No warning with uninitialized C string

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Category:Languages

I'm currently wondering why I don't get an error during GCC compilation/linking of a small C program.

I declared in version.h the following string:

const char* const VERSION; 

In version.c I have set the initialization of the variable:

const char* const VERSION = "0.8 rev 213"; 

No problem with that. I can use the string in the rest of the program.

If the c file is missing, no error occurs during compilation/linking but the program fails with SIGSEGV (of course) when it tries to access the variable.

Is my way of setting up the variable VERSION correct or is there a better way? Or is there a chance to get an error during compilation/linking?

 


You have defined (not just declared) a variable in the header.

If you ever include this header from more than one source file, the behaviour is undefined. Here's the relevant quote from the standard:

[J.2, list of undefined behaviours]

An identifier with external linkage is used, but in the program there does not exist exactly one external definition for the identifier, or the identifier is not used and there exist multiple external definitions for the identifier

You are relying on GCC-specific behaviour here (merging of duplicate tentative data definitions, see help for -fcommon and -fno-common compilation flags). Other compilers may behave differently.

One of the definitions (that in a .c file) initialises it, but if you omit it, it will remain zero-initialised. That is, it will be a constant null pointer. Not very useful.

To make long story short, never ever do that. In order to declare (but not define) a global variable in a header, use extern. If you do, and try to omit a definition elsewhere, you will likely get a linker error (though the standard does not require diagnostic for this violation, all known implementation produce one).

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