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qsort, qsort_r, heapsort, mergesort - sort functions



      qsort, qsort_r, heapsort, mergesort - sort functions


      Standard C Library (libc, -lc)


      #include <stdlib.h>
      qsort(void *base, size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));
      qsort_r(void *base, size_t nmemb, size_t size, void *thunk,
              int (*compar)(void *, const void *, const void *));
      heapsort(void *base, size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));
      mergesort(void *base, size_t nmemb, size_t size,
              int (*compar)(const void *, const void *));


      The qsort() function is a modified partition-exchange sort, or quicksort.
      The heapsort() function is a modified selection sort.  The mergesort()
      function is a modified merge sort with exponential search intended for
      sorting data with pre-existing order.
      The qsort() and heapsort() functions sort an array of nmemb objects, the
      initial member of which is pointed to by base.  The size of each object
      is specified by size.  The mergesort() function behaves similarly, but
      requires that size be greater than “sizeof(void *) / 2”.
      The contents of the array base are sorted in ascending order according to
      a comparison function pointed to by compar, which requires two arguments
      pointing to the objects being compared.
      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.
      The qsort_r() function behaves identically to qsort(), except that it
      takes an additional argument, thunk, which is passed unchanged as the
      first argument to function pointed to compar.  This allows the comparison
      function to access additional data without using global variables, and
      thus qsort_r() is suitable for use in functions which must be reentrant.
      The algorithms implemented by qsort(), qsort_r(), and heapsort() are not
      stable, that is, if two members compare as equal, their order in the
      sorted array is undefined.  The mergesort() algorithm is stable.
      The qsort() and qsort_r() functions are 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.  Quicksort takes O N lg N
      average time.  This implementation uses median selection to avoid its O
      N**2 worst-case behavior.
      The 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.  Heapsort takes O N lg N worst-case time.  Its
      only advantage over qsort() is that it uses almost no additional memory;
      while qsort() does not allocate memory, it is implemented using recur‐
      The function mergesort() requires additional memory of size nmemb * size
      bytes; it should be used only when space is not at a premium.  The
      mergesort() function is optimized for data with pre-existing order; its
      worst case time is O N lg N; its best case is O N.
      Normally, qsort() is faster than mergesort() is faster than heapsort().
      Memory availability and pre-existing order in the data can make this
      The qsort() and qsort_r() functions return no value.
      The heapsort() and mergesort() functions return the value 0 if success‐
      ful; otherwise the value -1 is returned and the global variable errno is
      set to indicate the error.


      Previous versions of qsort() did not permit the comparison routine itself
      to call qsort(3).  This is no longer true.


      The heapsort() and mergesort() functions succeed unless:
      [EINVAL]           The size argument is zero, or, the size argument to
                         mergesort() is less than “sizeof(void *) / 2”.
      [ENOMEM]           The heapsort() or mergesort() functions were unable to
                         allocate memory.
      sort(1), radixsort(3)
      Hoare, C.A.R., "Quicksort", The Computer Journal, 5:1, pp. 10-15, 1962.
      Williams, J.W.J, "Heapsort", Communications of the ACM, 7:1, pp. 347-348,
      Knuth, D.E., "Sorting and Searching", The Art of Computer Programming,
      Vol. 3, pp. 114-123, 145-149, 1968.
      McIlroy, P.M., "Optimistic Sorting and Information Theoretic Complexity",
      Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, January 1992.
      Bentley, J.L.  and McIlroy, M.D., "Engineering a Sort Function",
      Software--Practice and Experience, Vol. 23(11), pp. 1249-1265,
      November 1993.


      The qsort() function conforms to ISO/IEC 9899:1990 (“ISO C89”).


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