inline assembly with gcc to write a syscall function.

syscall_int.c

#include <stdio.h>
#include <sys/types.h>
#include <sys/syscall.h>
#include <errno.h>


int main()
{
        long rc;
        char *file_name = "/etc/passwd";
        unsigned short mode = 0444;

        asm(
                "int $0x80"
                : "=a" (rc)
                : "0" (SYS_chmod), "b" (file_name), "c" (mode)
        );

        if ((unsigned long)rc >= (unsigned long)-129) {
                errno = -rc;
                rc = -1;
        }

        if (rc == -1)
                fprintf(stderr, "chmode failed, errno = %d\n", errno);
        else
                printf("success!\n");

        return 0;
}