This is users_groups/check_password.c (Listing 8-2, page 164), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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> */
#define _XOPEN_SOURCE   /* Get crypt() declaration from <unistd.h> */
#include <unistd.h>
#include <limits.h>
#include <pwd.h>
#include <shadow.h>
#include "tlpi_hdr.h"
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)

    printf("Username: ");
    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)

    authOk = strcmp(encrypted, pwd->pw_passwd) == 0;
    if (!authOk) {
        printf("Incorrect password\n");

    printf("Successfully authenticated: UID=%ld\n", (long) pwd->pw_uid);

    /* Now do authenticated work... */



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