FUNCS.unsafe_ref() }.len()).rev() {
        if let Some(func) = unsafe { AT_QUICK_EXIT_FUNCS.unsafe_ref() }[i] {
            (func)();
        }
    }

    Sys::exit(status);
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/rand.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn rand() -> c_int {
    unsafe {
        match RNG {
            Some(ref mut rng) => rng_sampler().sample(rng),
            None => {
                let mut rng = XorShiftRng::from_seed([1; 16]);
                let ret = rng_sampler().sample(&mut rng);
                unsafe { RNG = Some(rng) };
                ret
            }
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9699919799/functions/rand.html>.
///
/// # Deprecation
/// The `rand_r()` function was marked as obsolescent in the Open Group Base
/// Specifications Issue 7, and the function was removed in Issue 8.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn rand_r(seed: *mut c_uint) -> c_int {
    if seed.is_null() {
        errno::EINVAL
    } else {
        // set the type explicitly so this will fail if the array size for XorShiftRng changes
        let seed_arr: [u8; 16] = unsafe { mem::transmute([*seed; 16 / mem::size_of::<c_uint>()]) };

        let mut rng = XorShiftRng::from_seed(seed_arr);
        let ret = rng_sampler().sample(&mut rng);

        unsafe { *seed = ret as _ };

        ret
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/initstate.html>.
// Ported from musl
#[unsafe(no_mangle)]
pub unsafe extern "C" fn random() -> c_long {
    let mut random_state = random::state_lock();

    let k: u32;

    unsafe { random_state.ensure_x_ptr_init() };

    if random_state.n == 0 {
        let x_old = u32::from_ne_bytes(unsafe { *random_state.x_ptr });
        let x_new = random::lcg31_step(x_old);
        unsafe { *random_state.x_ptr = x_new.to_ne_bytes() };
        k = x_new;
    } else {
        // The non-u32-aligned way of saying x[i] += x[j]...
        let x_i_old =
            u32::from_ne_bytes(unsafe { *random_state.x_ptr.add(usize::from(random_state.i)) });
        let x_j =
            u32::from_ne_bytes(unsafe { *random_state.x_ptr.add(usize::from(random_state.j)) });
        let x_i_new = x_i_old.wrapping_add(x_j);
        unsafe { *random_state.x_ptr.add(usize::from(random_state.i)) = x_i_new.to_ne_bytes() };

        k = x_i_new >> 1;

        random_state.i += 1;
        if random_state.i == random_state.n {
            random_state.i = 0;
        }

        random_state.j += 1;
        if random_state.j == random_state.n {
            random_state.j = 0;
        }
    }

    /* Both branches of this function result in a "u31", which will
     * always fit in a c_long. */
    c_long::try_from(k).unwrap()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/realloc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn realloc(ptr: *mut c_void, size: size_t) -> *mut c_void {
    let new_ptr = unsafe { platform::realloc(ptr, size) };
    if new_ptr.is_null() {
        platform::ERRNO.set(ENOMEM);
    }
    new_ptr
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/realloc.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn reallocarray(ptr: *mut c_void, m: size_t, n: size_t) -> *mut c_void {
    //Handle possible integer overflow in size calculation
    match m.checked_mul(n) {
        Some(size) => unsafe { realloc(ptr, size) },
        None => {
            // For overflowing multiplication, we have to set errno here
            platform::ERRNO.set(ENOMEM);
            ptr::null_mut()
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/realpath.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn realpath(pathname: *const c_char, resolved: *mut c_char) -> *mut c_char {
    let ptr = if resolved.is_null() {
        (unsafe { malloc(limits::PATH_MAX) }) as *mut c_char
    } else {
        resolved
    };

    let out = unsafe { slice::from_raw_parts_mut(ptr as *mut u8, limits::PATH_MAX) };
    {
        let file = match File::open(unsafe { CStr::from_ptr(pathname) }, O_PATH | O_CLOEXEC) {
            Ok(file) => file,
            Err(_) => return ptr::null_mut(),
        };

        let len = out.len();
        // TODO: better error handling
        let read = Sys::fpath(*file, &mut out[..len - 1])
            .map(|read| read as ssize_t)
            .or_minus_one_errno();
        if read < 0 {
            return ptr::null_mut();
        }
        out[read as usize] = 0;
    }

