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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


Start of target array.

Array size in elements.

Element size in bytes.

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.

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 number 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 value Description
< 0 element 1 less than element 2
0 element 1 equivalent to element 2
> 0 element 1 greater than element 2

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.

By default, this function's global state is scoped to the application. To change this behavior, see Global state in the CRT.

Error conditions

key base compare num width errno
NULL any any any any EINVAL
any NULL any != 0 any EINVAL
any any any any <= 0 EINVAL
any any NULL any any EINVAL

qsort_s has the same behavior as qsort but has the context parameter and sets errno. The context parameter allows comparison functions to 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_s more secure because context can be used to avoid reentrancy bugs introduced by using static variables to make shared information available to the compare function.


Routine Required header
qsort_s <stdlib.h> and <search.h>

For more compatibility information, see Compatibility.

Libraries: All versions of the C runtime libraries.


The following example demonstrates how to use the context parameter in the qsort_s function. 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"


#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"


// 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]);


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");

   // 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);

Sample output

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

See also

Searching and sorting