how to call sys_open rather than sys_openat when calling open

Question

I write a code to generate system call

void open_test(int fd, const char *filepath) {
    if (fd == -1) {
        printf(&qu         


        

Answer 1

You call via the syscall(2) wrapper: syscall(SYS_open, ...):

#define _GNU_SOURCE
#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <sys/syscall.h>

int main(void){
        char *path = "whatever.txt";
        int fd = syscall(SYS_open, path, O_RDONLY, 0);
        if(fd == -1) err(1, "SYS_open %s", path);
}

But why bother? SYS_openat is the canonical system call now, open(2) is just an API, and the SYS_open system call entry is only kept for backward binary compatibility.

On newer architectures, there may be no actual SYS_open system call at all.