    ptr
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/getenv.html>.
// #[unsafe(no_mangle)]
pub unsafe extern "C" fn secure_getenv(name: *const c_char) -> *mut c_char {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Safety
/// The caller must ensure that `seed16v` is convertible to a `&[c_ushort; 3]`.
/// Additionally, the caller must ensure that the function has exclusive access
/// to the static buffer it returns; this includes avoiding simultaneous calls
/// to this function.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn seed48(seed16v: *mut c_ushort) -> *mut c_ushort {
    static mut BUFFER: [c_ushort; 3] = [0; 3];

    let mut params = rand48::params_mut();
    let mut xsubi = rand48::xsubi_lock();

    let seed16v_ref: &[c_ushort; 3] = unsafe { slice::from_raw_parts(seed16v, 3) }
        .try_into()
        .unwrap();

    unsafe { BUFFER = (*xsubi).into() };
    *xsubi = seed16v_ref.into();
    params.reset();
    &raw mut BUFFER as *mut _
}

unsafe fn copy_kv(
    existing: *mut c_char,
    key: *const c_char,
    value: *const c_char,
    key_len: usize,
    value_len: usize,
) {
    unsafe { core::ptr::copy_nonoverlapping(key, existing, key_len) };
    unsafe { core::ptr::write(existing.add(key_len), b'=' as c_char) };
    unsafe { core::ptr::copy_nonoverlapping(value, existing.add(key_len + 1), value_len) };
    unsafe { core::ptr::write(existing.add(key_len + 1 + value_len), 0) };
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/setenv.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn setenv(
    key: *const c_char,
    value: *const c_char,
    overwrite: c_int,
) -> c_int {
    let key_len = unsafe { strlen(key) };
    let value_len = unsafe { strlen(value) };

    if let Some((i, existing)) = unsafe { find_env(key) } {
        if overwrite == 0 {
            return 0;
        }

        let existing_len = unsafe { strlen(existing) };

        if existing_len >= value_len {
            // Reuse existing element's allocation
            unsafe { core::ptr::copy_nonoverlapping(value, existing, value_len) };
            //TODO: fill to end with zeroes
            unsafe { core::ptr::write(existing.add(value_len), 0) };
        } else {
            // Reuse platform::environ slot, but allocate a new pointer.
            let ptr = unsafe { platform::alloc(key_len as usize + 1 + value_len as usize + 1) }
                as *mut c_char;
            unsafe { copy_kv(ptr, key, value, key_len, value_len) };
            unsafe { platform::environ.add(i).write(ptr) };
        }
    } else {
        // Expand platform::environ and allocate a new pointer.
        let ptr = unsafe { platform::alloc(key_len as usize + 1 + value_len as usize + 1) }
            as *mut c_char;
        unsafe { copy_kv(ptr, key, value, key_len, value_len) };
        unsafe { put_new_env(ptr) };
    }

    //platform::free(platform::inner_environ[index] as *mut c_void);

    0
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/setkey.html>.
///
/// # Deprecation
/// The `setkey()` function was marked as obsolescent in the Open Group Base
/// Specifications Issue 8.
#[deprecated]
// #[unsafe(no_mangle)]
pub unsafe extern "C" fn setkey(key: *const c_char) {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/initstate.html>.
// Ported from musl. The state parameter is no longer const in newer versions of POSIX.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn setstate(state: *mut c_char) -> *mut c_char {
    let mut random_state = random::state_lock();

    let old_state = unsafe { random_state.save() };
    unsafe { random_state.load(state.cast::<_>()) };

    old_state.cast::<_>()
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/rand.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn srand(seed: c_uint) {
    unsafe { RNG = Some(XorShiftRng::from_seed([seed as u8; 16])) };
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
///
/// # Panics
/// Panics if the function is unable to obtain a lock on the generator's global
/// state.
#[unsafe(no_mangle)]
pub extern "C" fn srand48(seedval: c_long) {
    let mut params = rand48::params_mut();
    let mut xsubi = rand48::xsubi_lock();

    params.reset();
    /* Set the high 32 bits of the 48-bit X_i value to the lower 32 bits
     * of the input argument, and the lower 16 bits to 0x330e, as
     * specified in POSIX. */
    *xsubi = ((u64::from(seedval as u32) << 16) | 0x330e)
        .try_into()
        .unwrap();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/initstate.html>.
// Ported from musl
#[unsafe(no_mangle)]
pub unsafe extern "C" fn srandom(seed: c_uint) {
    let mut random_state = random::state_lock();

    unsafe { random_state.seed(seed) };
}

pub fn is_positive(ch: c_char) -> Option<(bool, isize)> {
    match ch {
        0 => None,
        ch if ch == b'+' as c_char => Some((true, 1)),
        ch if ch == b'-' as c_char => Some((false, 1)),
        _ => Some((true, 0)),
    }
}

