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comp.lang.c FAQ list · Question 13.8

Q: I'm trying to sort an array of strings with qsort, using strcmp as the comparison function, but it's not working.

A: By ``array of strings'' you probably mean ``array of pointers to char.'' The arguments to qsort's comparison function are pointers to the objects being sorted, in this case, pointers to pointers to char. strcmp, however, accepts simple pointers to char. Therefore, strcmp can't be used directly. Write an intermediate comparison function like this:

/* compare strings via pointers */
int pstrcmp(const void *p1, const void *p2)
	return strcmp(*(char * const *)p1, *(char * const *)p2);

The comparison function's arguments are expressed as ``generic pointers,'' const void *. They are converted back to what they ``really are'' (pointers to pointers to char) and dereferenced, yielding char *'s which can be passed to strcmp.

The call to qsort might look like

#include <stdlib.h>
char *strings[NSTRINGS];
int nstrings;
/* nstrings cells of strings[] are to be sorted */
qsort(strings, nstrings, sizeof(char *), pstrcmp);

(Don't be misled by the discussion in K&R2 Sec. 5.11 pp. 119-20, which is not discussing the Standard library's qsort, and makes a quiet, unnecessary assumption about the equivalence of char * and void *).

For more information on qsort comparison functions--how they are called and how they must be declared--see question 13.9.

References: ISO Sec.
H&S Sec. 20.5 p. 419

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