How to retrieve the user name from the user ID

Question

I am implementing the (ls) command on Unix while learning from a book. During the coding part of my implementation of the (ls) command with the (-l) flag, I see that I have to prompt the user and group names of the file. So far I have the user and group IDs from the following lines:

struct stat statBuf;

statBuf.st_uid; //For the user id. 
statBuf.st_gid; //For the group id. 

In the default (ls) command on Unix, the information of the file is printed in such a way that the user name is shown instead of the user id.

Can anyone help me to find the correct methodology to retrieve the user and group names from their IDs?

Finecut

Answer 1

You use getpwuid to look up the password file entry for a particular UID (which includes the user name, but now not the password itself) and getgrgid to look up the group file entry for a particular GID.

Medrek

I presume that reading the manual page for those two functions will tell you the rest of what you need to know… – Medrek

Answer 2

check my code for username:

#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

string getUser(uid_t uid)
{
    struct passwd *pws;
    pws = getpwuid(uid);
        return pws->pw_name;
}

for groupname you can use getgrgid.

Firedog

Your sample has some flaws: There is no string type in C. And you do not check for NULL pointer that might be returned in case the uid is unknown. Apart from that is is essentially the same set of functions that was suggested in Donal Fellows' answer. – Broadspectrum