int fd; // file handle

while ((fd = open("file.txt", O_RDONLY)) == -1 && errno == EINTR)
{
    // repeat while fd is -1 and errno is EINTR
}

if (fd < -1) // if fd is still -1 that's an error
{
    // report error
}
// else fd is valid file descriptor

int fd;
TEMP_FAILURE_RETRY(fd = open("file.txt", O_RDONLY));
if (fd < 0)
{
   // error
}
// else success

#include <stddef.h>
#include <syscall.h>

// returns negative value for error (for example, if error is EINVAL, then -EINVAL is returned)
static int my_write(int fd, const void *buf, size_t size)
{
    long result;
    __asm__ __volatile__(
        "syscall"
        : "=a"(result)
        : "0"(__NR_write), "D"(fd), "S"(buf), "d"(size)
        : "cc", "rcx", "r11", "memory");
    return result;
}

static void my_exit(int code)
{
    __asm__ __volatile__(
        "syscall"
        :
        : "a"(__NR_exit)
        : "cc", "rcx", "r11", "memory");
    __builtin_unreachable(); // syscall above never returns
}

void _start()
{
    char text[] = "Hello, world!\n";

    // for this example let's ignore result of write
    // but you should really handle it
    // 1 is stdout file handle
    my_write(1, text, sizeof(text) - 1);

    my_exit(0);
}

$ gcc -s -Os -nostdlib -ffreestanding a.c

$ ./a.out
Hello, world!

$ objdump -t a.out

a.out:     file format elf64-x86-64

SYMBOL TABLE:
no symbols


$ objdump -S a.out

a.out:     file format elf64-x86-64


Disassembly of section .text:

0000000000400144 <.text>:
  400144:       b8 01 00 00 00          mov    $0x1,%eax
  400149:       48 8d 7c 24 f1          lea    -0xf(%rsp),%rdi
  40014e:       be 6f 01 40 00          mov    $0x40016f,%esi
  400153:       b9 0f 00 00 00          mov    $0xf,%ecx
  400158:       ba 0f 00 00 00          mov    $0xf,%edx
  40015d:       f3 a4                   rep movsb %ds:(%rsi),%es:(%rdi)
  40015f:       48 8d 74 24 f1          lea    -0xf(%rsp),%rsi
  400164:       89 c7                   mov    %eax,%edi
  400166:       0f 05                   syscall
  400168:       b8 3c 00 00 00          mov    $0x3c,%eax
  40016d:       0f 05                   syscall