Define functions in libc named __attribute__((weak))

c/ulibc.c

@@ -11,6 +11,7 @@
 #define HIGHS (ONES * (UCHAR_MAX / 2 + 1))
 #define HASZERO(x) (((x) - ONES) & ~(x) & HIGHS)
 
+__attribute__((weak)) 
 char *__stpcpy(char *restrict d, const char *restrict s)
 {
 #ifdef __GNUC__
@@ -36,19 +37,22 @@ char *__stpcpy(char *restrict d, const char *restrict s)
         return d;
 }
 
+__attribute__((weak)) 
 char *strcpy(char *restrict dest, const char *restrict src)
 {
         __stpcpy(dest, src);
         return dest;
 }
 
+__attribute__((weak)) 
 int strcmp(const char *l, const char *r)
 {
         for (; *l == *r && *l; l++, r++)
                 ;
         return *(unsigned char *)l - *(unsigned char *)r;
 }
 
+__attribute__((weak)) 
 int strncmp(const char *_l, const char *_r, size_t n)
 {
         const unsigned char *l = (void *)_l, *r = (void *)_r;
@@ -60,8 +64,10 @@ int strncmp(const char *_l, const char *_r, size_t n)
 }
 
 // fix me
+__attribute__((weak)) 
 FILE *const stdout = NULL;
 
+__attribute__((weak)) 
 int fflush(FILE *f)
 {
         // printf("fflush() is not implemented !\n");
@@ -70,6 +76,7 @@ int fflush(FILE *f)
 
 // +++++++++ uClibc +++++++++
 
+__attribute__((weak)) 
 void *memset(void *s, int c, size_t n)
 {
         register unsigned char *p = (unsigned char *)s;
@@ -84,6 +91,7 @@ void *memset(void *s, int c, size_t n)
 // musl, typedef int (*cmpfun)(const void *, const void *);
 typedef int (*__compar_fn_t)(const void *, const void *);
 typedef int (*__compar_d_fn_t)(const void *, const void *, void *);
+__attribute__((weak)) 
 void qsort_r(void *base,
              size_t nel,
              size_t width,
@@ -140,10 +148,11 @@ void qsort_r(void *base,
         }
 }
 
+__attribute__((weak)) 
 void qsort(void *base,
            size_t nel,
            size_t width,
            __compar_fn_t comp)
 {
         return qsort_r(base, nel, width, (__compar_d_fn_t)comp, NULL);
-}
+}

src/lib.rs

@@ -1,11 +1,12 @@
 #![no_std]
+#![feature(linkage)]
+#![feature(c_variadic, c_size_t)]
+#![feature(associated_type_defaults)]
+
 #![allow(non_upper_case_globals)]
 #![allow(non_camel_case_types)]
 #![allow(non_snake_case)]
 
-#![feature(c_variadic, c_size_t)]
-#![feature(associated_type_defaults)]
-
 extern crate alloc;
 
 #[macro_use]

src/ulibc.rs

@@ -5,6 +5,7 @@ use core::cmp::min;
 use core::ffi::{c_char, c_int, c_size_t, c_void};
 
 #[cfg(feature = "print")]
+#[linkage = "weak"]
 #[no_mangle]
 unsafe extern "C" fn printf(str: *const c_char, mut args: ...) -> c_int {
     // extern "C" { pub fn printf(arg1: *const c_char, ...) -> c_int; }
@@ -19,6 +20,7 @@ unsafe extern "C" fn printf(str: *const c_char, mut args: ...) -> c_int {
 }
 
 #[cfg(not(feature = "print"))]
+#[linkage = "weak"]
 #[no_mangle]
 unsafe extern "C" fn printf(str: *const c_char, mut args: ...) -> c_int {
     use core::ffi::CStr;
@@ -34,6 +36,7 @@ pub extern "C" fn ext4_user_malloc(size: c_size_t) -> *mut c_void {
     malloc(size)
 }
 
+#[linkage = "weak"]
 #[no_mangle]
 pub extern "C" fn calloc(m: c_size_t, n: c_size_t) -> *mut c_void {
     let mem = malloc(m * n);
@@ -44,6 +47,7 @@ pub extern "C" fn calloc(m: c_size_t, n: c_size_t) -> *mut c_void {
     unsafe { memset(mem, 0, m * n) }
 }
 
+#[linkage = "weak"]
 #[no_mangle]
 pub extern "C" fn realloc(memblock: *mut c_void, size: c_size_t) -> *mut c_void {
     if memblock.is_null() {
@@ -78,6 +82,7 @@ struct MemoryControlBlock {
 const CTRL_BLK_SIZE: usize = core::mem::size_of::<MemoryControlBlock>();
 
 /// Allocate size bytes memory and return the memory address.
+#[linkage = "weak"]
 #[no_mangle]
 pub extern "C" fn malloc(size: c_size_t) -> *mut c_void {
     // Allocate `(actual length) + 8`. The lowest 8 Bytes are stored in the actual allocated space size.
@@ -94,6 +99,7 @@ pub extern "C" fn malloc(size: c_size_t) -> *mut c_void {
 }
 
 /// Deallocate memory at ptr address
+#[linkage = "weak"]
 #[no_mangle]
 pub extern "C" fn free(ptr: *mut c_void) {
     if ptr.is_null() {