C snprintf specify user home directory

Question

I use snprintf to write formatted data to disk, but I have one problem, how do I save it to the user's home directory?

 snprintf(filename, sizeof(filename), "%s_%s.sho", client, id);

Answer 1

Use getenv(3) to query the value of the HOME environment variable. For example, this prints my home directory:

#include <stdlib.h>
#include <stdio.h>
int main(void) {
    printf("%s\n", getenv("HOME"));
    return 0;
}

You can set your variable filename to the return value, and then write whatever data you want there.

This should work on any Unix-like system, including Linux, macOS, BSD, and probably many more.

Answer 2

On Linux and POSIX systems the home directory is often from the HOME environment variable. So you might code

snprintf(filename, sizeof(filename), "%s/%s_%s.sho", 
         getenv("HOME"), client, id);

Pedantically the getenv(3) could fail (or be wrong). But that rarely happens. See environ(7).

(You might check, and or use getpwuid(3) with getuid(2)...)

With setuid executables things could become interestingly complex. You would need to define more precisely what the home is, and code appropriately (this is left as an exercise).

Answer 3

The user controls his environment - so the HOME environment variable may not be correct or it may not even be set.

Use getuid() and getpwuid() to get the user's home directory as specified by your system:

#include <unistd.h>
#include <pwd.h>

struct passwd *pwd = getpwuid( getuid() );

/* need to duplicate the string - we don't know
   what pw_dir points to */
const char *homeDir = strdup( pwd->pw_dir );

Error checking is left as an exercise...