#include <stdlib.h> void qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));
The qsort() function shall sort an array of nel objects, the initial element of which is pointed to by base. The size of each object, in bytes, is specified by the width argument. If the nel argument has the value zero, the comparison function pointed to by compar shall not be called and no rearrangement shall take place.
The application shall ensure that the comparison function pointed to by compar does not alter the contents of the array. The implementation may reorder elements of the array between calls to the comparison function, but shall not alter the contents of any individual element.
When the same objects (consisting of width bytes, irrespective of their current positions in the array) are passed more than once to the comparison function, the results shall be consistent with one another. That is, they shall define a total ordering on the array.
The contents of the array shall be sorted in ascending order according to a comparison function. The compar argument is a pointer to the comparison function, which is called with two arguments that point to the elements being compared. The application shall ensure that the function returns an integer less than, equal to, or greater than 0, if the first argument is considered respectively less than, equal to, or greater than the second. If two members compare as equal, their order in the sorted array is unspecified.
((char *)p - (char *)base) % width == 0 (char *)p >= (char *)base (char *)p < (char *)base + nel * width
The Base Definitions volume of POSIX.1-2008, <stdlib.h>
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