qsort is a C standard library function 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] It comes from <stdlib.h> (or <cstdlib> in C++ Standard Library).
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]
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 voidqsort(void*start,void*end,unsignedintlength) – 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]
Version 4 Unix adds a C implementation, with an interface equivalent to the standard. [5] It was rewritten in 1983 for the Berkeley Software Distribution. [2] The function was standardized in ANSI C (1989). The assembly implementation is removed in Version 6 Unix. [6]
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 adversarial data on-the-fly. [7]
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.intcompareInts(constvoid*p,constvoid*q){intx=*(constint*)p;inty=*(constint*)q;// Avoid returning x - y, which can cause undefined behaviour// because of signed integer overflow.if(x<y){// Return -1 for ascending, +1 for descending order. return-1;}elseif(x>y){// Return +1 for ascending, -1 for descending order.return1;}else{return0;}}// This could be more concisely written as:intcompareInts(constvoid*p,constvoid*q){intx=*(constint*)p;inty=*(constint*)q;return(x>y)-(x<y);}// Sort an array of n integers, pointed to by a.voidsortInts(int*a,size_tn){qsort(a,n,sizeof(*a),compareInts);}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 GNU Unix-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]
In C++, it is faster to use std::sort (or std::ranges::sort from C++20 and onwards). Compared to ::qsort, the templated std::sort is more type-safe since it does not require access to data items through unsafe void pointers, as ::qsort does. Also, ::qsort accesses the comparison function using a function pointer, necessitating large numbers of repeated function calls, whereas in std::sort, comparison functions may be inlined into the custom object code generated for a template instantiation. In practice, C++ code using std::sort is often considerably faster at sorting simple data like integers than equivalent C code using ::qsort. [9]