strtok_r(3) — Linux manual page


STRTOK(3)                 Linux Programmer's Manual                STRTOK(3)

NAME         top

       strtok, strtok_r - extract tokens from strings

SYNOPSIS         top

       #include <string.h>

       char *strtok(char *str, const char *delim);

       char *strtok_r(char *str, const char *delim, char **saveptr);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       strtok_r(): _POSIX_C_SOURCE
           || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION         top

       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

       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

       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.

RETURN VALUE         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see

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

CONFORMING TO         top

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

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

NOTES         top

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

BUGS         top

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

       * 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.

EXAMPLES         top

       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;
           int j;

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

           for (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(" --> %s\n", subtoken);


       Another example program using strtok() can be found in

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.08 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

GNU                              2020-06-09                        STRTOK(3)

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