[SOLVED] why does fdopen() modify the file permissions set by open()

Issue

I am using a simple C program where I am setting the file permissions while creating the file to 0664 with open() and then passing the file descriptor to fdopen() and doing fwrite() into the file followed by fclose(). But to my surprise, I see the file permissions change to 0644 after the program completes the execution.

My code:

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


int main (void)
{
    int fd = -1;
    FILE *fp = NULL;

    fd = open("/tmp/myfile.cfg", O_WRONLY | O_CREAT | O_CLOEXEC, 0664);
    fp = fdopen(fd, "w");
    fclose(fp);
    return 0;
}

Why do the file permissions get modified after the fd is handed over to fdopen()? How can I preserve the permissions that I set in the open() call?

Solution

The file mode is set to the value of (664 & ~umask), where umask is a file mode creation mask, typically set to 022 by default. That is why your file permission is set to 644, because 0666 & ~022 = 0644; i.e., rw-r--r--.

In order to keep your desired permissions you have to set your process umask to 0000.

Docs for open: https://man7.org/linux/man-pages/man2/open.2.html

Search for umask.

How to set umask: https://man7.org/linux/man-pages/man2/umask.2.html

Or better, set file permissions by chmod after close. See comment by Andrew Henle below.

Answered By - Andrey Ivanov

Answer Checked By - Willingham (WPSolving Volunteer)