NAME

qsort, qsort_r — sort a table of data

SYNOPSIS

#include <stdlib.h>

void qsort(void *base, size_t nel, size_t width,
       int (*compar)(const void *, const void *));


[CX] void qsort_r(void *base, size_t nel, size_t width,
       int (*compar)(const void *, const void *, void *), void *arg);

DESCRIPTION

For qsort(): ^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The qsort() function shall sort an array of nel objects, the initial element of which is pointed to by base. The size of each object, in bytes, is specified by the width argument. If the nel argument has the value zero, the comparison function pointed to by compar shall not be called and no rearrangement shall take place.

The application shall ensure that the comparison function pointed to by compar does not alter the contents of the array. The implementation may reorder elements of the array between calls to the comparison function, but shall not alter the contents of any individual element.

When the same objects (consisting of width bytes, irrespective of their current positions in the array) are passed more than once to the comparison function, the results shall be consistent with one another. That is, they shall define a total ordering on the array.

The contents of the array shall be sorted in ascending order according to a comparison function. The compar argument is a pointer to the comparison function, which is called with two arguments that point to the elements being compared. The application shall ensure that the function returns an integer less than, equal to, or greater than 0, if the first argument is considered respectively less than, equal to, or greater than the second. If two members compare as equal, their order in the sorted array is unspecified.

^[CX] The qsort_r() function shall be identical to qsort() except that the comparison function compar takes a third argument. The arg opaque pointer passed to qsort_r() shall in turn be passed as the third argument to the comparison function.

RETURN VALUE

These functions shall not return a value.

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

The comparison function need not compare every byte, so arbitrary data may be contained in the elements in addition to the values being compared.

If the compar callback function requires any additional state outside of the items being sorted, it can only access this state through global variables, making it potentially unsafe to use qsort() with the same compar function from separate threads at the same time. The qsort_r() function was added with the ability to pass through arbitrary arguments to the comparator, which avoids the need to access global variables and thus making it possible to safely share a stateful comparator across threads.

RATIONALE

The requirement that each argument (hereafter referred to as p) to the comparison function is a pointer to elements of the array implies that for every call, for each argument separately, all of the following expressions are non-zero:

((char *)p - (char *)base) % width == 0
(char *)p >= (char *)base
(char *)p < (char *)base + nel * width

FUTURE DIRECTIONS

None.

SEE ALSO

alphasort

XBD <stdlib.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/49 is applied, adding the last sentence to the first non-shaded paragraph in the DESCRIPTION, and the following two paragraphs. The RATIONALE is also updated. These changes are for alignment with the ISO C standard.

Issue 8

Austin Group Defect 900 is applied, adding the qsort_r() function.