library_5.html

Linux程序员的工作手册

<P>
The return value is the length of the entire transformed string.  This
value is not affected by the value of <VAR>size</VAR>, but if it is greater
than <VAR>size</VAR>, it means that the transformed string did not entirely
fit in the array <VAR>to</VAR>.  In this case, only as much of the string as
actually fits was stored.  To get the whole transformed string, call
<CODE>strxfrm</CODE> again with a bigger output array.
<P>
The transformed string may be longer than the original string, and it
may also be shorter.
<P>
If <VAR>size</VAR> is zero, no characters are stored in <VAR>to</VAR>.  In this
case, <CODE>strxfrm</CODE> simply returns the number of characters that would
be the length of the transformed string.  This is useful for determining
what size string to allocate.  It does not matter what <VAR>to</VAR> is if
<VAR>size</VAR> is zero; <VAR>to</VAR> may even be a null pointer.
<P>
Here is an example of how you can use <CODE>strxfrm</CODE> when
you plan to do many comparisons.  It does the same thing as the previous
example, but much faster, because it has to transform each string only
once, no matter how many times it is compared with other strings.  Even
the time needed to allocate and free storage is much less than the time
we save, when there are many strings.
<P>
<PRE>
struct sorter { char *input; char *transformed; };

/* This is the comparison function used with <CODE>qsort</CODE>
   to sort an array of <CODE>struct sorter</CODE>. */

int
compare_elements (struct sorter *p1, struct sorter *p2)
{
  return strcmp (p1-&#62;transformed, p2-&#62;transformed);
}

/* This is the entry point--the function to sort
   strings using the locale's collating sequence. */

void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;

  /* Set up <CODE>temp_array</CODE>.  Each element contains
     one input string and its transformed string. */
  for (i = 0; i &#60; nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;

      temp_array[i].input = array[i];

      /* Transform <CODE>array[i]</CODE>.
         First try a buffer probably big enough. */
      while (1)
        {
          char *transformed = (char *) xmalloc (length);
          if (strxfrm (transformed, array[i], length) &#60; length)
            {
              temp_array[i].transformed = transformed;
              break;
            }
          /* Try again with a bigger buffer. */
          free (transformed);
          length *= 2;
        }
    }

  /* Sort <CODE>temp_array</CODE> by comparing transformed strings. */
  qsort (temp_array, sizeof (struct sorter),
         nstrings, compare_elements);

  /* Put the elements back in the permanent array
     in their sorted order. */
  for (i = 0; i &#60; nstrings; i++)
    array[i] = temp_array[i].input;