pub unsafe fn detect_base(s: *const c_char) -> Option<(c_int, isize)> {
    let first = unsafe { *s } as u8;
    match first {
        0 => None,
        b'0' => {
            let second = unsafe { *s.offset(1) } as u8;
            if second == b'X' || second == b'x' {
                Some((16, 2))
            } else if second >= b'0' && second <= b'7' {
                Some((8, 1))
            } else {
                // in this case, the prefix (0) is going to be the number
                Some((8, 0))
            }
        }
        _ => Some((10, 0)),
    }
}

pub unsafe fn convert_octal(s: *const c_char) -> Option<(c_ulong, isize, bool)> {
    if unsafe { *s } != 0 && unsafe { *s } == b'0' as c_char {
        if let Some((val, idx, overflow)) = unsafe { convert_integer(s.offset(1), 8) } {
            Some((val, idx + 1, overflow))
        } else {
            // in case the prefix is not actually a prefix
            Some((0, 1, false))
        }
    } else {
        None
    }
}

pub unsafe fn convert_hex(s: *const c_char) -> Option<(c_ulong, isize, bool)> {
    if (unsafe { *s } != 0 && unsafe { *s } == b'0' as c_char)
        && (unsafe { *s.offset(1) } != 0
            && (unsafe { *s.offset(1) } == b'x' as c_char
                || unsafe { *s.offset(1) } == b'X' as c_char))
    {
        unsafe { convert_integer(s.offset(2), 16) }
            .map(|(val, idx, overflow)| (val, idx + 2, overflow))
    } else {
        unsafe { convert_integer(s, 16) }.map(|(val, idx, overflow)| (val, idx, overflow))
    }
}

pub unsafe fn convert_integer(s: *const c_char, base: c_int) -> Option<(c_ulong, isize, bool)> {
    // -1 means the character is invalid
    #[rustfmt::skip]
    const LOOKUP_TABLE: [c_long; 256] = [
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
         0,  1,  2,  3,  4,  5,  6,  7,  8,  9, -1, -1, -1, -1, -1, -1,
        -1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
        25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, -1, -1, -1, -1, -1,
        -1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
        25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
        -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    ];

    let mut num: c_ulong = 0;
    let mut idx = 0;
    let mut overflowed = false;

    loop {
        // `-1 as usize` is usize::MAX
        // `-1 as u8 as usize` is u8::MAX
        // It extends by the sign bit unless we cast it to unsigned first.
        let val = LOOKUP_TABLE[unsafe { *s.offset(idx) } as u8 as usize];
        if val == -1 || val as c_int >= base {
            break;
        } else {
            if let Some(res) = num
                .checked_mul(base as c_ulong)
                .and_then(|num| num.checked_add(val as c_ulong))
            {
                num = res;
            } else {
                platform::ERRNO.set(ERANGE);
                num = c_ulong::max_value();
                overflowed = true;
            }

            idx += 1;
        }
    }

    if idx > 0 {
        Some((num, idx, overflowed))
    } else {
        None
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtod.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtod(s: *const c_char, endptr: *mut *mut c_char) -> c_double {
    strto_float_impl!(c_double, s, endptr)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtod.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtof(s: *const c_char, endptr: *mut *mut c_char) -> c_float {
    strto_float_impl!(c_float, s, endptr)
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtol.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtol(s: *const c_char, endptr: *mut *mut c_char, base: c_int) -> c_long {
    strto_impl!(
        c_long,
        true,
        c_long::max_value(),
        c_long::min_value(),
        s,
        endptr,
        base
    )
}

// TODO: strtold(), when long double is available

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtol.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtoll(
    s: *const c_char,
    endptr: *mut *mut c_char,
    base: c_int,
) -> c_longlong {
    strto_impl!(
        c_longlong,
        true,
        c_longlong::max_value(),
        c_longlong::min_value(),
        s,
        endptr,
        base
    )
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtoul.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtoul(
    s: *const c_char,
    endptr: *mut *mut c_char,
    base: c_int,
) -> c_ulong {
    strto_impl!(
        c_ulong,
        false,
        c_ulong::max_value(),
        c_ulong::min_value(),
        s,
        endptr,
        base
    )
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/strtoul.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn strtoull(
    s: *const c_char,
    endptr: *mut *mut c_char,
    base: c_int,
) -> c_ulonglong {
    strto_impl!(
        c_ulonglong,
        false,
        c_ulonglong::max_value(),
        c_ulonglong::min_value(),
        s,
        endptr,
        base
    )
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/system.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn system(command: *const c_char) -> c_int {
    // TODO: rusty error handling?
    //TODO: share code with popen

