https://security.web.cern.ch/security/recommendations/en/codetools/c.shtml

safefile.c

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
 
#define MY_TMP_FILE "/tmp/file.tmp"
 
enum { FILE_MODE = 0600 };
 
int main(int argc, char* argv[])
{
    int fd;
    FILE* f;
 
    /* Remove possible symlinks */
    unlink(MY_TMP_FILE);
    /* Open, but fail if someone raced us and restored the symlink (secure version of fopen(path, "w") */
    fd = open(MY_TMP_FILE, O_WRONLY|O_CREAT|O_EXCL, FILE_MODE);
    if (fd == -1) {
        perror("Failed to open the file");
        return EXIT_FAILURE;
    }
    /* Get a FILE*, as they are easier and more efficient than plan file descriptors */
    f = fdopen(fd, "w");
    if (f == NULL) {
        perror("Failed to associate file descriptor with a stream");
        return EXIT_FAILURE;
    }
    fprintf(f, "Hello, world\n");
    fclose(f);
    /* fd is already closed by fclose()!!! */
    return EXIT_SUCCESS;
}