What's wrong with my pointer?

[Richardcavell]

Hi everyone. I'm using GCC to compile code according to the C99 standard.

I'm using libcurl to make HTTP requests. I need to call the function curl_easy_setopt where the third parameter is a "pointer to a zero-terminated string." GCC chokes on the following code, claiming that for the third parameter: invalid application of 'sizeof' to incomplete type 'const char[]'.

Note that if I put the szuseragent definition in the same translation unit, it works. Also, if I change szuseragent to &szuseragent[0], it works!

Is it my fault or gcc's?

#include "Variables.h"

int setuseragent ( void)
{
     curl_easy_setopt ( curl , CURLOPT_USERAGENT , szuseragent ) ;
     return 0 ;
}

//Variables.h

extern const char szuseragent [] ;

//Variables.c

const char szuseragent[]= "My user agent string." ;

[grumpy]

From the error message, curl_easy_setopt() is a macro, and uses sizeof() on its third argument.

The problem is with your code (or, more accurately, with youre code not matching expectations of the macro) not with gcc.

That means szuseragent needs to be a string literal or an array of char for which the size is known at compile time. Among other things, that means there cannot be a separation of declaration (in Variables.h) and definition (in Variables.c).

To fix the problem, change the declaration in Variables.h of szuseragent to

const char szuseragent[] = "My user agent string."

and don't define szuseragent in Variables.c.

Changing szuseragent to &szuseragent[0] will make your code compile, but - given what the documentation for libcurl says about the third argument of curl_easy_setopt() - it will probably misbehave at run time (as sizeof(szuseragent) and sizeof(&szuseragent[0]) will not be different values).

[Richardcavell]

Would a better solution be :

const char szuseragent [100] = "My user agent string." ;

[pheininger]

To fix the problem, change the declaration in Variables.h of szuseragent to

const char szuseragent[] = "My user agent string."

and don't define szuseragent in Variables.c.

But if variables.h is included in multiple translation units, won't there be multiple definitions for szusergent? That would result in multiple definition linker errors, right?

[grumpy]

Would a better solution be :

const char szuseragent [100] = "My user agent string." ;

Only if you wish to allocate 100 characters where only 22 is needed (which I would summarise, as an answer to your question, as "No"). The compiler can work out the size of string literals, and therefore can work out the actual array size needed - if allowed to.

But if variables.h is included in multiple translation units, won't there be multiple definitions for szusergent? That would result in multiple definition linker errors, right?

No. Each source file that #include's Variables.h will have its own local version of szuseragent.

Personally, I wouldn't put such a declaration in a header file at all - make it local to the function that uses the string. But it it is actually needed in a header file, and the size of the string needs to be found using sizeof(), then the approach I suggested above is what is needed.