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Showing content from http://en.cppreference.com/w/c/numeric/math/../../io/../language/../algorithm/qsort.html below:

qsort, qsort_s - cppreference.com

void qsort( void* ptr, size_t count, size_t size,
            int (*comp)(const void*, const void*) ); (1) errno_t qsort_s( void* ptr, rsize_t count, rsize_t size,

                 int (*comp)(const void*, const void*, void*),

                 void* context ); (2) (since C11)

1) Sorts the given array pointed to by ptr in ascending order. The array contains count elements of size bytes. Function pointed to by comp is used for object comparison.

2)

Same as

(1)

, except that the additional context parameter

context

is passed to

comp

and that the following errors are detected at runtime and call the currently installed

constraint handler

function:

As with all bounds-checked functions, qsort_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdlib.h>.

If comp indicates two elements as equivalent, their order in the resulting sorted array is unspecified.

[edit] Parameters ptr - pointer to the array to sort count - number of elements in the array size - size of each element in the array in bytes comp - comparison function which returns ​a negative integer value if the first argument is less than the second, a positive integer value if the first argument is greater than the second and zero if the arguments are equivalent.

The signature of the comparison function should be equivalent to the following:

 int cmp(const void *a, const void *b);

The function must not modify the objects passed to it and must return consistent results when called for the same objects, regardless of their positions in the array.

​

context - additional information (e.g., collating sequence), passed to comp as the third argument [edit] Return value

1) (none)

2) zero on success, non-zero if a runtime constraints violation was detected

[edit] Notes

Despite the name, neither C nor POSIX standards require this function to be implemented using quicksort or make any complexity or stability guarantees.

Unlike other bounds-checked functions, qsort_s does not treat arrays of zero size as a runtime constraint violation and instead returns successfully without altering the array (the other function that accepts arrays of zero size is bsearch_s).

The implementation of qsort_s in the Windows CRT is incompatible with the C standard. The Microsoft's version is declared as: void qsort_s(void *base, size_t num, size_t width,
             int (*compare )(void *, const void *, const void *), void * context);. It does not return a value, and the comparison function has a reversed parameter order with regard to the standard: the context is passed first.

[edit] Example 
#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
 
int compare_ints(const void* a, const void* b)
{
    int arg1 = *(const int*)a;
    int arg2 = *(const int*)b;
 
    if (arg1 < arg2) return -1;
    if (arg1 > arg2) return 1;
    return 0;
 
    // return (arg1 > arg2) - (arg1 < arg2); // possible shortcut
 
    // return arg1 - arg2; // erroneous shortcut: undefined behavior in case of
                           // integer overflow, such as with INT_MIN here
}
 
int main(void)
{
    int ints[] = {-2, 99, 0, -743, 2, INT_MIN, 4};
    int size = sizeof ints / sizeof *ints;
 
    qsort(ints, size, sizeof(int), compare_ints);
 
    for (int i = 0; i < size; i++)
        printf("%d ", ints[i]);
 
    printf("\n");
}

Output:

-2147483648 -743 -2 0 2 4 99
[edit] References
[edit] See also

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