Problem compiling K&R example - c

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

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 2

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 3

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:
Introduce a cast: (int (*)(void *, void *)), or
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 4

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:
change numcmp to match the strcmp prototype in terms of constness
push the (int (*)(void*, void*)) cast inside the ? :, e.g.
numeric ? (int (*)(void*, void*))numcmp : (int (*)(void*, void*))strcmp

Answer 5

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.