On Godbolt, opening /dev/null for writing fails when you use fopen but succeeds when you use open, and I have no idea why.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

int main() {
    if (fopen("/dev/null", "w") == NULL) {
        perror("fopen");
    }
    if (open("/dev/null", O_WRONLY) == -1) {
        perror("open");
    }
}

https://godbolt.org/z/1Y6x58Tv6