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qsort_s

 

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Performs a quick sort. A version of qsort with security enhancements as described in Security Features in the CRT.

void qsort_s(  
   void *base,  
   size_t num,  
   size_t width,  
   int (__cdecl *compare )(void *, const void *, const void *),  
   void * context  
);  

Parameters

base
Start of target array.

num
Array size in elements.

width
Element size in bytes.

compare
Comparison function. The first argument is the context pointer. The second argument is a pointer to the key for the search. The third argument is a pointer to the array element to be compared with key.

context
A pointer to a context, which can be any object that the compare routine needs to access.

The qsort_s function implements a quick-sort algorithm to sort an array of num elements, each of width bytes. The argument base is a pointer to the base of the array to be sorted. qsort_s overwrites this array with the sorted elements. The argument compare is a pointer to a user-supplied routine that compares two array elements and returns a value specifying their relationship. qsort_s calls the compare routine one or more times during the sort, passing pointers to two array elements on each call:

compare( context, (void *) & elem1, (void *) & elem2 );  

The routine must compare the elements and then return one of the following values:

Return valueDescription
< 0elem1 less than elem2
0elem1 equivalent to elem2
> 0elem1 greater than elem2

The array is sorted in increasing order, as defined by the comparison function. To sort an array in decreasing order, reverse the sense of "greater than" and "less than" in the comparison function.

If invalid parameters are passed to the function, the invalid parameter handler is invoked, as described in Parameter Validation. If execution is allowed to continue, then the function returns and errno is set to EINVAL. For more information, see errno, _doserrno, _sys_errlist, and _sys_nerr.

Error Conditions

keybasecomparenumwidtherrno
NULLanyanyanyanyEINVAL
anyNULLany!= 0anyEINVAL
anyanyanyany<= 0EINVAL
anyanyNULLanyanyEINVAL

qsort_s has the same behavior as qsort but has the context parameter and sets errno. By passing a context parameter, comparison functions can use an object pointer to access object functionality or other information not accessible through an element pointer. The addition of the context parameter makes qsort_smore secure because context can be used to avoid reentrancy bugs introduced by using static variables to make shared information available to the compare function.

RoutineRequired header
qsort_s<stdlib.h> and <search.h>

For additional compatibility information, see Compatibility in the Introduction.

Libraries: All versions of the CRT Library Features.

The following example demonstrates how to use the context parameter in the qsort_sfunction. The context parameter makes it easier to perform thread-safe sorts. Instead of using static variables that must be synchronized to ensure thread safety, pass a different context parameter in each sort. In this example, a locale object is used as the context parameter.

// crt_qsort_s.cpp  
// compile with: /EHsc /MT  
#include <stdlib.h>  
#include <stdio.h>  
#include <search.h>  
#include <process.h>  
#include <locale.h>  
#include <locale>  
#include <windows.h>  
using namespace std;  
  
// The sort order is dependent on the code page.  Use 'chcp' at the  
// command line to change the codepage.  When executing this application,  
// the command prompt codepage must match the codepage used here:  
  
#define CODEPAGE_850  
  
#ifdef CODEPAGE_850  
// Codepage 850 is the OEM codepage used by the command line,  
// so \x00e1 is the German Sharp S in that codepage and \x00a4  
// is the n tilde.  
  
char *array1[] = { "wei\x00e1", "weis", "annehmen", "weizen", "Zeit",  
                   "weit" };  
char *array2[] = { "Espa\x00a4ol", "Espa\x00a4" "a", "espantado" };  
char *array3[] = { "table", "tableux", "tablet" };  
  
#define GERMAN_LOCALE "German_Germany.850"  
#define SPANISH_LOCALE "Spanish_Spain.850"  
#define ENGLISH_LOCALE "English_US.850"  
  
#endif  
  
#ifdef CODEPAGE_1252  
   // If using codepage 1252 (ISO 8859-1, Latin-1), use \x00df  
   // for the German Sharp S and \x001f for the n tilde.  
char *array1[] = { "wei\x00df", "weis", "annehmen", "weizen", "Zeit",  
                   "weit" };  
char *array2[] = { "Espa\x00f1ol", "Espa\x00f1" "a", "espantado" };  
char *array3[] = { "table", "tableux", "tablet" };  
  
#define GERMAN_LOCALE "German_Germany.1252"  
#define SPANISH_LOCALE "Spanish_Spain.1252"  
#define ENGLISH_LOCALE "English_US.1252"  
  
#endif  
  
// The context parameter lets you create a more generic compare.  
// Without this parameter, you would have stored the locale in a  
// static variable, thus making sort_array vulnerable to thread  
// conflicts.  
  
int compare( void *pvlocale, const void *str1, const void *str2)  
{  
    char s1[256];  
    char s2[256];  
    strcpy_s(s1, 256, *(char**)str1);  
    strcpy_s(s2, 256, *(char**)str2);  
    _strlwr_s( s1, sizeof(s1) );  
    _strlwr_s( s2, sizeof(s2) );  
  
    locale& loc = *( reinterpret_cast< locale * > ( pvlocale));  
  
    return use_facet< collate<char> >(loc).compare(s1,   
       &s1[strlen(s1)], s2, &s2[strlen(s2)]);  
  
}  
  
void sort_array(char *array[], int num, locale &loc)  
{  
    qsort_s(array, num, sizeof(char*), compare, &loc);  
}  
  
void print_array(char *a[], int c)  
{  
   for (int i = 0; i < c; i++)  
     printf("%s ", a[i]);  
   printf("\n");  
  
}  
  
void sort_german(void * Dummy)  
{  
   sort_array(array1, 6, locale(GERMAN_LOCALE));  
}  
  
void sort_spanish(void * Dummy)  
{     
   sort_array(array2, 3, locale(SPANISH_LOCALE));       
}  
  
void sort_english(void * Dummy)  
{     
   sort_array(array3, 3, locale(ENGLISH_LOCALE));     
}  
  
int main( )  
{  
  
   int i;  
   HANDLE threads[3];  
  
   printf("Unsorted input:\n");  
   print_array(array1, 6);  
   print_array(array2, 3);  
   print_array(array3, 3);  
  
   // Create several threads that perform sorts in different  
   // languages at the same time.   
  
   threads[0] = reinterpret_cast<HANDLE>(  
                 _beginthread( sort_german , 0, NULL));  
   threads[1] = reinterpret_cast<HANDLE>(  
                 _beginthread( sort_spanish, 0, NULL));  
   threads[2] = reinterpret_cast<HANDLE>(  
                 _beginthread( sort_english, 0, NULL));  
  
   for (i = 0; i < 3; i++)  
   {  
      if (threads[i] == reinterpret_cast<HANDLE>(-1))  
      {  
         printf("Error creating threads.\n");  
         exit(1);  
      }  
   }  
  
   // Wait until all threads have terminated.  
   WaitForMultipleObjects(3, threads, true, INFINITE);  
  
   printf("Sorted output: \n");  
  
   print_array(array1, 6);  
   print_array(array2, 3);  
   print_array(array3, 3);  
  
}  

Unsorted input:  
weiß weis annehmen weizen Zeit weit   
Español España espantado   
table tableux tablet   
Sorted output:   
annehmen weiß weis weit weizen Zeit   
España Español espantado   
table tablet tableux  

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