A simple read file function for x86_64 Linux implemented in asm

readfile.c

#include <sys/syscall.h>
#include <stddef.h>
#include <fcntl.h>

int readfile(const char *file, char *buf, size_t size)
{
    int ret;
    asm (
        "mov  %[mode], %%rsi \n\t"
        "mov  %[open], %%rax \n\t"
        "syscall \n\t"
        "cmp  $0, %%rax \n\t"
        "jl   100f \n\t"

        "mov  %%rax, %%rdi \n\t"
        "mov  %[buf], %%rsi \n\t"
        "mov  %[size], %%rdx \n\t"
        "mov  %[read], %%rax \n\t"
        "syscall \n\t"
    "100: \n\t"
        : "=a"(ret)
        : [file]"D"(file), [buf]"r"(buf), [size]"r"(size), [open]"i"(__NR_open), [read]"i"(__NR_read), [mode]"i"(O_RDONLY)
        : "memory"
    );
    return ret;
}

// test
#include <stdio.h>
int main()
{
    char buf[128];
    buf[0] = 0;
    int ret = readfile("test_file", buf, 128);
    printf("%s\n", buf);
    return ret;
}