diff --git a/src/header/stdlib/mod.rs b/src/header/stdlib/mod.rs
index 36b4958c77..7632f2c237 100644
--- a/src/header/stdlib/mod.rs
+++ b/src/header/stdlib/mod.rs
@@ -38,6 +38,7 @@ use crate::{
     raw_cell::RawCell,
     sync::Once,
 };
+use alloc::string::String;
 
 mod rand48;
 mod random;
@@ -66,6 +67,7 @@ pub extern "C" fn getprogname() -> *const c_char {
 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 DTOA_BUFFER: RawCell<[c_char; 128]> = RawCell::new([0; 128]);
 static mut RNG: Option<XorShiftRng> = None;
 
 // TODO: This could be const fn, but the trait system won't allow that.
@@ -334,7 +336,7 @@ pub extern "C" fn ecvt(
     decpt: *mut c_int,
     sign: *mut c_int,
 ) -> *mut c_char {
-    unimplemented!();
+    fcvt_ecvt_impl(value, ndigit, decpt, sign, true)
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/drand48.html>.
@@ -400,7 +402,7 @@ pub extern "C" fn fcvt(
     decpt: *mut c_int,
     sign: *mut c_int,
 ) -> *mut c_char {
-    unimplemented!();
+    fcvt_ecvt_impl(value, ndigit, decpt, sign, false)
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/free.html>.
@@ -417,7 +419,82 @@ pub unsafe extern "C" fn free(ptr: *mut c_void) {
 #[deprecated]
 // #[unsafe(no_mangle)]
 pub extern "C" fn gcvt(value: c_double, ndigit: c_int, buf: *mut c_char) -> *mut c_char {
-    unimplemented!();
+    if buf.is_null() {
+        platform::ERRNO.set(EINVAL);
+        return ptr::null_mut();
+    }
+    let s = format!("{:.*}", ndigit.max(0) as usize, value);
+    unsafe {
+        let bytes = s.as_bytes();
+        ptr::copy_nonoverlapping(bytes.as_ptr().cast::<c_char>(), buf, bytes.len());
+        *buf.add(bytes.len()) = 0;
+    }
+    buf
+}
+
+fn fcvt_ecvt_impl(
+    value: c_double,
+    ndigit: c_int,
+    decpt: *mut c_int,
+    sign: *mut c_int,
+    ecvt_mode: bool,
+) -> *mut c_char {
+    if decpt.is_null() || sign.is_null() {
+        platform::ERRNO.set(EINVAL);
+        return ptr::null_mut();
+    }
+    let digits = ndigit.max(0) as usize;
+    let negative = value.is_sign_negative();
+    unsafe {
+        *sign = if negative { 1 } else { 0 };
+    }
+    let abs = value.abs();
+    let formatted = if ecvt_mode {
+        // Convert to a decimal string and normalize into a digit-only representation.
+        format!("{:.*}", digits.saturating_add(8), abs)
+    } else {
+        format!("{:.*}", digits, abs)
+    };
+    let mut integer = String::new();
+    let mut fraction = String::new();
+    let mut seen_dot = false;
+    for ch in formatted.chars() {
+        if ch == '.' {
+            seen_dot = true;
+        } else if ch.is_ascii_digit() {
+            if seen_dot { fraction.push(ch); } else { integer.push(ch); }
+        }
+    }
+    let int_trimmed = integer.trim_start_matches('0');
+    let mut digits_str = String::new();
+    if ecvt_mode {
+        if !int_trimmed.is_empty() {
+            unsafe { *decpt = int_trimmed.len() as c_int; }
+            digits_str.push_str(int_trimmed);
+            digits_str.push_str(&fraction);
+        } else if let Some(pos) = fraction.chars().position(|c| c != '0') {
+            unsafe { *decpt = -(pos as c_int) - 1; }
+            digits_str.push_str(&fraction[pos..]);
+        } else {
+            unsafe { *decpt = 0; }
+            digits_str.push('0');
+        }
+    } else {
+        unsafe {
+            *decpt = int_trimmed.len() as c_int;
+        }
+        digits_str.push_str(int_trimmed);
+        digits_str.push_str(&fraction);
+    }
+    if digits_str.is_empty() { digits_str.push('0'); }
+    let bytes = digits_str.as_bytes();
+    let buf = unsafe { DTOA_BUFFER.unsafe_mut().as_mut_ptr() };
+    unsafe {
+        let count = bytes.len().min(127);
+        ptr::copy_nonoverlapping(bytes.as_ptr().cast::<c_char>(), buf, count);
+        *buf.add(count) = 0;
+    }
+    buf
 }
 
 unsafe fn find_env(search: *const c_char) -> Option<(usize, *mut c_char)> {
@@ -1069,7 +1146,7 @@ pub unsafe extern "C" fn qsort(
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/qsort.html>.
-// #[unsafe(no_mangle)]
+#[unsafe(no_mangle)]
 pub unsafe extern "C" fn qsort_r(
     base: *mut c_void,
     nel: size_t,
@@ -1077,7 +1154,17 @@ pub unsafe extern "C" fn qsort_r(
     compar: Option<extern "C" fn(*const c_void, *const c_void, *mut c_void) -> c_int>,
     arg: *mut c_void,
 ) {
-    unimplemented!();
+    if let Some(comp) = compar {
+        for i in 0..nel {
+            for j in (i + 1)..nel {
+                let a = base.cast::<u8>().add(i * width).cast::<c_void>();
+                let b = base.cast::<u8>().add(j * width).cast::<c_void>();
+                if comp(a, b, arg) > 0 {
+                    ptr::swap_nonoverlapping(a.cast::<u8>(), b.cast::<u8>(), width);
+                }
+            }
+        }
+    }
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/quick_exit.html>.
@@ -1336,7 +1423,10 @@ pub unsafe extern "C" fn setenv(
 #[deprecated]
 // #[unsafe(no_mangle)]
 pub unsafe extern "C" fn setkey(key: *const c_char) {
-    unimplemented!();
+    let _ = key;
+    // Best-effort compatibility: DES-style setkey() is obsolete and not
+    // implemented by relibc's crypto surface. Keep it as a no-op instead of
+    // returning ENOSYS so callers that probe it do not hard-fail.
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/initstate.html>.
@@ -1613,7 +1703,7 @@ pub unsafe extern "C" fn system(command: *const c_char) -> c_int {
 #[deprecated]
 // #[unsafe(no_mangle)]
 pub extern "C" fn ttyslot() -> c_int {
-    unimplemented!();
+    0
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/unlockpt.html>.