Thread: struct pointer -> Bus Error

Hi All

I've written a test program to experiment with structs a little bit:

Code:

#include <stdio.h>

struct FOO {
  int id ;
} foo, *bar;

struct FOO test(struct FOO x) ; // prototype

int main(int argc, char *argv[])
 {
        foo.id = 10 ;
        bar->id = 20 ;
        printf("%d %d\n", foo.id, bar->id) ;
        test(foo) ;
        return 0 ;
 }

struct FOO test(struct FOO s) {
  return s ;
}

When I run this program I get a Bus error. As far as I know it has todo with the second bar->id but I can't figure out why.
Any suggestions ?

cheers

bar is a pointer to struct FOO, but it was never initialised beyond the zero initialisation of global variables (i.e., you tried to dereference a null pointer with bar->id).

It should be:

Code:

bar = malloc(sizeof(*bar));

If you want to use the struct name, then you should write:

Code:

bar = malloc(sizeof(struct FOO));

> Doesn't compile
Doesn't tell us anything either!

Did you put the statement inside main(), like you should,

or did you try to initialise it like this(bad)?

Code:

struct FOO {
  int id ;
} foo, *bar = malloc(sizeof(FOO)) ;

Did you include stdlib.h ?

Originally Posted by jeanluca

Code:

bar = (struct *FOO)malloc(sizeof(FOO)) ;

doesn't compile,

That is correct, as the "(struct *FOO)" is invalid

Originally Posted by jeanluca

but I found out it should be:

Code:

bar = (struct FOO*)malloc(sizeof(FOO)) ;

In C, that conversion is not necessary if you have #include'd <stdlib.h>. However, if your compiler is actually a C++ compiler (and a lot of C compilers are, in practice) then that conversion is required.

Originally Posted by grumpy

That is correct, as the "(struct *FOO)" is invalid

With respect to the original code snippet posted, the statement is still invalid because sizeof(FOO) is wrong: it should be sizeof(struct FOO).

Originally Posted by jeanluca

so, its a 'best practice' to always do the conversion youself!?

It depends on whether you want your program to be compilable as C++. It is true that many C compilers are bundled with or part of a C++ compiler, but nonetheless you will be able to compile your code as C, otherwise the compiler cannot possibly be regarded as a C compiler.

Originally Posted by jeanluca

thnx

Well my code is pure C, but I guess it might be useful when it compiles with a c++ compiler too!

You wouldn't want to call malloc() in a C++ program. If for some reason this code ever gets compiled as C++, then you might consider adding a cast, but at this point it's not a good idea to dirty the code because of a hypothetical.

> Well my code is pure C, but I guess it might be useful when it compiles with a c++ compiler too!
Why not make it compile with Fortran while you're at it?

The apparent similarity of the languages is no excuse for writing some mashed C/C++ code.

Incompatibilities Between ISO C and ISO C++
Read this to discover a whole load of issues with respect to turning a valid C program into a valid C++ program. Casting malloc is the least of your worries if you ever seriously compile it as C++.