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'.
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.
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
(see section 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. See section 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.
See section Signal Handling and Nonreentrant Functions, 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.
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.
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 */
basename
function returns the last
component of the path in filename. This function is the prefered
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.
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); } ... }
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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