CODEWITHSUNDEEP
cstringgccprintf
Reuben Crimp
Reuben Crimp

Reputation: 331

Passing printf() as a parameter

I'm trying to make a simple BST ADT, and I'm having some problems since I'm still new to C.

It compiles, but with warnings and a 'note', if I run the progam it only prints one element, the root node ( i want it to print all elements inorder).

I only provided code snippets that I thought was necessary, if you want all the code just ask.

bst.c - BST traversal method

41    void bst_inorder(bst b, void f(char *str)) {
42       if (b->key == NULL) {
43          return;
44       }
45       bst_inorder(b->left, f);
46       f(b->key);
47       bst_inorder(b->right, f);
48    }

TEST.c

14    bst_inorder(my_bst, printf);

bst.h

10    extern void bst_inorder(bst b, void f(char *str));

I'm compiling it like this

gcc -O2 -W -Wall -ansi -pedantic *.c -o TEST

and I get these warnings

TEST.c: In function ‘main’:
TEST.c:14:4: warning: passing argument 2 of ‘bst_inorder’ from incompatible pointer type [enabled by default]

In file included from TEST.c:3:0:
bst.h:10:13: note: expected ‘void (*)(char *)’ but argument is of type ‘int (*)(const char * __ restrict__)’

Upvotes: 0

Views: 888

Answers (3)

Yu Hao
Yu Hao

Reputation: 122383

The warning is clear, the 2nd argument of bst_inorder type mismatch.

I assume that you are trying to use printf to print strings only(i.e, not using the variable argument part), in that case, you can wrap it like this:

void my_printf(char *str)
{
    printf("%s", str);
}

And call it with:

bst_inorder(my_bst, my_printf);

Upvotes: 0

aragaer
aragaer

Reputation: 17848

What it says - your function is expecting a function returning void, while printf returns int.

Other problem is that the correct syntax for pointer to function is like this:

void (*f)(char *str)

Or in case of printf:

int (*f)(const char *)

Upvotes: 2

unwind
unwind

Reputation: 399793

The warning is simply because there really is a mis-match between your argument and the printf() function.

Your function expects void (*)(char *), but printf()'s signature is int (*)(const char *, ...). Clearly these are not the same.

It's probably OK, but the cleanest way to fix it is to write a "shim" or "trampoline" function:

static void print_node(char *str)
{
  printf("%s", str);
}

Then use that instead of printf directly in the call to bst_inorder().

Not sure about the other problem, I think there's not enough code present to help with that.

Upvotes: 2

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