getlogin() c function returns NULL and error "No such file or directory"

Question

I have a question regarding the getlogin() function (). I tried to get the login name of my account from the c program using this function. But the function returns a NULL. Using perror shows that the error is "No such file or directory".

I don't get what is the problem. Is there a way to get user login name in a program.

Here is a sample code:

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

int main()
{
  char *name;
  name = getlogin();
  perror("getlogin() error");
  //printf("This is the login info: %s\n", name);
  return 0;
}

And this is the output: getlogin() error: No such file or directory

Please let me know how to get this right.

Thanks.

Answer 1

getlogin is an unsafe and deprecated way of determining the logged-in user. It's probably trying to open a record of logged-in users, perhaps utmp or something. The correct way to determine the user you're running as (which might not be the same as the logged-in user, but is almost always better to use anyway) is getpwuid(getuid()).

Answer 2

Here is a good link I found explaining that it may not work: getlogin

Here is a quote from it:

Unfortunately, it is often rather easy to fool getlogin(). Sometimes it does not work at all, because some program messed up the utmp file

Answer 3

It works fine for me if I comment perror call.

From man:

getlogin() returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined.'

So you should do:

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

int main()
{
  char *name;
  name = getlogin();
  if (!name)
    perror("getlogin() error");
  else
    printf("This is the login info: %s\n", name);
  return 0;
}