Implement confstr (unistd.h)

The implementation for confstr is straightforward. Most of the constants
return 1 on both musl and glibc. The only constant that doesn't is
required for Fish.

I also switched an #[unsafe(no_mangle)] from my last patch back
to a #[no_mangle] because we need to bump cbindgen. #[unsafe(no_mangle)]
is required for Rust 2024.

src/header/unistd/mod.rs

@@ -34,7 +34,10 @@ pub use crate::header::stdio::{ctermid, cuserid};
 // TODO: implement and reexport fcntl functions:
 //pub use crate::header::fcntl::{faccessat, fchownat, fexecve, linkat, readlinkat, symlinkat, unlinkat};
 
-use super::errno::{E2BIG, ENOMEM};
+use super::{
+    errno::{E2BIG, ENOMEM},
+    stdio::snprintf,
+};
 
 mod brk;
 mod getopt;
@@ -62,6 +65,45 @@ pub const STDERR_FILENO: c_int = 2;
 
 pub const L_cuserid: usize = 9;
 
+// confstr constants
+// These are copied from Rust's libc and match musl as well.
+pub const _CS_PATH: c_int = 0;
+pub const _CS_POSIX_V6_WIDTH_RESTRICTED_ENVS: c_int = 1;
+pub const _CS_POSIX_V5_WIDTH_RESTRICTED_ENVS: c_int = 4;
+pub const _CS_POSIX_V7_WIDTH_RESTRICTED_ENVS: c_int = 5;
+pub const _CS_POSIX_V6_ILP32_OFF32_CFLAGS: c_int = 1116;
+pub const _CS_POSIX_V6_ILP32_OFF32_LDFLAGS: c_int = 1117;
+pub const _CS_POSIX_V6_ILP32_OFF32_LIBS: c_int = 1118;
+pub const _CS_POSIX_V6_ILP32_OFF32_LINTFLAGS: c_int = 1119;
+pub const _CS_POSIX_V6_ILP32_OFFBIG_CFLAGS: c_int = 1120;
+pub const _CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS: c_int = 1121;
+pub const _CS_POSIX_V6_ILP32_OFFBIG_LIBS: c_int = 1122;
+pub const _CS_POSIX_V6_ILP32_OFFBIG_LINTFLAGS: c_int = 1123;
+pub const _CS_POSIX_V6_LP64_OFF64_CFLAGS: c_int = 1124;
+pub const _CS_POSIX_V6_LP64_OFF64_LDFLAGS: c_int = 1125;
+pub const _CS_POSIX_V6_LP64_OFF64_LIBS: c_int = 1126;
+pub const _CS_POSIX_V6_LP64_OFF64_LINTFLAGS: c_int = 1127;
+pub const _CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS: c_int = 1128;
+pub const _CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS: c_int = 1129;
+pub const _CS_POSIX_V6_LPBIG_OFFBIG_LIBS: c_int = 1130;
+pub const _CS_POSIX_V6_LPBIG_OFFBIG_LINTFLAGS: c_int = 1131;
+pub const _CS_POSIX_V7_ILP32_OFF32_CFLAGS: c_int = 1132;
+pub const _CS_POSIX_V7_ILP32_OFF32_LDFLAGS: c_int = 1133;
+pub const _CS_POSIX_V7_ILP32_OFF32_LIBS: c_int = 1134;
+pub const _CS_POSIX_V7_ILP32_OFF32_LINTFLAGS: c_int = 1135;
+pub const _CS_POSIX_V7_ILP32_OFFBIG_CFLAGS: c_int = 1136;
+pub const _CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS: c_int = 1137;
+pub const _CS_POSIX_V7_ILP32_OFFBIG_LIBS: c_int = 1138;
+pub const _CS_POSIX_V7_ILP32_OFFBIG_LINTFLAGS: c_int = 1139;
+pub const _CS_POSIX_V7_LP64_OFF64_CFLAGS: c_int = 1140;
+pub const _CS_POSIX_V7_LP64_OFF64_LDFLAGS: c_int = 1141;
+pub const _CS_POSIX_V7_LP64_OFF64_LIBS: c_int = 1142;
+pub const _CS_POSIX_V7_LP64_OFF64_LINTFLAGS: c_int = 1143;
+pub const _CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS: c_int = 1144;
+pub const _CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS: c_int = 1145;
+pub const _CS_POSIX_V7_LPBIG_OFFBIG_LIBS: c_int = 1146;
+pub const _CS_POSIX_V7_LPBIG_OFFBIG_LINTFLAGS: c_int = 1147;
+
 #[thread_local]
 pub static mut fork_hooks_static: Option<[LinkedList<extern "C" fn()>; 3]> = None;
 
@@ -154,9 +196,26 @@ pub extern "C" fn close(fildes: c_int) -> c_int {
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/confstr.html>.
-// #[no_mangle]
+#[no_mangle]
 pub extern "C" fn confstr(name: c_int, buf: *mut c_char, len: size_t) -> size_t {
-    unimplemented!();
+    // confstr returns the number of bytes required to hold the string INCLUDING the NUL
+    // terminator. This is different from other C functions hence the + 1.
+    match name {
+        _CS_PATH => {
+            let posix2_path = c"/usr/bin";
+            unsafe { snprintf(buf, len, c"%s".as_ptr(), posix2_path.as_ptr()) + 1 }
+                .try_into()
+                .unwrap_or_default()
+        }
+        _CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
+        | _CS_POSIX_V5_WIDTH_RESTRICTED_ENVS
+        | _CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
+        | _CS_POSIX_V6_LP64_OFF64_LIBS..=_CS_POSIX_V7_LPBIG_OFFBIG_LINTFLAGS => 1,
+        _ => {
+            platform::ERRNO.set(errno::EINVAL);
+            0
+        }
+    }
 }
 
 /// See <https://pubs.opengroup.org/onlinepubs/9799919799/functions/crypt.html>.

src/header/unistd/sysconf.rs

@@ -12,7 +12,7 @@ pub use sys::*;
 
 use core::ffi::{c_int, c_long};
 
-#[unsafe(no_mangle)]
+#[no_mangle]
 pub unsafe extern "C" fn sysconf(name: c_int) -> c_long {
     sysconf_impl(name)
 }

tests/Makefile

@@ -124,6 +124,7 @@ EXPECT_NAMES=\
 	unistd/access \
 	unistd/brk \
 	unistd/constants \
+	unistd/confstr \
 	unistd/dup \
 	unistd/exec \
 	unistd/fchdir \

tests/unistd/confstr.c

@@ -0,0 +1,34 @@
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#define CSPATHSZ 9
+#define BADSIZ 4
+
+int main(void) {
+    char actual[CSPATHSZ] = {0};
+    assert(confstr(_CS_PATH, actual, CSPATHSZ) == CSPATHSZ);
+    const char expected[] = "/usr/bin";
+    assert(strncmp(expected, actual, CSPATHSZ) == 0);
+
+    // The constants other than _CS_PATH just return an empty str (no support).
+    char empty[] = "";
+    assert(
+        confstr(
+            _CS_POSIX_V6_LP64_OFF64_LIBS,
+           empty,
+           0
+        ) == 1
+    );
+
+    // Buffers that are too small should return the expected size.
+    char small[BADSIZ] = {0};
+    assert(confstr(_CS_PATH, small, BADSIZ) == CSPATHSZ);
+
+    // Null buffer and zero length should return the expected size
+    // for the constant.
+    assert(confstr(_CS_PATH, NULL, 0) == CSPATHSZ);
+
+    return EXIT_SUCCESS;
+}