diff options
Diffstat (limited to 'manual/string.texi')
| -rw-r--r-- | manual/string.texi | 109 |
1 files changed, 87 insertions, 22 deletions
diff --git a/manual/string.texi b/manual/string.texi index c638912..8b7e9da 100644 --- a/manual/string.texi +++ b/manual/string.texi @@ -6,7 +6,7 @@ many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the @code{memcpy} -function can be used to copy the contents of any kind of array. +function can be used to copy the contents of any kind of array. It's fairly common for beginning C programmers to ``reinvent the wheel'' by duplicating this functionality in their own code, but it pays to @@ -158,7 +158,7 @@ get the allocation size of the character array that holds a string using the @code{sizeof} operator: @smallexample -char string[32] = "hello, world"; +char string[32] = "hello, world"; sizeof (string) @result{} 32 strlen (string) @@ -411,7 +411,7 @@ return a nonzero value if the strings are @emph{not} equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is ``less'' than the -second, while a positive value indicates that the first string is +second, while a positive value indicates that the first string is ``greater''. The most common use of these functions is to check only for equality. @@ -623,10 +623,10 @@ overlap; see @ref{Copying and Concatenation}. The return value is the length of the entire transformed string. This value is not affected by the value of @var{size}, but if it is greater -than @var{size}, it means that the transformed string did not entirely -fit in the array @var{to}. In this case, only as much of the string as -actually fits was stored. To get the whole transformed string, call -@code{strxfrm} again with a bigger output array. +or equal than @var{size}, it means that the transformed string did not +entirely fit in the array @var{to}. In this case, only as much of the +string as actually fits was stored. To get the whole transformed +string, call @code{strxfrm} again with a bigger output array. The transformed string may be longer than the original string, and it may also be shorter. @@ -671,23 +671,32 @@ sort_strings_fast (char **array, int nstrings) for (i = 0; i < nstrings; i++) @{ size_t length = strlen (array[i]) * 2; + char *transformed; + size_t transformed_lenght; temp_array[i].input = array[i]; - /* @r{Transform @code{array[i]}.} - @r{First try a buffer probably big enough.} */ - while (1) + /* @r{First try a buffer perhaps big enough.} */ + transformed = (char *) xmalloc (length); + + /* @r{Transform @code{array[i]}.} */ + transformed_length = strxfrm (transformed, array[i], length); + + /* @r{If the buffer was not large enough, resize it} + @r{and try again.} */ + if (transformed_length >= length) @{ - char *transformed = (char *) xmalloc (length); - if (strxfrm (transformed, array[i], length) < length) - @{ - temp_array[i].transformed = transformed; - break; - @} - /* @r{Try again with a bigger buffer.} */ - free (transformed); - length *= 2; + /* @r{Allocate the needed space. +1 for terminating} + @r{@code{NUL} character.} */ + transformed = (char *) xrealloc (transformed, + transformed_length + 1); + + /* @r{The return value is not interesting because we know} + @r{how long the transformed string is.} */ + (void) strxfrm (transformed, array[i], transformed_length + 1); @} + + temp_array[i].transformed = transformed; @} /* @r{Sort @code{temp_array} by comparing transformed strings.} */ @@ -741,7 +750,7 @@ strchr ("hello, world", 'l') @result{} "llo, world" strchr ("hello, world", '?') @result{} NULL -@end smallexample +@end smallexample The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by @@ -857,8 +866,6 @@ strpbrk ("hello, world", " \t\n,.;!?") @node Finding Tokens in a String, , Search Functions, String and Array Utilities @section Finding Tokens in a String -@c !!! Document strsep, which is a better thing to use than strtok. - @cindex tokenizing strings @cindex breaking a string into tokens @cindex parsing tokens from a string @@ -945,3 +952,61 @@ token = strtok (NULL, delimiters); /* token => "and" */ token = strtok (NULL, delimiters); /* token => "punctuation" */ token = strtok (NULL, delimiters); /* token => NULL */ @end smallexample + +The GNU C library contains two more functions for tokenizing a string +which overcome the limitation of non-reentrancy. + +@comment string.h +@comment POSIX +@deftypefun {char *} strtok_r (char *@var{newstring}, const char *@var{delimiters}, char **@var{save_ptr}) +Just like @code{strtok} this function splits the string into several +tokens which can be accessed be successive calls to @code{strtok_r}. +The difference is that the information about the next token is not set +up in some internal state information. Instead the caller has to +provide another argument @var{save_ptr} which is a pointer to a string +pointer. Calling @code{strtok_r} with a null pointer for +@var{newstring} and leaving @var{save_ptr} between the calls unchanged +does the job without limiting reentrancy. + +This function was proposed for POSIX.1b and can be found on many systems +which support multi-threading. +@end deftypefun + +@comment string.h +@comment BSD +@deftypefun {char *} strsep (char **@var{string_ptr}, const char *@var{delimiter}) +A second reentrant approach is to avoid the additional first argument. +The initialization of the moving pointer has to be done by the user. +Successive calls of @code{strsep} move the pointer along the tokens +separated by @var{delimiter}, returning the address of the next token +and updating @var{string_ptr} to point to the beginning of the next +token. + +This function was introduced in 4.3BSD and therefore is widely available. +@end deftypefun + +Here is how the above example looks like when @code{strsep} is used. + +@comment Yes, this example has been tested. +@smallexample +#include <string.h> +#include <stddef.h> + +@dots{} + +char string[] = "words separated by spaces -- and, punctuation!"; +const char delimiters[] = " .,;:!-"; +char *running; +char *token; + +@dots{} + +running = 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 => "and" */ +token = strsep (&running, delimiters); /* token => "punctuation" */ +token = strsep (&running, delimiters); /* token => NULL */ +@end smallexample |