strtok(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

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 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.

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
       attributes(7).

       ┌───────────┬───────────────┬───────────────────────┐
       │Interface  Attribute     Value                 │
       ├───────────┼───────────────┼───────────────────────┤
       │strtok()   │ Thread safety │ MT-Unsafe race:strtok │
       ├───────────┼───────────────┼───────────────────────┤
       │strtok_r() │ Thread safety │ MT-Safe               │
       └───────────┴───────────────┴───────────────────────┘

CONFORMING TO         top

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

       strtok_r()
              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
       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.

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>

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

           if (argc != 4) {
               fprintf(stderr, "Usage: %s string delim subdelim\n",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

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

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

           exit(EXIT_SUCCESS);
       }

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

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 5.10 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
       https://www.kernel.org/doc/man-pages/.

GNU                            2020-11-01                      STRTOK(3)

Pages that refer to this page: index(3)strchr(3)string(3)strpbrk(3)strsep(3)strspn(3)strstr(3)wcstok(3)signal-safety(7)