  /* Free the strings we allocated. */
  for (i = 0; i &#60; nstrings; i++)
    free (temp_array[i].transformed);
}
</PRE>
<P>
<STRONG>Compatibility Note:</STRONG>  The string collation functions are a new
feature of ANSI C.  Older C dialects have no equivalent feature.
<P>
<H2><A NAME="SEC64" HREF="library_toc.html#SEC64" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC64">Search Functions</A></H2>
<P>
This section describes library functions which perform various kinds
of searching operations on strings and arrays.  These functions are
declared in the header file <TT>`string.h'</TT>.
<A NAME="IDX315"></A>
<A NAME="IDX316"></A>
<A NAME="IDX314"></A>
<P>
<A NAME="IDX317"></A>
<U>Function:</U> void * <B>memchr</B> <I>(const void *<VAR>block</VAR>, int <VAR>c</VAR>, size_t <VAR>size</VAR>)</I><P>
This function finds the first occurrence of the byte <VAR>c</VAR> (converted
to an <CODE>unsigned char</CODE>) in the initial <VAR>size</VAR> bytes of the
object beginning at <VAR>block</VAR>.  The return value is a pointer to the
located byte, or a null pointer if no match was found.
<P>
<A NAME="IDX318"></A>
<U>Function:</U> char * <B>strchr</B> <I>(const char *<VAR>string</VAR>, int <VAR>c</VAR>)</I><P>
The <CODE>strchr</CODE> function finds the first occurrence of the character
<VAR>c</VAR> (converted to a <CODE>char</CODE>) in the null-terminated string
beginning at <VAR>string</VAR>.  The return value is a pointer to the located
character, or a null pointer if no match was found.
<P>
For example,
<PRE>
strchr ("hello, world", 'l')
    => "llo, world"
strchr ("hello, world", '?')
    => NULL
</PRE>
<P>
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
specifying a null character as the value of the <VAR>c</VAR> argument.
<P>
<A NAME="IDX319"></A>
<U>Function:</U> char * <B>strrchr</B> <I>(const char *<VAR>string</VAR>, int <VAR>c</VAR>)</I><P>
The function <CODE>strrchr</CODE> is like <CODE>strchr</CODE>, except that it searches
backwards from the end of the string <VAR>string</VAR> (instead of forwards
from the front).
<P>
For example,
<PRE>
strrchr ("hello, world", 'l')
    => "ld"
</PRE>
<P>
<A NAME="IDX320"></A>
<U>Function:</U> char * <B>strstr</B> <I>(const char *<VAR>haystack</VAR>, const char *<VAR>needle</VAR>)</I><P>
This is like <CODE>strchr</CODE>, except that it searches <VAR>haystack</VAR> for a
substring <VAR>needle</VAR> rather than just a single character.  It
returns a pointer into the string <VAR>haystack</VAR> that is the first
character of the substring, or a null pointer if no match was found.  If
<VAR>needle</VAR> is an empty string, the function returns <VAR>haystack</VAR>.
<P>
For example,
<PRE>
strstr ("hello, world", "l")
    => "llo, world"
strstr ("hello, world", "wo")
    => "world"
</PRE>
<P>
<A NAME="IDX321"></A>
<U>Function:</U> void * <B>memmem</B> <I>(const void *<VAR>needle</VAR>, size_t <VAR>needle_len</VAR>,<BR>const void *<VAR>haystack</VAR>, size_t <VAR>haystack_len</VAR>)</I><P>
This is like <CODE>strstr</CODE>, but <VAR>needle</VAR> and <VAR>haystack</VAR> are byte
arrays rather than null-terminated strings.  <VAR>needle_len</VAR> is the
length of <VAR>needle</VAR> and <VAR>haystack_len</VAR> is the length of
<VAR>haystack</VAR>.<P>
This function is a GNU extension.
<P>
<A NAME="IDX322"></A>
<U>Function:</U> size_t <B>strspn</B> <I>(const char *<VAR>string</VAR>, const char *<VAR>skipset</VAR>)</I><P>
The <CODE>strspn</CODE> ("string span") function returns the length of the
initial substring of <VAR>string</VAR> that consists entirely of characters that
are members of the set specified by the string <VAR>skipset</VAR>.  The order
of the characters in <VAR>skipset</VAR> is not important.
<P>
For example,
<PRE>
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
    => 5
</PRE>
<P>
<A NAME="IDX323"></A>
<U>Function:</U> size_t <B>strcspn</B> <I>(const char *<VAR>string</VAR>, const char *<VAR>stopset</VAR>)</I><P>
The <CODE>strcspn</CODE> ("string complement span") function returns the length
of the initial substring of <VAR>string</VAR> that consists entirely of characters
that are <EM>not</EM> members of the set specified by the string <VAR>stopset</VAR>.
(In other words, it returns the offset of the first character in <VAR>string</VAR>
that is a member of the set <VAR>stopset</VAR>.)
<P>
For example,
<PRE>
strcspn ("hello, world", " \t\n,.;!?")
    => 5
</PRE>
<P>
<A NAME="IDX324"></A>
