crypt(3) Example Encrypting Passwords https://www.gnu.org/software/libc/manual/html_node/crypt.html

check.c

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

/* 
 To compile:
 $ gcc check.c -lcrypt -o check
 $ ./check
 */
int
main(void)
{
  /* Hashed form of "GNU libc manual". */
  const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/";

  char *result;
  int ok;

  /* Read in the user’s password and encrypt it,
     passing the expected password in as the salt. */
  result = crypt(getpass("Password:"), pass);
  /* Test the result. */
  ok = strcmp (result, pass) == 0;

  puts(ok ? "Access granted." : "Access denied.");
  return ok ? 0 : 1;
} 

crypt.c

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

/* 
 To compile:
 $ gcc crypt.c -lcrypt -o crypt
 $ ./crypt
 */
int
main(void)
{
  unsigned long seed[2];
  char salt[] = "$1$........";
  const char *const seedchars =
    "./0123456789ABCDEFGHIJKLMNOPQRST"
    "UVWXYZabcdefghijklmnopqrstuvwxyz";
  char *password;
  int i;

  /* Generate a (not very) random seed.
     You should do it better than this... */
  seed[0] = time(NULL);
  seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000);

  /* Turn it into printable characters from ‘seedchars’. */
  for (i = 0; i < 8; i++)
    salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f];

  /* Read in the user’s password and encrypt it. */
  password = crypt(getpass("Password:"), salt);

  /* Print the results. */
  puts(password);
  return 0;
}