CODEWITHSUNDEEP
cpointerscasting
tautvilas
tautvilas

Reputation: 137

cast to void**, for what reason?

here is C code from the book "The C programming language":

#include <stdio.h>
#include <string.h> 

#define MAXLINES 5000    /* max #lines to be sorted */
char *lineptr[MAXLINES]; /* pointers to text lines */

int readlines(char *lineptr[], int nlines);
void writelines(char *lineptr[], int nlines);

void qsort(void *lineptr[], int left, int right, 
        int (*comp)(void *, void *));

int numcmp(char *, char *);

/* sort input lines */
main(int argc, char *argv[])
{
    int nlines;

    int numeric = 0; /* number of input lines read */

    if (argc > 1 && strcmp(argv[1], "-n") == 0)  /* 1 if numeric sort */
        numeric = 1;
    if ((nlines = readlines(lineptr, MAXLINES)) >= 0) {
        qsort((void**) lineptr, 0, nlines-1, // MY QUESTION: WHY lineptr IS CAST TO POINTER TO A VOID POINTER
                (int (*)(void*,void*))(numeric ? numcmp : strcmp));
        writelines(lineptr, nlines);
        return 0;
    } else {
        printf("input too big to sort\n");
        return 1;
    }
}


void qsort(void *v[], int left, int right,
        int (*comp)(void *, void *))
{
    int i, last;
    void swap(void *v[], int, int);
    // rest of code
}

Why when qsort function is called first argument is being cast to pointer to a void pointer (void **) and not just to pointer to void (void *) . Please tell me why is it so?

Thanks

Upvotes: 2

Views: 979

Answers (2)

XB.Wang
XB.Wang

Reputation: 1

Here, theqsort function defines an int (*comp)(void *, void *) parameter and *comp is a function pointer that can be used to call a function of the comparison. Even they don't have the same function name (comp and numcmp), it's still valid.

Upvotes: 0

djhaskin987
djhaskin987

Reputation: 10057

Compilers will warn you these days unless you cast non-void pointers to void pointers when you pass them as void pointers (if that made any sense). I think you knew that, though.

The signature of void **lineptr is equivalent in C to void *lineptr[], which is the type of pointer that qsort takes as an argument. See this site for more details.

Upvotes: 1

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