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# qsort, qsort_s

From cppreference.com
< c‎ | algorithm

Defined in header `<stdlib.h>` |  |
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:

  - `count` or `size` is greater than RSIZE_MAX
  - `ptr` or `comp` is a null pointer (unless `count` is zero)
  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 undefined.

### [edit] Parameters

ptr | - | pointer to the array to sort
count | - | number of element 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 equal. 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).

Until `qsort_s`, users of `qsort` often used global variables to pass additional context to the comparison function.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <stdlib.h>
    #include <limits.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 (fails if INT_MIN is present)
    }

    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

- C11 standard (ISO/IEC 9899:2011):

  - 7.22.5.2 The qsort function (p: 355-356)

  - K.3.6.3.2 The qsort_s function (p: 609)

- C99 standard (ISO/IEC 9899:1999):

  - 7.20.5.2 The qsort function (p: 319)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.10.5.2 The qsort function

### [edit] See also

bsearch bsearch_s (C11) | searches an array for an element of unspecified type (function) [edit]
C++ documentation for qsort

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