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STRTOK(3) Library Functions Manual STRTOK(3)

strtok, strtok_r
string token operations

#include <string.h>
char *
strtok(char *str, const char *sep);
char *
strtok_r(char *str, const char *sep, char **last);

This interface is obsoleted by strsep(3).
The strtok() function is used to isolate sequential tokens in a NUL-terminated string, str. These tokens are separated in the string by at least one of the characters in sep. The first time that strtok() is called, str should be specified; subsequent calls, wishing to obtain further tokens from the same string, should pass a null pointer instead. The separator string, sep, must be supplied each time, and may change between calls.
The strtok_r() function is a version of strtok() that takes an explicit context argument and is reentrant.
Since strtok() and strtok_r() modify the string, str should not point to an area in the initialized data segment.

The strtok() and strtok_r() functions return a pointer to the beginning of each subsequent token in the string, after replacing the separator character itself with a NUL character. When no more tokens remain, a null pointer is returned.

The following will construct an array of pointers to each individual word in the string s:
#define MAXTOKENS	128 
 
char s[512], *p, *tokens[MAXTOKENS]; 
char *last; 
int i = 0; 
 
snprintf(s, sizeof(s), "cat dog horse cow"); 
 
for ((p = strtok_r(s, " ", &last)); p; 
    (p = strtok_r(NULL, " ", &last))) { 
	if (i < MAXTOKENS - 1) 
		tokens[i++] = p; 
} 
tokens[i] = NULL;
That is, tokens[0] will point to “cat”, tokens[1] will point to “dog”, tokens[2] will point to “horse”, and tokens[3] will point to “cow”.

memchr(3), strchr(3), strcspn(3), strpbrk(3), strrchr(3), strsep(3), strspn(3), strstr(3), wcstok(3)

The strtok() function conforms to ANSI X3.159-1989 (“ANSI C89”). The strtok_r() function conforms to IEEE Std 1003.1-2001 (“POSIX.1”).

The strtok() function first appeared in AT&T System III UNIX and was reimplemented for 4.3BSD-Tahoe. The strtok_r() function first appeared in NetBSD 1.3 and was reimplemented for OpenBSD 2.7.

The System V strtok(), if handed a string containing only delimiter characters, will not alter the next starting point, so that a call to strtok() with a different (or empty) delimiter string may return a non-null value. Since this implementation always alters the next starting point, such a sequence of calls would always return NULL.
September 2, 2017 OpenBSD-current