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	Crypt (password or data encoding)
Related functions	Getpass
Header file	# DEFINE _ xopen_source # Include <unistd. h>
Define functions	Char * crypt (const char * Key, const char * salt );
Function Description	Crypt () uses the Data Encryption Standard (DES) algorithm to encode the string referred to by the parameter key. The length of the key string is only the first 8 characters. It is meaningless to exceed the length. The salt parameter is a string consisting of a-Z, a-Z, 0-9, ". "and"/"are used to determine which of the 4096 different built-in tables to use. After the function is successfully executed, a pointer to the encoded string is returned, and the string referred to by the parameter key is not changed. The encoded string is 13 characters long and the first two characters are strings represented by the salt parameter.
Return Value	Returns a password string that points to the end of null.
Additional instructions	-Lcrypt must be added during GCC compilation.
Example	# Include <unistd. h> Main () { Char passwd [13]; Char * key; Char slat [2]; Key = getpass ("input first Password :"); Slat [0] = Key [0]; Slat [1] = Key [1]; Strcpy (passwd, crypt (Key slat )); Key = getpass ("Input second password :"); Slat [0] = passwd [0]; Slat [1] = passwd [1]; Printf ("After crypt (), 1st passwd: % s/n", passwd ); Printf ("After crypt (), 2nd passwd: % s/n", crypt (Key slat )); }
Run	Input first Password:/* enter test, which is encoded and stored in passwd [] */ Input second password/* enter test, and the password will be the same after the same encoding */ After crypt () 1st passwd: teh0wlipw0gyq After crypt () 2nd passwd: teh0wlipw0gyq
	Bsearch)
Related functions	Qsort
Header file	# Include <stdlib. h>
Define functions	Void * bsearch (const void * Key, const void * base, size_t nmemb, size_tsize, INT (* COMPAR) (const void *, const void *));
Function Description	Bsearch () uses binary search to find data from sorted arrays. The parameter key points to the key data to be searched. The parameter base points to the address starting with the array to be searched. The parameter nmemb indicates the number of elements in the array. The size of each element is determined by the parameter size, the last COMPAR parameter is a function pointer used to determine the size relationship between two elements, if the element data referred to by the first parameter passed to COMPAR is greater than the element data referred to by the second parameter, the value greater than 0 must be returned. If the two elements are equal, the return value is 0.
Additional instructions	If key data is found, the address is returned. If no key data is found in the array, null is returned.
Example	# Include <stdio. h> # Include <stdlib. h> # Define nmemb 5 # Define size 10 Int COMPAR (const void * a, const void * B) { Return (strcmp (char *) A, (char *) B )); } Main () { Char data [50] [size] = {"Linux", "FreeBSD", "Solaris", "SunOS", "windows "}; Char key [80], * base, * offset; Int I, nmemb = nmemb, size = size; While (1 ){ Printf ("> "); Fgets (Key, sizeof (key), stdin ); Key [strlen (key)-1] = '/0 '; If (! Strcmp (Key, "exit") break; If (! Strcmp (Key, "list ")){ For (I = 0; I <nmemb; I ++) Printf ("% s/n", data [I]); Continue; } Base = data [0]; Qsort (base, nmemb, size, COMPAR ); Offset = (char *) bsearch (Key, base, nmemb, size, COMPAR ); If (offset = NULL ){ Printf ("% s not found! /N ", key ); Strcpy (data [nmemb ++], key ); Printf ("Add % s to data array/N", key ); } Else { Printf ("found: % s/n", offset ); } } }
Run	> Hello/* enter the hello string */ Hello not found! /* The Hello string cannot be found */ Add hello to data array/* Add the hello string */ >. List/* list all data */ FreeBSD Linux Solaris SunOS Windows Hello > Hello Found: Hello
	Lfind (linear search)
Related functions	Lsearch
Header file	# Include <stdlib. h>
Define functions	Void * lfind (const void * Key, const void * base, size_t * nmemb, size_t Size, INT (* COMPAR) (const void *, const void *));
Function Description	Lfind () uses linear search to search for data from the beginning to the end of an array. The parameter key points to the key data to be searched. The parameter base points to the address starting with the array to be searched. The parameter nmemb indicates the number of elements in the array. The size of each element is determined by the parameter size, the last COMPAR parameter is a function pointer, which is used to determine whether two elements are the same, if the element data referred to by the remote parameter passed to COMPAR is the same as the element data referred to by the second parameter, 0 is returned. If the two elements are different, a non-0 value is returned. The difference between lfind () and lsearch () is that when key data cannot be found, lfind () Only returns NULL, rather than actively adding the data to the end of the array.
Return Value	If the key data is found, the address of the element is returned. If the key data cannot be found in the array, a null pointer is returned ).
Example	Refer to lsearch ().
