CODEWITHSUNDEEP
cfileunixstdoutfile-descriptor
user1217715
user1217715

Reputation: 23

Default file descriptor table not working properly

I am having some trouble with file descriptors. The standard fd table on a POSIX-compliant system should, for each process, have stdin as 0, stdout as 1, and stderr as 2. Thereafter, file descriptors should be issued sequentially, so a request to open() a new file should return the file descriptor 3.

This does not happen on my system. Code which was working a few hours ago has ceased to do so. Because it is very long and messy code, I have written a quick example of the sort of thing I am seeing.

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

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

{
    int fToWrite;
    char *outFileName = "chuckles.txt";

    if (fToWrite = open(outFileName,O_WRONLY|O_CREAT|O_TRUNC) < 0) {
            fprintf(stderr,"error during open!: %s\n",strerror(errno));
            exit(EXIT_FAILURE);
    }

    write(fToWrite,"kekeke",6);
    fprintf(stderr,"fToWrite = %i\n",fToWrite);
    close(fToWrite);
}

On my machine, after running this code, chuckles.txt is created, but nothing is written to it. To the console is printed:

kekekefToWrite = 0

Note that kekeke was not written to chuckles.txt, and that fToWrite, the file descriptor of the file I open()ed, is 0. But 0 is reserved for stdin. Likewise, if I try to fprintf to STDOUT_FILENO (which is the int 1) or STDERR_FILENO (which is 2), I get a segfault. In the main program I am trying to write, the program segfaults when I try to write() to STDOUT_FILENO. A few hours ago, the same code snippets would happily put letters on the console; I don't know what changed. I've rebooted, but I don't know what else to do about it.

Strangely, printf still works. I was under the impression that printf just wrote to whatever was in the fd table's #2 entry.

Upvotes: 2

Views: 452

Answers (2)

caf
caf

Reputation: 239011

< has higher precedence than =, so you are assigning the result of open(outFileName,O_WRONLY|O_CREAT|O_TRUNC) < 0 to fToWrite, which is either 0 or 1.

So you end up writing to your terminal.

Upvotes: 0

Thomas Padron-McCarthy
Thomas Padron-McCarthy

Reputation: 27632

You need an extra pair of parentheses:

if ((fToWrite = open(outFileName,O_WRONLY|O_CREAT|O_TRUNC)) < 0) {

That's why fToWrite gets the value 0.

Upvotes: 2

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