GNU Info
(libc.info)Finding Tokens in a String
Finding Tokens in a String
==========================
It's fairly common for programs to have a need to do some simple
kinds of lexical analysis and parsing, such as splitting a command
string up into tokens. You can do this with the `strtok' function,
declared in the header file `string.h'.
- Function: char * strtok (char *restrict NEWSTRING, const char
*restrict DELIMITERS)
A string can be split into tokens by making a series of calls to
the function `strtok'.
The string to be split up is passed as the NEWSTRING argument on
the first call only. The `strtok' function uses this to set up
some internal state information. Subsequent calls to get
additional tokens from the same string are indicated by passing a
null pointer as the NEWSTRING argument. Calling `strtok' with
another non-null NEWSTRING argument reinitializes the state
information. It is guaranteed that no other library function ever
calls `strtok' behind your back (which would mess up this internal
state information).
The DELIMITERS argument is a string that specifies a set of
delimiters that may surround the token being extracted. All the
initial characters that are members of this set are discarded.
The first character that is _not_ a member of this set of
delimiters marks the beginning of the next token. The end of the
token is found by looking for the next character that is a member
of the delimiter set. This character in the original string
NEWSTRING is overwritten by a null character, and the pointer to
the beginning of the token in NEWSTRING is returned.
On the next call to `strtok', the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters DELIMITERS do not have to be the
same on every call in a series of calls to `strtok'.
If the end of the string NEWSTRING is reached, or if the remainder
of string consists only of delimiter characters, `strtok' returns
a null pointer.
Note that "character" is here used in the sense of byte. In a
string using a multibyte character encoding (abstract) character
consisting of more than one byte are not treated as an entity.
Each byte is treated separately. The function is not
locale-dependent.
Note that "character" is here used in the sense of byte. In a
string using a multibyte character encoding (abstract) character
consisting of more than one byte are not treated as an entity.
Each byte is treated separately. The function is not
locale-dependent.
- Function: wchar_t * wcstok (wchar_t *NEWSTRING, const char
*DELIMITERS)
A string can be split into tokens by making a series of calls to
the function `wcstok'.
The string to be split up is passed as the NEWSTRING argument on
the first call only. The `wcstok' function uses this to set up
some internal state information. Subsequent calls to get
additional tokens from the same wide character string are
indicated by passing a null pointer as the NEWSTRING argument.
Calling `wcstok' with another non-null NEWSTRING argument
reinitializes the state information. It is guaranteed that no
other library function ever calls `wcstok' behind your back (which
would mess up this internal state information).
The DELIMITERS argument is a wide character string that specifies
a set of delimiters that may surround the token being extracted.
All the initial wide characters that are members of this set are
discarded. The first wide character that is _not_ a member of
this set of delimiters marks the beginning of the next token. The
end of the token is found by looking for the next wide character
that is a member of the delimiter set. This wide character in the
original wide character string NEWSTRING is overwritten by a null
wide character, and the pointer to the beginning of the token in
NEWSTRING is returned.
On the next call to `wcstok', the searching begins at the next
wide character beyond the one that marked the end of the previous
token. Note that the set of delimiters DELIMITERS do not have to
be the same on every call in a series of calls to `wcstok'.
If the end of the wide character string NEWSTRING is reached, or
if the remainder of string consists only of delimiter wide
characters, `wcstok' returns a null pointer.
Note that "character" is here used in the sense of byte. In a
string using a multibyte character encoding (abstract) character
consisting of more than one byte are not treated as an entity.
Each byte is treated separately. The function is not
locale-dependent.
*Warning:* Since `strtok' and `wcstok' alter the string they is
parsing, you should always copy the string to a temporary buffer before
parsing it with `strtok'/`wcstok' (Note: Copying and Concatenation).
If you allow `strtok' or `wcstok' to modify a string that came from
another part of your program, you are asking for trouble; that string
might be used for other purposes after `strtok' or `wcstok' has
modified it, and it would not have the expected value.
The string that you are operating on might even be a constant. Then
when `strtok' or `wcstok' tries to modify it, your program will get a
fatal signal for writing in read-only memory. Note: Program Error
Signals. Even if the operation of `strtok' or `wcstok' would not
require a modification of the string (e.g., if there is exactly one
token) the string can (and in the GNU libc case will) be modified.
This is a special case of a general principle: if a part of a program
does not have as its purpose the modification of a certain data
structure, then it is error-prone to modify the data structure
temporarily.
