strtok(3) Library Functions Manual strtok(3)
NAME
strtok, strtok_r - extract tokens from strings
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
strtok_r():
_POSIX_C_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
DESCRIPTION
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 main-
tains 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 inter-
nally 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 sub-
sequent 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
The strtok() and strtok_r() functions return a pointer to the next to-
ken, or NULL if there are no more tokens.
ATTRIBUTES
For an explanation of the terms used in this section, see attrib-
utes(7).
┌─────────────────────────────┬───────────────┬───────────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────┼───────────────┼───────────────────────┤
│ strtok() │ Thread safety │ MT-Unsafe race:strtok │
├─────────────────────────────┼───────────────┼───────────────────────┤
│ strtok_r() │ Thread safety │ MT-Safe │
└─────────────────────────────┴───────────────┴───────────────────────┘
VERSIONS
On some implementations, *saveptr is required to be NULL on the first
call to strtok_r() that is being used to parse str.
STANDARDS
strtok()
C11, POSIX.1-2008.
strtok_r()
POSIX.1-2008.
HISTORY
strtok()
POSIX.1-2001, C89, SVr4, 4.3BSD.
strtok_r()
POSIX.1-2001.
BUGS
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
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 speci-
fies the delimiter byte(s) to be used to separate that string into "ma-
jor" 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;
int j;
if (argc != 4) {
fprintf(stderr, "Usage: %s string delim subdelim\n",
argv[0]);
exit(EXIT_FAILURE);
}
for (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("\t --> %s\n", subtoken);
}
}
exit(EXIT_SUCCESS);
}
Another example program using strtok() can be found in getad-
drinfo_a(3).
SEE ALSO
memchr(3), strchr(3), string(3), strpbrk(3), strsep(3), strspn(3),
strstr(3), wcstok(3)
Linux man-pages 6.7 2023-10-31 strtok(3)
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