qsort is a C standard libraryfunction that implements a sorting algorithm for arrays of arbitrary objects according to a user-provided comparison function. It is named after the "quicker sort" algorithm[1] (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.[2]
The ability to operate on different kinds of data (polymorphism) is achieved 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.[3]
History
A qsort function appears in Version 2 Unix in 1972 as a library assembly language subroutine. Its interface is unlike the modern version, in that it can be pseudo-prototyped as qsort(void * start, void * end, unsigned length) – sorting contiguously-stored length-long byte strings from the range [start, end).[1] This, and the lack of a replaceable comparison function, makes it unsuitable to properly sort the system's little-endian integers, or any other data structures.
In Version 3 Unix, the interface is extended by calling compar(III), with an interface identical to modern-day memcmp. This function may be overridden by the user's program to implement any kind of ordering, in an equivalent fashion to the compar argument to standard qsort (though program-global, of course).[4]
In 1991, Bell Labs employees observed that AT&T and BSD versions of qsort would consume quadratic time for some simple inputs. Thus Jon Bentley and Douglas McIlroy engineered a new faster and more robust implementation.[2] McIlroy would later produce a more complex quadratic-time input, termed AntiQuicksort, in 1998. This function constructs adversary data on-the-fly.[7]
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. */intcompare_ints(constvoid*p,constvoid*q){intx=*(constint*)p;inty=*(constint*)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. elseif(x>y)return1;// Return 1 if you want ascending, -1 if you want descending order.return0;// All the logic is often alternatively written:return(x>y)-(x<y);}/* Sort an array of n integers, pointed to by a. */voidsort_ints(int*a,size_tn){qsort(a,n,sizeof(*a),compare_ints);}
Extensions
Since the comparison function of the original qsort only accepts two pointers, passing in additional parameters (e.g. producing a comparison function that compares by the two value's difference with another value) must be done using global variables. The issue was solved by the BSD and GNUUnix-like systems by introducing a qsort_r function, which allows for an additional parameter to be passed to the comparison function. The two versions of qsort_r have different argument orders. C11 Annex K defines a qsort_s essentially identical to GNU's qsort_r. The macOS and FreeBSD libcs also contain qsort_b, a variant that uses blocks, an analogue to closures, as an alternate solution to the same problem.[8]
