Bad file descriptor

Question

I'm learning about file descriptors and I wrote this code:

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

int fdrd, fdwr, fdwt;
char c;

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

    if((fdwt = open("output", O_CREAT, 0777)) == -1) {
        perror("Error opening the file:");
        exit(1);
    }

    char c = 'x';

    if(write(fdwt, &c, 1) == -1) {
        perror("Error writing the file:");
    }

    close(fdwt);
    exit(0);

}

, but I'm getting: Error writing the file:: Bad file descriptor

I don't know what could be wrong, since this is a very simple example.

Solution 1

Try this:

open("output", O_CREAT|O_WRONLY, 0777)

Solution 2

I think O_CREAT alone is not enough. Try adding O_WRONLY as flag to the open command.

Solution 3

According to the open(2) man page:

The argument flags must include one of the following access modes: O_RDONLY, O_WRONLY, or O_RDWR.

So yes, as suggested by others, please change your open to open("output", O_CREAT|O_WRONLY, 0777));. Use O_RDWR if you need to read from the file. You may also want O_TRUNC -- see the man page for details.

Comments

[Lucy]

Wow! So fast! :) That worked! Now I have to find out why! Thank you very much!

[asveikau]

@Lucy - It gave you a file descriptor, so the open didn't fail ... but the descriptor was not valid for writing.

[PyWalker2797]

Could you explain why O_WRONLY is required, and where 0777 comes from?