[lug] C programming Q.

[rm at fabula.de]

On Wed, Apr 17, 2002 at 11:54:41AM -0500, Michael J. Hammel wrote:
> I hate this.  We're using some gawd-awful args to gcc that force all
> warnings into errors so compiles fail.  Ick.  

No, this is a _good_ habbit, really ;-) Since i made this the default
in my projects (and cranked up the warning switches) i had way less 
bugs sneak into my production code.

> I've cleaned up most of them,
> but now I've hit one that I've never hit before.
> 
> We have a struct that has a function pointer as a member, such as:
> 
> typedef struct teststruct{
>     char *name;
>     int (*fp) (void);
>     int test_type;
> } teststruct_0;
> 
> An initialization using this struct looks like this:
> 
> teststruct_0 tests_noarg[] =
> {
> 	{"Stuck Address", test_stuck_address, someval}
> }
> 
> The function "test_stuck_address" has this prototype:
> 
>     int test_stuck_address (uint *val1, uint *val2, uint count);
> 
> During compile, the error we get is this:
> 
>     file.c:xxx: warning: initialization from incompatible pointer type


Well, a pointer to 'int f(uint *val1, uint *val2, uint count);' _is_ different
from a pointer to 'int f(void)'. 

> So how do I specify the initialization so that this error (re: warning)
> goes away?  I've tried these:
> 
> 	{"Stuck Address", test_stuck_address(), someval}
> 	{"Stuck Address", &test_stuck_address, someval}
> 
> But neither works.  Does it need a special casting?  Anyone got a suggestion?

No, you can either change the signature of the function in the structures
declaration:

typedef struct teststruct{
     char *name;
     int (*fp) (uint *val1, uint *val2, uint count);
     int test_type;
} teststruct_0;

or cast the function to the appropriate type during inititalisation . I would 
expect the first solution in situations where the functions signature is constant
in the program and the second one in places where the function might have a different
signature depending on other information (callbacks would come to mind). 


Hope that helps

   Ralf
> -- 
> Michael J. Hammel           |
> The Graphics Muse           |          Ambivalent? Well, yes and no.
> mjhammel at graphics-muse.org  |
> http://www.graphics-muse.com 
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