wasi-libc/libc-bottom-half/sources/sbrk.c

#include <unistd.h>
#include <stdlib.h>
#include <errno.h>
#include <__macro_PAGESIZE.h>

/* Bare-bones implementation of sbrk. */
void *sbrk(intptr_t increment) {
    /* sbrk(0) returns the current memory size. */
    if (increment == 0) {
        /* The wasm spec doesn't guarantee that memory.grow of 0 always succeeds. */
        return (void *)(__builtin_wasm_memory_size(0) * PAGESIZE);
    }

    /* We only support page-size increments. */
    if (increment % PAGESIZE != 0) {
        abort();
    }

    /* WebAssembly doesn't support shrinking linear memory. */
    if (increment < 0) {
        abort();
    }

    uintptr_t old = __builtin_wasm_memory_grow(0, (uintptr_t)increment / PAGESIZE);
   
    if (old == SIZE_MAX) {
        errno = ENOMEM;
        return (void *)-1;
    }

    return (void *)(old * PAGESIZE);
}