qsort

qsort is a C standard library function that implements a polymorphic sorting algorithm for arrays of arbitrary objects according to a user-provided comparison function. It is named after the "quicker sort" algorithm (a quicksort variant due to R. S. Scowen), which was originally used to implement it in the Unix C library, although the C standard does not require it to implement quicksort.^[1]

Implementations of the qsort function achieve polymorphism, the ability to sort different kinds of data, by taking a function pointer to a three-way comparison function, as well as a parameter that specifies the size of its individual input objects. The C standard requires the comparison function to implement a total order on the items in the input array.^[2]

A qsort function was in place in Version 3 Unix of 1973, but was then an assembler subroutine.^[3] A C version, with roughly the interface of the standard C version, was in-place in Version 6 Unix.^[4] It was rewritten in 1983 at Berkeley.^[1] The function was standardized in ANSI C (1989).

Example

The following piece of C code shows how to sort a list of integers using qsort.

#include <stdlib.h>

/* Comparison function. Receives two generic (void) pointers to the items under comparison. */
int compare_ints(const void *p, const void *q) {
    int x = *(const int *)p;
    int y = *(const int *)q;

    /* Avoid return x - y, which can cause undefined behaviour
       because of signed integer overflow. */
    if (x < y)
        return -1;  // Return -1 if you want ascending, 1 if you want descending order. 
    else if (x > y)
        return 1;   // Return 1 if you want ascending, -1 if you want descending order. 

    return 0;
}

/* Sort an array of n integers, pointed to by a. */
void sort_ints(int *a, size_t n) {
    qsort(a, n, sizeof *a, &compare_ints);
}

References

1. Bentley, Jon L.; McIlroy, M. Douglas (1993). "Engineering a sort function". Software—Practice and Experience. 23 (11): 1249–1265. doi:10.1002/spe.4380231105.
2. ISO/IEC 9899:201x, Programming Languages—C (draft). §7.22.5. November 16, 2010.
3. "qsort(III), from UNIX Programmer's Manual, Third Edition". Unix Archive.
4. "qsort(III), from UNIX Programmer's Manual, Sixth Edition". Unix Archive.