Programatically getting UID and GID from username in Unix?

Question

I'm trying to use setuid() and setgid() to set the respective id's of a program to drop privileges down from root, but to use them I need to know the uid and gid of the user I want to change to.

Is there a system call to do this? I don't want to hardcode it or parse from /etc/passwd .

Also I'd like to do this programatically rather than using:

id -u USERNAME

Any help would be greatly appreciated

Answer 1

Have a look at the getpwnam() and getgrnam() functions.

Answer 2

You want to use the getpw* family of system calls, generally in pwd.h. It's essentially a C-level interface to the information in /etc/passwd.

Answer 3

Look at getpwnam and struct passwd.

Answer 4

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

int main()
{
    char *username = ...

    struct passwd *pwd = calloc(1, sizeof(struct passwd));
    if(pwd == NULL)
    {
        fprintf(stderr, "Failed to allocate struct passwd for getpwnam_r.\n");
        exit(1);
    }
    size_t buffer_len = sysconf(_SC_GETPW_R_SIZE_MAX) * sizeof(char);
    char *buffer = malloc(buffer_len);
    if(buffer == NULL)
    {
        fprintf(stderr, "Failed to allocate buffer for getpwnam_r.\n");
        exit(2);
    }
    getpwnam_r(username, pwd, buffer, buffer_len, &pwd);
    if(pwd == NULL)
    {
        fprintf(stderr, "getpwnam_r failed to find requested entry.\n");
        exit(3);
    }
    printf("uid: %d\n", pwd->pw_uid);
    printf("gid: %d\n", pwd->pw_gid);
    return 0;
}