Question

So I’m following a tutorial on C and I’m stuck at structs since they use the malloc function and that function doesn’t seem to go well with my compiler (Visual Studio C++ 10.0). So I followed the instructions exactly and I can compile C except that in this particular code, it gives me an error (code taken literally from the tutorial website):

#include <stdio.h>
#include <stdlib.h>

struct node {
  int x;
  struct node *next;
};

int main()
{
    /* This won't change, or we would lose the list in memory */
    struct node *root;       
    /* This will point to each node as it traverses the list */
    struct node *conductor;  

    root = malloc( sizeof(struct node) );  
    root->next = 0;   
    root->x = 12;
    conductor = root; 
    if ( conductor != 0 ) {
        while ( conductor->next != 0)
        {
            conductor = conductor->next;
        }
    }
    /* Creates a node at the end of the list */
    conductor->next = malloc( sizeof(struct node) );  

    conductor = conductor->next; 

    if ( conductor == 0 )
    {
        printf( "Out of memory" );
        return 0;
    }
    /* initialize the new memory */
    conductor->next = 0;         
    conductor->x = 42;

    return 0;
}

The malloc function kept giving trouble: “a value of type void cannot be assigned to an entity of type “node *” so I cast (node *) to every malloc-containing line, i.e.:

root = malloc( sizeof(struct node) );  

etc. This seemed to solve the above mentioned error but then when I did that and tried to compile a new error came up:

1>------ Build started: Project: TutorialTest, Configuration: Debug Win32 ------
1>  TutorialTest.c
1>c:\users\ahmed\documents\visual studio 2010\projects\tutorialtest\tutorialtest\tutorialtest.c(16): error C2065: 'node' : undeclared identifier
1>c:\users\ahmed\documents\visual studio 2010\projects\tutorialtest\tutorialtest\tutorialtest.c(16): error C2059: syntax error : ')'
1>c:\users\ahmed\documents\visual studio 2010\projects\tutorialtest\tutorialtest\tutorialtest.c(27): error C2065: 'node' : undeclared identifier
1>c:\users\ahmed\documents\visual studio 2010\projects\tutorialtest\tutorialtest\tutorialtest.c(27): error C2059: syntax error : ')'
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

So yeah, after (as a complete C newbie) trying to figure this out for half an hour, I couldn’t come up with the solution. How can I solve this error? I’m starting to think it’s a compiler issue, but I don’t want to change compiler if it’s not a necessity.

Answer 1

The problem is that you’re compiling C code with a C++ compiler. C allows that conversion from void * to an object pointer; C++ doesn’t.

You say that you added a cast, but didn’t show us what it looked like. If it looks like this, then the code should compile as both C and C++:

root = (struct node *) malloc(sizeof (struct node));

Alternatively, there might be a way to tell the compiler to treat it as C, but I don’t know enough about that compiler to help you there.