<U>Function:</U> char * <B>strpbrk</B> <I>(const char *<VAR>string</VAR>, const char *<VAR>stopset</VAR>)</I><P>
The <CODE>strpbrk</CODE> ("string pointer break") function is related to
<CODE>strcspn</CODE>, except that it returns a pointer to the first character
in <VAR>string</VAR> that is a member of the set <VAR>stopset</VAR> instead of the
length of the initial substring.  It returns a null pointer if no such
character from <VAR>stopset</VAR> is found.
<P>
For example,
<P>
<PRE>
strpbrk ("hello, world", " \t\n,.;!?")
    => ", world"
</PRE>
<P>
<H2><A NAME="SEC65" HREF="library_toc.html#SEC65" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_toc.html#SEC65">Finding Tokens in a String</A></H2>
<P>
<A NAME="IDX325"></A>
<A NAME="IDX326"></A>
<A NAME="IDX327"></A>
<P>
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 <CODE>strtok</CODE> function, declared
in the header file <TT>`string.h'</TT>.
<A NAME="IDX328"></A>
<P>
<A NAME="IDX329"></A>
<U>Function:</U> char * <B>strtok</B> <I>(char *<VAR>newstring</VAR>, const char *<VAR>delimiters</VAR>)</I><P>
A string can be split into tokens by making a series of calls to the
function <CODE>strtok</CODE>.
<P>
The string to be split up is passed as the <VAR>newstring</VAR> argument on
the first call only.  The <CODE>strtok</CODE> 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 <VAR>newstring</VAR> argument.  Calling <CODE>strtok</CODE> with another
non-null <VAR>newstring</VAR> argument reinitializes the state information.
It is guaranteed that no other library function ever calls <CODE>strtok</CODE>
behind your back (which would mess up this internal state information).
<P>
The <VAR>delimiters</VAR> 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
<EM>not</EM> 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 <VAR>newstring</VAR> is overwritten by a null character, and the
pointer to the beginning of the token in <VAR>newstring</VAR> is returned.
<P>
On the next call to <CODE>strtok</CODE>, the searching begins at the next
character beyond the one that marked the end of the previous token.
Note that the set of delimiters <VAR>delimiters</VAR> do not have to be the
same on every call in a series of calls to <CODE>strtok</CODE>.
<P>
If the end of the string <VAR>newstring</VAR> is reached, or if the remainder of
string consists only of delimiter characters, <CODE>strtok</CODE> returns
a null pointer.
<P>
<STRONG>Warning:</STRONG> Since <CODE>strtok</CODE> alters the string it is parsing,
you always copy the string to a temporary buffer before parsing it with
<CODE>strtok</CODE>.  If you allow <CODE>strtok</CODE> to modify a string that came
from another part of your program, you are asking for trouble; that
string may be part of a data structure that could be used for other
purposes during the parsing, when alteration by <CODE>strtok</CODE> makes the
data structure temporarily inaccurate.
<P>
The string that you are operating on might even be a constant.  Then
when <CODE>strtok</CODE> tries to modify it, your program will get a fatal
signal for writing in read-only memory.  See section <A HREF="library_21.html#SEC336" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_21.html#SEC336">Program Error Signals</A>.
<P>
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.
<P>
The function <CODE>strtok</CODE> is not reentrant.  See section <A HREF="library_21.html#SEC357" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_21.html#SEC357">Signal Handling and Nonreentrant Functions</A>, for
a discussion of where and why reentrancy is important.
<P>
Here is a simple example showing the use of <CODE>strtok</CODE>.
<P>
<PRE>
#include &#60;string.h&#62;
#include &#60;stddef.h&#62;

...

char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token;

...

token = strtok (string, delimiters);  /* token =&#62; "words" */
token = strtok (NULL, delimiters);    /* token =&#62; "separated" */
token = strtok (NULL, delimiters);    /* token =&#62; "by" */
token = strtok (NULL, delimiters);    /* token =&#62; "spaces" */
token = strtok (NULL, delimiters);    /* token =&#62; "and" */
token = strtok (NULL, delimiters);    /* token =&#62; "punctuation" */
token = strtok (NULL, delimiters);    /* token =&#62; NULL */
</PRE>
<P>Go to the <A HREF="library_4.html" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_4.html">previous</A>, <A HREF="library_6.html" tppabs="http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_6.html">next</A> section.<P>