View source for c/algorithm/qsort

{{c/title|qsort|qsort_s}}
{{c/algorithm/navbar}}
{{dcl begin}}
{{dcl header|stdlib.h}}
{{dcl|num=1|
void qsort( void* ptr, size_t count, size_t size,
            int (*comp)(const void*, const void*) );
}}
{{dcl|num =2|since=c11|
errno_t qsort_s( void* ptr, rsize_t count, rsize_t size,
                 int (*comp)(const void*, const void*, void*),
                 void* context );
}}
{{dcl end}}

@1@ Sorts the given array pointed to by {{c|ptr}} in ascending order. The array contains {{c|count}} elements of {{c|size}} bytes. Function pointed to by {{c|comp}} is used for object comparison.
@2@ Same as {{v|1}}, except that the additional context parameter {{c|context}} is passed to {{c|comp}} and that the following errors are detected at runtime and call the currently installed [[c/error/set_constraint_handler_s|constraint handler]] function:
:* {{c|count}} or {{c|size}} is greater than {{lc|RSIZE_MAX}}
:* {{c|ptr}} or {{c|comp}} is a null pointer (unless {{c|count}} is zero)
:{{c/ext1 availability|qsort_s}}

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

===Parameters===
{{par begin}}
{{par|ptr|pointer to the array to sort}}
{{par|count|number of elements in the array}}
{{par|size|size of each element in the array in bytes}}
{{par ccmp|comp}}
{{par|context|additional information (e.g., collating sequence), passed to {{c|comp}} as the third argument}}
{{par end}}

===Return value===
@1@ (none)
@2@ zero on success, non-zero if a runtime constraints violation was detected

===Notes===
Despite the name, neither C nor POSIX standards require this function to be implemented using [[enwiki:Quicksort|quicksort]] or make any complexity or stability guarantees.

Unlike other bounds-checked functions, {{tt|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 {{lc|bsearch_s}}).

The implementation of {{tt|qsort_s}} in the [https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/qsort-s Windows CRT] is incompatible with the C standard. The Microsoft's version
is declared as:
{{c multi|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 {{c|context}} is passed first.

===Example===
{{example
|code=
#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
}}

===References===
{{ref std c23}}
{{ref std|section=7.22.5.2|title=The qsort function|p=TBD}}
{{ref std|section=K.3.6.3.2|title=The qsort_s function|p=TBD}}
{{ref std end}}
{{ref std c17}}
{{ref std|section=7.22.5.2|title=The qsort function|p=258-259}}
{{ref std|section=K.3.6.3.2|title=The qsort_s function|p=442-443}}
{{ref std end}}
{{ref std c11}}
{{ref std|section=7.22.5.2|title=The qsort function|p=355-356}}
{{ref std|section=K.3.6.3.2|title=The qsort_s function|p=609}}
{{ref std end}}
{{ref std c99}}
{{ref std|section=7.20.5.2|title=The qsort function|p=319}}
{{ref std end}}
{{ref std c89}}
{{ref std|section=4.10.5.2|title=The qsort function}}
{{ref std end}}

===See also===
{{dsc begin}}
{{dsc inc|c/algorithm/dsc bsearch}}
{{dsc see cpp|cpp/algorithm/qsort}}
{{dsc end}}

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