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ccastingqsortmarkov

"programming pearls": Strings of Pearls

In column 15.3, the author introduced how to generate text randomly from an input document. The author also gave the source code.

qsort(word, nword, sizeof(word[0]), sortcmp);
int sortcmp(char **p, char **q)
{   return wordncmp(*p, *q);
}

I've been confused by the above lines in the source code. The last argument of qsort is:

int comparator ( const void * elem1, const void * elem2 ).

But the definition of sortcmp is different. Actually, the source code cannot compiled in my VS2010.

Solution

  • It seems this code was originally compiled with a more forgiving (or less standard-compliant) compiler. The idea seems to be that the canonical void * arguments of the comparator function are interpreted as char ** so that wordncmp(), which is an implementation of lexicographical comparison of up to length n, can be applied to them.

    Declaring the function as expected (i.e. taking two const void * arguments) and making the type casts explicit appears to solve the problem (tested with GCC 4.7.0):

    int sortcmp(const void *p, const void *q) {
  return wordncmp(*(const char **)p, *(const char **)q);
}

    I also had to modify the declaration of the wordncmp() function:

    int wordncmp(const char *p, const char* q)
{
  /*.. Definition unchanged.. */
}