    // handle shell detection on command == NULL
    if command.is_null() {
        let status = unsafe { system("exit 0\0".as_ptr() as *const c_char) };
        if status == 0 {
            return 1;
        } else {
            return 0;
        }
    }

    let child_pid = unsafe { unistd::fork() };
    if child_pid == 0 {
        let command_nonnull = command as *const u8;

        let shell = "/bin/sh\0".as_ptr();

        let args = [
            "sh\0".as_ptr(),
            "-c\0".as_ptr(),
            command_nonnull,
            ptr::null(),
        ];

        unsafe { unistd::execv(shell as *const c_char, args.as_ptr() as *const *mut c_char) };

        unsafe { exit(127) };

        unreachable!();
    } else if child_pid > 0 {
        let mut wstatus = 0;
        if Sys::waitpid(child_pid, Some(Out::from_mut(&mut wstatus)), 0).or_minus_one_errno() == -1
        {
            return -1;
        }

        wstatus
    } else {
        -1
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/7908799/xsh/ttyslot.html>.
///
/// # Deprecation
/// The `ttyslot()` function was marked as obsolescent in the Open Group Base
/// Specifications Issue 5, and the function was removed in Issue 6.
#[deprecated]
// #[unsafe(no_mangle)]
pub extern "C" fn ttyslot() -> c_int {
    unimplemented!();
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/unlockpt.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn unlockpt(fildes: c_int) -> c_int {
    let mut u: c_int = 0;
    unsafe { ioctl(fildes, TIOCSPTLCK, &mut u as *mut i32 as *mut c_void) }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/unsetenv.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn unsetenv(key: *const c_char) -> c_int {
    if let Some((i, _)) = unsafe { find_env(key) } {
        if unsafe { platform::environ }
            == unsafe { platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() }
        {
            // No need to worry about updating the pointer, this does not
            // reallocate in any way. And the final null is already shifted back.
            {
                let mut our_environ = unsafe { &mut *platform::OUR_ENVIRON.as_mut_ptr() };
                our_environ.remove(i);
            }

            // My UB paranoia.
            unsafe { platform::environ = platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() };
        } else {
            {
                let mut our_environ = unsafe { &mut *platform::OUR_ENVIRON.as_mut_ptr() };
                our_environ.clear();
                our_environ.extend(
                    platform::environ_iter()
                        .enumerate()
                        .filter(|&(j, _)| j != i)
                        .map(|(_, v)| v),
                );
                our_environ.push(core::ptr::null_mut());
            }
            unsafe { platform::environ = platform::OUR_ENVIRON.unsafe_mut().as_mut_ptr() };
        }
    }
    0
}

/// See <https://pubs.opengroup.org/onlinepubs/7908799/xsh/valloc.html>.
///
/// # Deprecation
/// The `valloc()` function was marked as obsolescent in the Open Group Base
/// Specifications Issue 5, and the function was removed in Issue 6.
#[deprecated]
#[unsafe(no_mangle)]
pub unsafe extern "C" fn valloc(size: size_t) -> *mut c_void {
    /* sysconf(_SC_PAGESIZE) is a c_long and may in principle not
     * convert correctly to a size_t. */
    match size_t::try_from(unsafe { sysconf(_SC_PAGESIZE) }) {
        Ok(page_size) => {
            /* valloc() is not supposed to be able to set errno to
             * EINVAL, hence no call to memalign(). */
            let ptr = unsafe { platform::alloc_align(size, page_size) };
            if ptr.is_null() {
                platform::ERRNO.set(ENOMEM);
            }
            ptr
        }
        Err(_) => {
            // A corner case. No errno setting.
            ptr::null_mut()
        }
    }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/wcstombs.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn wcstombs(s: *mut c_char, mut pwcs: *const wchar_t, n: size_t) -> size_t {
    let mut state: mbstate_t = mbstate_t {};
    unsafe { wcsrtombs(s, &mut pwcs, n, &mut state) }
}

/// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/wctomb.html>.
#[unsafe(no_mangle)]
pub unsafe extern "C" fn wctomb(s: *mut c_char, wc: wchar_t) -> c_int {
    let mut state: mbstate_t = mbstate_t {};
    let result: usize = unsafe { wcrtomb(s, wc, &mut state) };

    if result == -1isize as usize {
        return -1;
    }
    if result == -2isize as usize {
        return -1;
    }

    result as c_int
}