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from: @(#)qsort.3 8.1 (Berkeley) 6/4/93
.Dd July 20, 2025 .Dt QSORT 3 .Os .Sh NAME .Nm qsort , .Nm heapsort , .Nm mergesort , .Nm qsort_r , .Nm heapsort_r , .Nm mergesort_r .Nd sort functions .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdlib.h .Ft void .Fn qsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)" .Ft void .Fn qsort_r "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *, void *)" "void *cookie" .Ft int .Fn heapsort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)" .Ft int .Fn heapsort_r "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *, void *)" "void *cookie" .Ft int .Fn mergesort "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *)" .Ft int .Fn mergesort_r "void *base" "size_t nmemb" "size_t size" "int (*compar)(const void *, const void *, void *)" "void *cookie" .Sh DESCRIPTION The .Fn qsort function is a modified partition-exchange sort, or quicksort. The .Fn heapsort function is a modified selection sort. The .Fn mergesort function is a modified merge sort with exponential search intended for sorting data with pre-existing order.
p The .Fn qsort and .Fn heapsort functions sort an array of .Fa nmemb objects, the initial member of which is pointed to by .Fa base . The size of each object is specified by .Fa size . .Fn mergesort behaves similarly, but .Em requires that .Fa size be greater than .Dq "sizeof(void *) / 2" .
p The contents of the array .Fa base are sorted in ascending order according to a comparison function pointed to by .Fa compar , which requires two arguments pointing to the objects being compared.
p The comparison function must return an integer less than, equal to, or greater than zero if the first argument is considered to be respectively less than, equal to, or greater than the second.
p The functions .Fn qsort and .Fn heapsort are .Em not stable, that is, if two members compare as equal, their order in the sorted array is undefined. The function .Fn mergesort is stable.
p The .Fn qsort_r , .Fn heapsort_r , and .Fn mergesort_r functions pass an additional cookie argument through to the comparison function.
p The .Fn qsort function is an implementation of C.A.R. Hoare's ``quicksort'' algorithm, a variant of partition-exchange sorting; in particular, see D.E. Knuth's Algorithm Q. .Fn qsort takes O N lg N average time. This implementation uses median selection to avoid its O N**2 worst-case behavior.
p The .Fn heapsort function is an implementation of J.W.J. William's ``heapsort'' algorithm, a variant of selection sorting; in particular, see D.E. Knuth's Algorithm H. .Fn heapsort takes O N lg N worst-case time. Its .Em only advantage over .Fn qsort is that it uses almost no additional memory; while .Fn qsort does not allocate memory, it is implemented using recursion.
p The function .Fn mergesort requires additional memory of size .Fa nmemb * .Fa size bytes; it should be used only when space is not at a premium. .Fn mergesort is optimized for data with pre-existing order; its worst case time is O N lg N; its best case is O N.
p Normally, .Fn qsort is faster than .Fn mergesort is faster than .Fn heapsort . Memory availability and pre-existing order in the data can make this untrue. .Sh RETURN VALUES The .Fn qsort and .Fn qsort_r functions return no value.
p Upon successful completion, .Fn heapsort , .Fn mergesort .Fn heapsort_r , and .Fn mergesort_r return 0. Otherwise, they return -1 and the global variable .Va errno is set to indicate the error. .Sh COMPATIBILITY Previous versions of .Fn qsort did not permit the comparison routine itself to call .Fn qsort . This is no longer true. .Sh ERRORS The .Fn heapsort , .Fn mergesort .Fn heapsort_r , and .Fn heapsort_r functions succeed unless: l -tag -width Er t Bq Er EINVAL The .Fa size argument is zero, or, the .Fa size argument to .Fn mergesort or .Fn mergesort_r is less than .Dq "sizeof(void *) / 2" . t Bq Er ENOMEM .Fn heapsort , .Fn heapsort_r , .Fn mergesort , or .Fn mergesort_r were unable to allocate memory. .El .Sh SEE ALSO .Xr sort 1 , .Xr radixsort 3 .Rs .%A Hoare, C.A.R. .%D 1962 .%T "Quicksort" .%J "The Computer Journal" .%V 5:1 .%P pp. 10-15 .Re .Rs .%A Williams, J.W.J .%D 1964 .%T "Heapsort" .%J "Communications of the ACM" .%V 7:1 .%P pp. 347-348 .Re .Rs .%A Knuth, D.E. .%D 1968 .%B "The Art of Computer Programming" .%V Vol. 3 .%T "Sorting and Searching" .%P pp. 114-123, 145-149 .Re .Rs .%A McIlroy, P.M. .%D 1993 .%T "Optimistic Sorting and Information Theoretic Complexity" .%J "Proceedings of the Fourth Annual ACM-SIAM Symposium on Discrete Algorithms" .%P pp. 467-474 .Re .Rs .%A Bentley, J.L. and McIlroy, M.D. .%D 1993 .%T "Engineering a Sort Function" .%J "Software-Practice and Experience" .%V Vol. 23 .%P pp. 1249-1265 .Re .Sh STANDARDS The .Fn qsort function conforms to .St -ansiC . .Sh HISTORY The .Fn qsort_r , .Fn heapsort_r , and .Fn mergesort_r functions were added in .Nx 11.0 .
p The .Fn qsort_r function conforms to .St -p1003.1-2024 .
Indexes created Sun Oct 05 05:09:57 GMT 2025