Qsort
quick sort, defined by ANSI C standard
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 (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. 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.
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 void qsort(void* start, void* end, unsigned int length) – sorting contiguously-stored length-long byte strings from the range [start, end). 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).
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