CODEWITHSUNDEEP
cpointersstructure
upgrayedd
upgrayedd

Reputation: 129

C - Error returning structure pointer across multiple files

I am trying to learn how to separate C code into multiple files, but I am running into an error when doing so.

Relevant code (separated by file):

ex6.h:

#ifndef __ex6_h__
#define __ex6_h__

struct nlist { /* table entry: */
    struct nlist *next; /* next entry in chain */
    char *name; /* defined name */
    char *defn; /* replacement text */
};

#endif

list.c:

#include "ex6.h"

struct nlist *install(char *name, char *defn)
{
    struct nlist *np;
    unsigned hashval;

    if ((np = lookup(name)) == NULL) { /* not found */
        np = (struct nlist *) malloc(sizeof(*np));
        if (np == NULL || (np->name = strdup(name) == NULL)
            return NULL;
        hashval = hash(name);
        np->next = hashtab[hashval];
        hashtab[hashval] = np;
    } else /* already there */
        free((void *) np->defn); /*free previous defn */
    if ((np->defn = strdup(defn)) == NULL)
        return NULL;
    return np;
}

main.c:

#include "ex6.h"

int main (int argc, char* argv[])
{
    struct nlist *np1;

    np1 = install("This", "That");

    return 0;
}

When I compile this code, I get this:

cc -g main.c list.c -o main
main.c: In function ‘main’:
main.c:10:6: warning: assignment makes pointer from integer without a cast [enabled by default]
  np1 = install("This", "That");
  ^

There is obviously more code than this (will post if requested), but every other part of the code seems to work fine except this snippets. Also, when I put the code from my "main.c" file and "list.c" into the same file the the code works fine.

Any help is appreciated!

Upvotes: 0

Views: 157

Answers (3)

R Sahu
R Sahu

Reputation: 206707

You need to add a declaration for install in ex6.h. Something like:

extern struct nlist *install(char *name, char *defn);

Without a declaration, the assumed return values of functions are int. The compiler complains because the type of np1 is struct nlist* and it tries to assign an int to np1.

When you provide the declaration, it knows the return type of install is struct nlist*. Hence, it is OK to assign the return value of install to np1.

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 311078

The compilation unit with main does not see the declaration of function install. So the compiler supposes that the function by default has return type int and that in this statement

np1 = install("This", "That");.

a value of type int is assigned to pointer to the structure.

You should include the function declaration in header "ex6.h" because the function is used in more than one compilation unit.

Upvotes: 0

Joni
Joni

Reputation: 111349

You're missing a declaration of the install function in the header file. This makes the compiler assume it returns int rather than a pointer, which causes this warning. Add to ex6.h:

struct nlist *install(char *name, char *defn);

Upvotes: 2

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