The functions `strtok' and `wcstok' are not reentrant. Note:
Nonreentrancy, for a discussion of where and why reentrancy is
important.
Here is a simple example showing the use of `strtok'.
#include <string.h>
#include <stddef.h>
...
const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token, *cp;
...
cp = strdupa (string); /* Make writable copy. */
token = strtok (cp, delimiters); /* token => "words" */
token = strtok (NULL, delimiters); /* token => "separated" */
token = strtok (NULL, delimiters); /* token => "by" */
token = strtok (NULL, delimiters); /* token => "spaces" */
token = strtok (NULL, delimiters); /* token => "and" */
token = strtok (NULL, delimiters); /* token => "punctuation" */
token = strtok (NULL, delimiters); /* token => NULL */
The GNU C library contains two more functions for tokenizing a string
which overcome the limitation of non-reentrancy. They are only
available for multibyte character strings.
- Function: char * strtok_r (char *NEWSTRING, const char *DELIMITERS,
char **SAVE_PTR)
Just like `strtok', this function splits the string into several
tokens which can be accessed by successive calls to `strtok_r'.
The difference is that the information about the next token is
stored in the space pointed to by the third argument, SAVE_PTR,
which is a pointer to a string pointer. Calling `strtok_r' with a
null pointer for NEWSTRING and leaving SAVE_PTR between the calls
unchanged does the job without hindering reentrancy.
This function is defined in POSIX.1 and can be found on many
systems which support multi-threading.
- Function: char * strsep (char **STRING_PTR, const char *DELIMITER)
This function has a similar functionality as `strtok_r' with the
NEWSTRING argument replaced by the SAVE_PTR argument. The
initialization of the moving pointer has to be done by the user.
Successive calls to `strsep' move the pointer along the tokens
separated by DELIMITER, returning the address of the next token
and updating STRING_PTR to point to the beginning of the next
token.
One difference between `strsep' and `strtok_r' is that if the
input string contains more than one character from DELIMITER in a
row `strsep' returns an empty string for each pair of characters
from DELIMITER. This means that a program normally should test
for `strsep' returning an empty string before processing it.
This function was introduced in 4.3BSD and therefore is widely
available.
Here is how the above example looks like when `strsep' is used.
#include <string.h>
#include <stddef.h>
...
const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *running;
char *token;
...
running = strdupa (string);
token = strsep (&running, delimiters); /* token => "words" */
token = strsep (&running, delimiters); /* token => "separated" */
token = strsep (&running, delimiters); /* token => "by" */
token = strsep (&running, delimiters); /* token => "spaces" */
token = strsep (&running, delimiters); /* token => "" */
token = strsep (&running, delimiters); /* token => "" */
token = strsep (&running, delimiters); /* token => "" */
token = strsep (&running, delimiters); /* token => "and" */
token = strsep (&running, delimiters); /* token => "" */
token = strsep (&running, delimiters); /* token => "punctuation" */
token = strsep (&running, delimiters); /* token => "" */
token = strsep (&running, delimiters); /* token => NULL */
- Function: char * basename (const char *FILENAME)
The GNU version of the `basename' function returns the last
component of the path in FILENAME. This function is the preferred
usage, since it does not modify the argument, FILENAME, and
respects trailing slashes. The prototype for `basename' can be
found in `string.h'. Note, this function is overriden by the XPG
version, if `libgen.h' is included.
Example of using GNU `basename':
#include <string.h>
int
main (int argc, char *argv[])
{
char *prog = basename (argv[0]);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
*Portability Note:* This function may produce different results on
different systems.
- Function: char * basename (char *PATH)
This is the standard XPG defined `basename'. It is similar in
spirit to the GNU version, but may modify the PATH by removing
trailing '/' characters. If the PATH is made up entirely of '/'
characters, then "/" will be returned. Also, if PATH is `NULL' or
an empty string, then "." is returned. The prototype for the XPG
version can be found in `libgen.h'.
Example of using XPG `basename':
#include <libgen.h>
int
main (int argc, char *argv[])
{
char *prog;
char *path = strdupa (argv[0]);
prog = basename (path);
if (argc < 2)
{
fprintf (stderr, "Usage %s <arg>\n", prog);
exit (1);
}
...
}
- Function: char * dirname (char *PATH)
The `dirname' function is the compliment to the XPG version of
`basename'. It returns the parent directory of the file specified
by PATH. If PATH is `NULL', an empty string, or contains no '/'
characters, then "." is returned. The prototype for this function
can be found in `libgen.h'.
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