We can use getpwnam to get user's system UID by its login name, the function is:
#include <pwd.h>
struct passwd* getpwnam(const char*name);
And the passwd struct is:
struct passwd {
char* pw_name; // login name (username)
char* pw_passwd; // Encryped password
uid_t pw_uid; // User ID
gid_t pw_gid; // Group ID
char* pw_gecos; // Comment
char* pw_dir; // Initial working (home) dir
char* pw_shell; // login shell
}
Example:
#include <stdio.h>
#include <pwd.h>
#include <string.h>
int main() {
char name[30] = {0};
struct passwd* pwd;
size_t nread = 0;
while (1) {
printf("Query UID for login name: ");
fgets(name, sizeof(name), stdin);
nread = strlen(name);
if (name[nread-1] == '\n') {
name[strlen(name)-1] = 0;
}
// or use: name[strcspn(name, "\n")] = 0;
if (strcmp(name, "quit") == 0) {
break;
}
pwd = getpwnam(name);
if (pwd == NULL) {
printf("Cannot find UID for name %s\n", name);
continue;
}
printf("UID: %ld, Login Shell: %s\n",
(long)pwd->pw_uid, pwd->pw_shell);
}
return 0;
}
Compile gcc main.c -o main and run:
Query UID for login name: root
UID: 0, Login Shell: /bin/bash
Query UID for login name: nobody
UID: 65534, Login Shell: /usr/sbin/nologin
Query UID for login name: quit
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