	Lsearch)
Related functions	Lfind
Header file	# Include <stdlib. h>
Define functions	Void * lsearch (const void * Key, const void * base, size_t * nmemb, size_t size, INT (* COMPAR) (const void *, const void *));
Function Description	Lsearch () uses linear search to search for data from the beginning to the end of an array. The parameter key points to the key data to be searched. The parameter base points to the address starting with the array to be searched. The parameter nmemb indicates the number of elements in the array. The size of each element is determined by the parameter size, the last COMPAR parameter is a function pointer, which is used to determine whether two elements are the same, if the element data referred to by the first parameter passed to COMPAR is the same as the element data referred to by the second parameter, 0 is returned. If the two elements are different, a non-0 value is returned. If the key data cannot be found by lsearch (), the data is automatically added to the array.
Return Value	If the key data is found, the limbs of the element are returned. If the key data cannot be found in the array, the key data is added to the array, and the address after the array is returned.
Example	# Include <stdio. h> # Include <stdlib. h> # Define nmemb 50 # Define size 10 Int COMPAR (comst void * a, const void * B) { Return (strcmp (char *) A, (char *) B )); } Main () { Char data [nmemb] [size] = {"Linux", "FreeBSD", "solzris", "SunOS", "windows "}; Char key [80], * base, * offset; Int I, nmemb = nmemb, size = size; For (I = 1; I <5; I ++ ){ Fgets (Key, sizeof9key), stdin ); Key [strlen (key)-1] = '/0 '; Base = data [0]; Offset = (char *) lfind (Key, base, & nmemb, size, COMPAR ); If (offset = NULL ){ Printf ("% s not found! /N ", key ); Offset = (char *) lsearch (Key, base, & nmemb, size, COMPAR ); Printf ("Add % s to data array/N", offset ); } Else { Printf ("found: % s/n", offset ); } } }
Run	Linux Found: Linux OS/2 OS/2 not found! Add OS/2 to data array OS/2 Found: OS/2
	Qsort (sort arrays by quick sorting)
Related functions	Bsearch
Header file	# Include <stdlib. h>
Define functions	Void qsort (void * base, size_t nmemb, size_t size, INT (* COMPAR) (const void *, const void *));
Function Description	The base parameter points to the address starting with the array to be sorted. The nmemb parameter indicates the number of elements in the array. The size of each element is determined by the parameter size. The final parameter COMPAR is a function pointer, this function is used to determine the size relationship between two elements. If the element data passed to the first parameter of COMPAR is greater than the element data referred to by the second parameter, it must return a value greater than zero, if the two elements are equal, 0 is returned.
Return Value
Additional instructions
Example	# Define nmemb 7 # Include <stdlib. h> Int COMPAR (const void * a, const void * B) { Int * AA = (int *) A, * BB = (int *) B; If (* AA> * bb) return 1; If (* AA = * bb) return 0; If (* AA <* bb) Return-1; } Main () { Int base [nmemb] = {3,102, 5 }; Int I; For (I = 0; I <nmemb; I ++) Printf ("% d", base [I]); Printf ("/N "); Qsort (base, nmemb, sizeof (INT), COMPAR ); For (I = 0; I <nmemb; I ++) Printf ("% d" base [I]); Printf ("/N "); }
Run	3 102 5-2 98 52 18 -2 3 5 18 52 98 102
	Rand (Random Number Generation)
Related functions	Srand, random, srandom
Header file	# Include <stdlib. h>
Define functions	Int rand (void)
Function Description	Rand () returns a random value ranging from 0 to rand_max. Before calling this function to generate a random number, you must use srand () to set the random number seed. If no random number seed is set, Rand () will automatically set the random number seed to 1. For details about random seed, see srand ().
Return Value	Returns a random value between 0 and rand_max. rand_max is defined in stdlib. h and its value is 2147483647.
Example	/* Generate a random value ranging from 1 to 10. In this example, no random seed is set. For the complete random number generation, see Srand ()*/ # Include <stdlib. h> Main () { Int I, J; For (I = 0; I <10; I ++) { J = 1 + (INT) (10.0 * rand ()/(rand_max + 1.0 )); Printf ("% d", J ); } }
Run	9 4 8 8 10 2 4 8 3 6 9 4 8 8 10 2 4 8 3 6
	Srand (set Random Seed)
Related functions	Rand, random srandom
Header file	# Include <stdlib. h>
Define functions	Void srand (unsigned int seed );
Function Description	Srand () is used to set the random number seed when rand () generates a random number. The seed parameter must be an integer. Generally, the return value of geypid () or time (0) can be used as seed. If the same value is set for each seed, the random values generated by rand () are the same each time.
Return Value
Example	/* Generate a random number ranging from 1 to 10. This example and execution result can be referenced with RAND */ # Include <time. h> # Include <stdlib. h> Main () { Int I, J; Srand (INT) time (0 )); For (I = 0; I <10; I ++) { J = 1 + (INT) (10.0 * rand ()/(rand_max + 1.0 )); Printf ("% d", J ); } }
Run	5 8 8 8 10 2 10 8 9 9 2 9 7 4 10 3 2 10 8 7