How to write to a file using open() and printf()?

There’s a trick to this.

You need to duplicate the open file descriptor to file descriptor 1, i.e. stdout. Then you can use printf:

int main(int argc, char *argv[]){

    int fd;
    char *name = "helloworld";
    fd = open(name, O_WRONLY | O_CREAT, 0644);
    if (fd == -1) {
        perror("open failed");
        exit(1);
    }

    if (dup2(fd, 1) == -1) {
        perror("dup2 failed"); 
        exit(1);
    }

    // file descriptor 1, i.e. stdout, now points to the file
    // "helloworld" which is open for writing
    // You can now use printf which writes specifically to stdout

    char *hi = "Hello World";
    printf("%s\n", hi);

    exit(0);

}