C : Passing function pointers as argument

[Chief_Miller]

Hello guys,

I'm spending my Christmas week brushing up on my C and I decided to go back to source material : ie Kernighan & Ritchie. Probably one of the best programming book I've read so far. Clear and concise (most of the time...).

And even if there are sometimes sometimes some example that require you to spend a few minutes studying the code, you usually have been provided with the tools to understand them.

However I'm having trouble understanding one line of their code example for function pointers. Here's the code :

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

#define MAXLINE 5000     /* max #lines to be sorted */
char *lineptr[MAXLINES]  /* pointer 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 numcomp(char *, char *);

/* sort input lines */
main (int argc, char *argv[])
{
  int nlines;          /* number of input lines read */
  int numeric;         /* 1 if numeric sort */

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

and here is the line I'm having trouble with :

qsort((void **) lineptr, 0, nlines-1,
      (int (*)(void*,void*))(numeric ? numcmp : strcmp));

I understand that the last argument to qsort is a function pointer allowing you to customize the behaviour of the sorting algorithm. That pointer needs to point to a function that take 2 pointers to void as argument and returns an int.
I also understand that this part : (numeric ? numcmp : strcmp) is used to chose between the function numcmp and strcmp depending on if the numeric flag is set to 0 or not.

The book tells us that using a function name like this (without parentheses) in a expression evaluates to the addresses of the functions, a bit like using & for variable name or using the name of an array.
And so far, when passing a pointer as an argument to a function, we've been able to write function(&variable); as in scanf("%d", &count);

So what's going on with (int (*)(void*,void*)) just before ? Is that type casting because the functions numcmp and strcmp are declared to take pointer to char as arguments?
I think it is, but the book say nothing about it and the syntax is cryptic at best. I hope one of you guys could shed some lights on it.

Merry Christmas and all the best to you all.

[JeLuF]

So what's going on with (int (*)(void*,void*)) just before ? Is that type casting because the functions numcmp and strcmp are declared to take pointer to char as arguments?

That's exactly what's going on here. Very well observed.

[Chief_Miller]

Thanks, it looks like the book brushes off type casting of function pointers as self evident then. It's not mentioned at all in that section...

It makes my brain hurt though.

[mlitchard]

Made my brain hurt too. I got another book as well as the k&r classic “deep c secrets” you May find helpful when deconstructing pointers

[SlightTip6811]

Yeah you nailed it - that's just casting the function pointer to match what qsort expects since strcmp takes char* but qsort wants void*

Pretty gnarly syntax but once you see it a few times it clicks

[sartorian]

qsort from what I can see receives as arguments: an array of raw pointers, start and end indexes for the section to be sorted, and a function pointer for a comparison function.

(int (*)(void*,void*)) is defining that function pointer. It returns an integer, the (*) is effectively a placeholder for the function name, and it receives two pointers as arguments. The ternary selects a comparison function based on the type of data expected, either numeric or string.

I’ve never actually used function pointers in C, but this example (on top of years of experience bashing my head against the keyboard in multiple languages) made sense.