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NAME
strtok, strtok_r — split string into tokens
SYNOPSIS
#include <string.h>
char *strtok(char *restrict s, const char *restrict sep);
char *strtok_r(char *restrict s, const char *restrict sep,
char **restrict state);
DESCRIPTION
For strtok(): The functionality described on this reference page is aligned with the ISO C standard. Any
conflict between the requirements described here and the ISO C standard is unintentional. This volume of
POSIX.1‐2017 defers to the ISO C standard.
A sequence of calls to strtok() breaks the string pointed to by s into a sequence of tokens, each of
which is delimited by a byte from the string pointed to by sep. The first call in the sequence has s as
its first argument, and is followed by calls with a null pointer as their first argument. The separator
string pointed to by sep may be different from call to call.
The first call in the sequence searches the string pointed to by s for the first byte that is not
contained in the current separator string pointed to by sep. If no such byte is found, then there are no
tokens in the string pointed to by s and strtok() shall return a null pointer. If such a byte is found,
it is the start of the first token.
The strtok() function then searches from there for a byte that is contained in the current separator
string. If no such byte is found, the current token extends to the end of the string pointed to by s, and
subsequent searches for a token shall return a null pointer. If such a byte is found, it is overwritten
by a NUL character, which terminates the current token. The strtok() function saves a pointer to the
following byte, from which the next search for a token shall 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 function defined in this volume of POSIX.1‐2017 calls strtok().
The strtok() function need not be thread-safe.
The strtok_r() function shall be equivalent to strtok(), except that strtok_r() shall be thread-safe and
the argument state points to a user-provided pointer that allows strtok_r() to maintain state between
calls which scan the same string. The application shall ensure that the pointer pointed to by state is
unique for each string (s) being processed concurrently by strtok_r() calls. The application need not
initialize the pointer pointed to by state to any particular value. The implementation shall not update
the pointer pointed to by state to point (directly or indirectly) to resources, other than within the
string s, that need to be freed or released by the caller.
RETURN VALUE
Upon successful completion, strtok() shall return a pointer to the first byte of a token. Otherwise, if
there is no token, strtok() shall return a null pointer.
The strtok_r() function shall return a pointer to the token found, or a null pointer when no token is
found.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
Searching for Word Separators
The following example searches for tokens separated by <space> characters.
#include <string.h>
...
char *token;
char line[] = "LINE TO BE SEPARATED";
char *search = " ";
/* Token will point to "LINE". */
token = strtok(line, search);
/* Token will point to "TO". */
token = strtok(NULL, search);
Find First two Fields in a Buffer
The following example uses strtok() to find two character strings (a key and data associated with that
key) separated by any combination of <space>, <tab>, or <newline> characters at the start of the array of
characters pointed to by buffer.
#include <string.h>
...
char *buffer;
...
struct element {
char *key;
char *data;
} e;
...
// Load the buffer...
...
// Get the key and its data...
e.key = strtok(buffer, " \t\n");
e.data = strtok(NULL, " \t\n");
// Process the rest of the contents of the buffer...
...
APPLICATION USAGE
Note that if sep is the empty string, strtok() and strtok_r() return a pointer to the remainder of the
string being tokenized.
The strtok_r() function is thread-safe and stores its state in a user-supplied buffer instead of possibly
using a static data area that may be overwritten by an unrelated call from another thread.
RATIONALE
The strtok() function searches for a separator string within a larger string. It returns a pointer to the
last substring between separator strings. This function uses static storage to keep track of the current
string position between calls. The new function, strtok_r(), takes an additional argument, state, to keep
track of the current position in the string.
FUTURE DIRECTIONS
None.
SEE ALSO
The Base Definitions volume of POSIX.1‐2017, <string.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .