input output no libc

README.txt

Compile with gcc
gcc -nostdlib main.c 

input-output-nolibc.c

#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/syscall.h>
#include <sys/stat.h>
#define BUFSIZ (1 << 12)

int my_open(const char *filename, int flags, mode_t mode)
{
    int ret;
    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(SYS_open), "D"(filename), "S"(flags), "d"(mode)
        : "rcx", "r11", "memory");
    if (ret < 0)
    {
        return -1;
    }
    return ret;
}

ssize_t my_read(int fd, void *buf, size_t count)
{
    ssize_t ret;
    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(SYS_read), "D"(fd), "S"(buf), "d"(count)
        : "rcx", "r11", "memory");
    if (ret < 0)
    {
        return -1;
    }
    return ret;
}

ssize_t my_write(int fd, const void *buf, size_t count)
{
    ssize_t ret;
    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(SYS_write), "D"(fd), "S"(buf), "d"(count)
        : "rcx", "r11", "memory");
    if (ret < 0)
    {
        return -1;
    }
    return ret;
}

int my_close(int fd)
{
    int ret;
    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(SYS_close), "D"(fd)
        : "rcx", "r11", "memory");
    if (ret < 0)
    {
        return -1;
    }
    return ret;
}

int _start()
{
    const char input_file[] = "/tmp/inputfile";
    const char output_file[] = "/tmp/outputfile";

    int fd_input, fd_output;
    ssize_t bytes_read, bytes_written;

    fd_input = my_open(input_file, O_RDONLY, 0);
    if (fd_input == -1)
    {
        return 1;
    }

    fd_output = my_open(output_file, O_WRONLY | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
    if (fd_output == -1)
    {
        my_close(fd_input);
        return 1;
    }

    char buffer[BUFSIZ];

    while ((bytes_read = my_read(fd_input, buffer, BUFSIZ)) > 0)
    {
        bytes_written = my_write(fd_output, buffer, bytes_read);
        if (bytes_written != bytes_read)
        {
            my_close(fd_input);
            my_close(fd_output);
            return 1;
        }
    }

    if (bytes_read == -1)
    {
        my_close(fd_input);
        my_close(fd_output);
        return 1;
    }

    if (my_close(fd_input) == -1)
    {
        return 1;
    }
    if (my_close(fd_output) == -1)
    {
        return 1;
    }

    return 0;
}