users_groups/check_password.c

+/* check_password.c
+
+   Read a user name and password and check if they are valid.
+
+   This program uses the shadow password file. Some UNIX implementations
+   don't support this feature.
+*/
+/* Compile with -lcrypt */
+#if ! defined(__sun)
 #define _BSD_SOURCE     /* Get getpass() declaration from <unistd.h> */
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE   /* Get crypt() declaration from <unistd.h> */
+#endif
+#endif
 #include <unistd.h>
 #include <limits.h>
 #include <pwd.h>
 #include <shadow.h>
+#include <crypt.h>
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     char *username, *password, *encrypted, *p;
     struct passwd *pwd;
     struct spwd *spwd;
     Boolean authOk;
     size_t len;
     long lnmax;
 
+    /* Determine size of buffer required for a username, and allocate it */
+
     lnmax = sysconf(_SC_LOGIN_NAME_MAX);
     if (lnmax == -1)                    /* If limit is indeterminate */
         lnmax = 256;                    /* make a guess */
 
     username = malloc(lnmax);
     if (username == NULL)
         errExit("malloc");
 
     printf("Username: ");
     fflush(stdout);
     if (fgets(username, lnmax, stdin) == NULL)
         exit(EXIT_FAILURE);             /* Exit on EOF */
 
     len = strlen(username);
     if (username[len - 1] == '\n')
         username[len - 1] = '\0';       /* Remove trailing '\n' */
 
+    /* Look up password and shadow password records for username */
+
     pwd = getpwnam(username);
     if (pwd == NULL)
         fatal("couldn't get password record");
     spwd = getspnam(username);
     if (spwd == NULL && errno == EACCES)
         fatal("no permission to read shadow password file");
 
     if (spwd != NULL)           /* If there is a shadow password record */
         pwd->pw_passwd = spwd->sp_pwdp;     /* Use the shadow password */
 
     password = getpass("Password: ");
 
     /* Encrypt password and erase cleartext version immediately */
 
     encrypted = crypt(password, pwd->pw_passwd);
     for (p = password; *p != '\0'; )
         *p++ = '\0';
 
     if (encrypted == NULL)
         errExit("crypt");
 
     authOk = strcmp(encrypted, pwd->pw_passwd) == 0;
     if (!authOk) {
         printf("Incorrect password\n");
         exit(EXIT_FAILURE);
     }
 
     printf("Successfully authenticated: UID=%ld\n", (long) pwd->pw_uid);
 
     /* Now do authenticated work... */
 
     exit(EXIT_SUCCESS);
 }