strtok, strtok_r — extract tokens from strings


#include <string.h>
char *strtok( char *restrict str,
  const char *restrict delim);
char *strtok_r( char *restrict str,
  const char *restrict delim,
  char **restrict saveptr);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_POSIX_C_SOURCE || /* Glibc <= 2.19:


The strtok() function breaks a string into a sequence of zero or more nonempty tokens. On the first call to strtok(), the string to be parsed should be specified in str. In each subsequent call that should parse the same string, str must be NULL.

The delim argument specifies a set of bytes that delimit the tokens in the parsed string. The caller may specify different strings in delim in successive calls that parse the same string.

Each call to strtok() returns a pointer to a null-terminated string containing the next token. This string does not include the delimiting byte. If no more tokens are found, strtok() returns NULL.

A sequence of calls to strtok() that operate on the same string maintains a pointer that determines the point from which to start searching for the next token. The first call to strtok() sets this pointer to point to the first byte of the string. The start of the next token is determined by scanning forward for the next nondelimiter byte in str. If such a byte is found, it is taken as the start of the next token. If no such byte is found, then there are no more tokens, and strtok() returns NULL. (A string that is empty or that contains only delimiters will thus cause strtok() to return NULL on the first call.)

The end of each token is found by scanning forward until either the next delimiter byte is found or until the terminating null byte ('\0') is encountered. If a delimiter byte is found, it is overwritten with a null byte to terminate the current token, and strtok() saves a pointer to the following byte; that pointer will be used as the starting point when searching for the next token. In this case, strtok() returns a pointer to the start of the found token.

From the above description, it follows that a sequence of two or more contiguous delimiter bytes in the parsed string is considered to be a single delimiter, and that delimiter bytes at the start or end of the string are ignored. Put another way: the tokens returned by strtok() are always nonempty strings. Thus, for example, given the string "aaa;;bbb,", successive calls to strtok() that specify the delimiter string ";," would return the strings "aaa" and "bbb", and then a null pointer.

The strtok_r() function is a reentrant version of strtok(). The saveptr argument is a pointer to a char * variable that is used internally by strtok_r() in order to maintain context between successive calls that parse the same string.

On the first call to strtok_r(), str should point to the string to be parsed, and the value of *saveptr is ignored (but see NOTES). In subsequent calls, str should be NULL, and saveptr (and the buffer that it points to) should be unchanged since the previous call.

Different strings may be parsed concurrently using sequences of calls to strtok_r() that specify different saveptr arguments.


The strtok() and strtok_r() functions return a pointer to the next token, or NULL if there are no more tokens.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
strtok() Thread safety MT-Unsafe race:strtok
strtok_r() Thread safety MT-Safe



POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.


POSIX.1-2001, POSIX.1-2008.


On some implementations, *saveptr is required to be NULL on the first call to strtok_r() that is being used to parse str.


Be cautious when using these functions. If you do use them, note that:

  • These functions modify their first argument.

  • These functions cannot be used on constant strings.

  • The identity of the delimiting byte is lost.

  • The strtok() function uses a static buffer while parsing, so it's not thread safe. Use strtok_r() if this matters to you.


The program below uses nested loops that employ strtok_r() to break a string into a two-level hierarchy of tokens. The first command-line argument specifies the string to be parsed. The second argument specifies the delimiter byte(s) to be used to separate that string into "major" tokens. The third argument specifies the delimiter byte(s) to be used to separate the "major" tokens into subtokens.

An example of the output produced by this program is the following:

$ ./a.out 'a/bbb///cc;xxx:yyy:' ':;' '/'
1: a/bbb///cc
         −−> a
         −−> bbb
         −−> cc
2: xxx
         −−> xxx
3: yyy
         −−> yyy

Program source

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

main(int argc, char *argv[])
    char *str1, *str2, *token, *subtoken;
    char *saveptr1, *saveptr2;

    if (argc != 4) {
        fprintf(stderr, "Usage: %s string delim subdelim\n",

    for (int j = 1, str1 = argv[1]; ; j++, str1 = NULL) {
        token = strtok_r(str1, argv[2], &saveptr1);
        if (token == NULL)
        printf("%d: %s\n", j, token);

        for (str2 = token; ; str2 = NULL) {
            subtoken = strtok_r(str2, argv[3], &saveptr2);
            if (subtoken == NULL)
            printf("\t −−> %s\n", subtoken);


Another example program using strtok() can be found in getaddrinfo_a(3).


index(3), memchr(3), rindex(3), strchr(3), string(3), strpbrk(3), strsep(3), strspn(3), strstr(3), wcstok(3)


This page is part of release 5.11 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at−pages/.

  Copyright (C) 2005, 2013 Michael Kerrisk <>
a few fragments from an earlier (1996) version by
Andries Brouwer ( remain.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Rewritten old page, 960210,
Updated, added strtok_r. 2000-02-13 Nicolás Lichtmaier <>
2005-11-17, mtk: Substantial parts rewritten
2013-05-19, mtk: added much further detail on the operation of strtok()