These functions are powerful and widely used. They are probably more widely used than the functions that concatenate and copy strings. You may have trouble guessing the name of the search function you need in the Stand- ard C library, but it is probably there. At least you can usually find a search function that comes close enough to meet your needs.

#include <string.h>
void *memchr(const void *s, int c, size_t n);

#include <string.h>
char *strchr(const char *s, int c);

#include <string.h>
size_t strcspn(const char *s1, const char *s2);

#include <string.h>
char *strpbrk(const char *s1, const char *s2);

#include <string.h>
char *strrchr(const char *s, int c);

#include <string.h>
size_t strspn(const char *s1, const char *s2);

#include <string.h>
char *strstr(const char *s1, const char *s2);

#include <string.h>
char *strtok(char *s1, const char *s2);

The first call in the sequence searches the string pointed to by s1 for the first character that is not contained in the current separator string pointed to by s2. If no such character is found, then there are no tokens in the string pointed to by s1 and the strtok function returns a null pointer. If such a character is found, it is the start of the first token.

The strtok function then searches from there for a character that is contained in the current separator string. If no such character is found, the current token extends to the end of the string pointed to by s1, and subsequent searches for a token will return a null pointer. If such a character is found, it is overwritten by a null character, which terminates the current token. The strtok function saves a pointer to the following character, from which the next search for a token will start.

Each subsequent call, with a null pointer as the value of the first argument, starts searching from the saved pointer and behaves as described above.

The implementation shall behave as if no library function calls the strtok function.

#include <string.h>
static char str[] = "?a???b,,,#c";
char *t;

t = strtok(str, "?");   /* t points to the token "a" */
t = strtok(NULL, ",");  /* t points to the token "??b" */
t = strtok(NULL, "#,"); /* t points to the token "c" */
t = strtok(NULL, "?");  /* t is a null pointer */

strchr — Use this function to locate the first occurrence (the one having the lowest subscript) of a character in a null-terminated string. The function type casts the second (search character) argument to char. That ensures that an argument expression of any character type behaves sensibly and predictably. A search failure returns a null pointer, however. Be sure to test the return value before you try to use it to access storage. Note that the call strchr(s, '\0') returns a pointer to the terminating null. See also strcspn, strpbrk, and strrchr, described below.

strcspn — You can think of strcspn as a companion to strchr that matches any of a set of characters instead of just one. That makes it similar to strpbrk as well. Note, however, that strcspn returns an index into the string instead of a pointer to an element. If it finds no match, it returns the index of the terminating null instead of a null pointer. Thus, you may find that the call strcspn(s, "a"), for example, is more convenient than either strchr(s, 'a') or strpbrk(s, "a").

strpbrk — You can think of strpbrk as a companion to strchr that matches any of a set of characters instead of just one. That makes it similar to strcspn as well. Note, however, that strcspn returns an index into the string instead of a pointer to an element. If it finds no match, it returns the index of the terminating null instead of a null pointer. Thus, you may find that the call strcspn(s, "abc"), for example, is more convenient than strpbrk(s, "abc").

strrchr — Use this function to locate the last occurrence (the one having the highest subscript) of a character in a null-terminated string. The function type casts the second (search character) argument to char. That ensures that an argument expression of any character type behaves sensibly and predictably. A search failure returns a null pointer, however. Be sure to test the return value before you try to use it to access storage. Note that the call strrchr(s, '\0') returns a pointer to the terminating null. See also strchr, strcspn, and strpbrk, described above.

strspn — You can think of strspn as the complement to strcspn. It searches for a character that matches none of the elements in a set of characters instead of any one of them. strspn also returns an index into the string or, if it finds no match, the index of the terminating null. Thus, the call strspn(s, "abc"), for example, finds the longest possible span of characters from the set "abc".

strstr — You write strstr(s1, s2) to locate the first occurrence of the substring s2 in the string s1. A successful search returns a pointer to the start of the substring within s1. Note that a search failure returns a null pointer.

strtok — This is an intricate function designed to help you parse a null-terminated string into tokens. You specify the set of separator characters. Sequences of one or more separators occur between tokens. Such sequences can also occur before the first token and after the last. strtok maintains an internal memory of where it left off parsing a string. Hence, you can process only one string at a time using strtok. Here, for example, is a code sequence that calls the function word for each "word" in the string line. The code sequence defines a word as the longest possible sequence of characters not containing "white-space" — defined here as a space, horizontal tab, or newline:

#include <string.h>
char *s;
for (s=line; (s=strtok(s," \t\n"))!=NULL;s=NULL)
   word(s);

You can specify a different set of separators on each call to strtok that processes a given string, by the way.