‘Hello, World!’ C program made without the use of C's Standard Library

headers.h

#define WRITE        1
#define STDOUT       1
#define EXIT         60
#define EXIT_SUCCESS 0

main.c

#include "headers.h"
 
typedef unsigned long int size_t;  // 64-bit, Absence of an extra 's' infers an unsigned value.
typedef long int ssize_t;          // 64-bit, Extra 's' infers a signed value. 

extern _Noreturn void exit(int code);
extern         size_t strlen(const char *s);
extern        ssize_t write(int fd, void const* data, size_t nbytes);

void _start(void)
{
    const char *msg = "Hello, World!\n";
    write(STDOUT, msg, strlen(msg));
    exit(EXIT_SUCCESS);
}

main.S

#include "headers.h"

.globl write, exit, strlen

.text
    
write:
        mov    $WRITE, %rax
        syscall
        ret

strlen:
        xor    %rax, %rax         /* 'rax' register will serve as our return value.                                               */
        .loop:
                cmpb   $0, (%rdi) /* Comparing the byte value's stored in 'rdi' to '0', with '0' being the ASCII null terminator. */
                je     .end       /* If we meet an ASCII null term, jump to '.end' label.                                         */
                inc    %rdi       /* Incrementing the index of our character array.                                               */
                inc    %rax       /* Incrementing our return value in correlation.                                                */
                jmp    .loop      /* Looping back by jumping to our '.loop' label.                                                */
        .end:
                ret               /* Returns the value stored in the 'rax' register.                                              */

exit:
        mov    $EXIT, %rax
        syscall