Problem compiling K&R example

Question

I'm having trouble compiling the example program presented in section 5.11 of the book. I have removed most of the code and left only the relevant stuff.

#define MAXLINES 5000
char *lineptr[MAXLINES];

void qsort1(void *lineptr[], int left, int right, int (*comp)(void *, void *));
int numcmp(char *, char *);

main(int argc, char *argv[]) {
    int numeric = 1;
    /* ... */
    qsort1((void**) lineptr, 0, 100, (int (*)(void*, void*))(numeric ? numcmp : strcmp));
}

void qsort1(void *v[], int left, int right, int (*comp)(void *, void *)) {
    /* ... */
}

int numcmp(char *s1, char *s2) {
    /* ... */
}

The problem is that the code doesn't compile (I'm using Digital Mars compiler). The error I get is this:

        qsort1((void**) lineptr, 0, nlines - 1, (int (*)(void*, void*))(numeric
? numcmp : strcmp));

                 ^
go.c(19) : Error: need explicit cast to convert
from: int (*C func)(char const *,char const *)
to  : int (*C func)(char *,char *)
--- errorlevel 1

There must be something wrong with the declarations although I pasted the code from the book correctly. I don't know enough to make the right changes (the section about the function pointers could certainly have been written more extensively).

EDIT: I should have mentioned that I'm reading the ANSI version of the book.

Answer 1

Its been awhile since I have done any pure C programming, I'm not certain on the new standard.

However casting to void ** creates a pointer to a pointer where the function requires a pointer to an array. Sure, they are the same thing internally, but strong typechecking will catch that as an error.

rewrite the qsort to accept ** instead of *[] and you should be ok.

Answer 2

The standard function pointer expected by qsort() or bsearch() has the prototype:

int comparator(const void *v1, const void *v2);

The qsort1() defined in the code expects:

int comparator(void *v1, void *v2);

The comparator functions defined in the code do not have that prototype, and there is no automatic conversion between different function pointer types.

So, fixes for qsort1() are either:

1. Introduce a cast: (int (*)(void *, void *)), or

2. Rewrite the comparators:

   int numcmp(void *v1, void *v2)
   {
       char *s1 = v1;
       char *s2 = v2;
       ...
   }
   
   
   int str_cmp(void *v1, void *v2)  // Note new function name!
   {
       return(strcmp(v1, v2));
   }
   

Obviously, the call to qsort1() would reference str_cmp instead of strcmp. The authors sought to avoid an intermediate function, but run foul of the (legitimately) fussier compilers in use nowadays.

The standard version of qsort() would require a bunch of const qualifiers, as in the first version of this answer.

Answer 3

That's a common problem :)

The following line tells qsort to expect a pointer to a function with two void* parameters. Unfortunately, strcmp takes two non-modifiable strings hence it's signature is

int (*comp)(const char*, const char*)

instead of what you have:

int (*comp)(void *, void *)

Change the signature of both qsort1 and numeric:

qsort1(void *v[], int left, int right, int (*comp)(const void *, const void *))

and:

int numcmp(const char*, const char*)

Answer 4

I think the error comes from the fact that old C did not know const yet: strcmp there took two pointers to non-const characters (char *) i think (which could be the reason why it compiled back then, but not with your compiler). However, nowadays strcmp takes char const* (const char* is the same thing). Change your function prototype to this:

int numcmp(char const*, char const*);

Answer 5

Note that strcmp takes two const arguments, whereas your numcmp does not. Therefore, the two functions' types do not match, and the ? : operator will complain.

Do one of:

1. change numcmp to match the strcmp prototype in terms of constness

2. push the (int (*)(void*, void*)) cast inside the ? :, e.g.

   numeric ? (int (*)(void*, void*))numcmp : (int (*)(void*, void*))strcmp
   

Cheers, V.