C : Warning about visibility of a struct

Question

I have a complex C project. In a file message.h I declare this structure

struct message  
{
    int err;
    struct header
    {
        char *protocol_version;         
        char *type;                     
        long int sequence_number;       
    } header;                           
    struct body
    {
        int num_tag;                     
        char *tag_labels[LEN];          
        int num_attr_tag[LEN];          
        char *attr_labels[LEN][LEN];    
        char *attr_values[LEN][LEN];    
        char *attr_types[LEN][LEN];     
    } body;                             
};

In the file "castfunctions.h", I include the file "message.h" and I declare the function "setClientNat"

#include <message.h>
void *setClientNat(struct message *msg);

When I compile, I have this warning

castfunctions.h:warning: 
  declaration of 'struct message' will not be visible outside of this function [-Wvisibility]
  void *setClientNat(struct message *msg);

Can anyone help me?

Did you put the `#include` inside the function body by mistake? — Arkku, Nov 15 '13 at 17:48
There is nothing obviously wrong with the structure you show. Can you construct a [sscce](http://sscce.org/) that demonstrates this problem? — simonc, Nov 15 '13 at 17:50
I insert the declaration of setClientNat at line 36. No I don't put the include inside the function body. — EngAndreaR, Nov 15 '13 at 17:51
No I declare that struct in the header file 'message.h'. After, I include this file in another header file 'castfunctions.h', where I declare the function 'setClientNat'. — EngAndreaR, Nov 15 '13 at 17:55
If you are not deliberately including either header inside a function, my guess is that you are missing a semicolon or `}` somewhere above `#include `, but it is impossible to answer accurately with the information available. — Arkku, Nov 15 '13 at 17:59
If I declare `typedef struct Message`, I delete this warning. But I cannot do this because this change generate errors related to other file of the project. — EngAndreaR, Nov 15 '13 at 18:10
`message.h` is a pretty generic-looking name. Are you sure you're getting the right one? Maybe another one is hiding in `/usr/include` — , Nov 15 '13 at 18:24
Wait, wait, wait. I can't say with 100% certainty for C++, but if you're compiling this as C, could this be because you have `#include ` and not `#include "message.h"`? — Dennis Meng, Nov 15 '13 at 18:42

nos · Answer 1 · 2013-11-15T18:38:23.710

declaration of 'struct message' will not be visible outside of this function [-Wvisibility]

That warning means that struct message was not declared at that point, so it serves as a useless forward declaration.

This means that the code you show is not the complete truth, your files have a lot more in them that what you show - the error is in the code not shown to us.

Here is a few reasons as to why you might get the warning;

#include <message.h> includes an entierly different file than what you think it does, go look for another message.h elsewhere.
You have include guards in your message.h like so

#ifndef MESSAGE_H
#define MESSAGE_H 
struct message { 
....
};
#endif`

Then you use the headerfiles in a source file like so:

   #include <thisnthat.h>
   #include <message.h>

And it just so happened that the <thisnthat.h> file also defined a
MESSAGE_H macro, rendering the entire message.h invisible. Alternatively the thisnthat.h header have a #define message something_else

There's a syntax error somewhere in the header files directly or indirectly included together with message.h. Go hunt for missing ; or { or }
You misspelled something. Your comment states the error is gone when you did a typedef struct Message which for some reason have Message with a capital M. So somewhere you're mixing up struct Message vs struct message

score 1 · Answer 2 · answered Nov 15 '13 at 18:58

1

In addition to nos' answer you should run gcc with the -E option instead of -c. This will output the preprocessed translation unit, so you can see what the compiler really sees. The output also mentions each file that gets included.

answered Nov 15 '13 at 18:58

Roland Illig

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C : Warning about visibility of a struct

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