CODEWITHSUNDEEP
c
Heatsink
Heatsink

Reputation: 7761

Cannot use standard library function name for a global variable, even when no headers are included

Declaring a global variable with the same name as a standard function produces an error in clang (but not gcc). It is not due to a previous declaration in a header file. I can get the error by compiling the following one-line file:

extern void *memcpy[];

Clang says

foo.c:1:14: error: redefinition of 'memcpy' as different kind of symbol
foo.c:1:14: note: previous definition is here

Apparently this only happens for a few standard functions. printf produces an error, fprintf produces a warning, fseek just works.

Why is this an error? Is there a way to work around it?

Motivation. I am using the C compiler as a compiler backend. C code is programmatically generated. The generated code relies on byte-level address arithmetic and pointer type casting. All external symbols are declared as extern void *variablename[];.

Upvotes: 2

Views: 396

Answers (2)

Diomidis Spinellis
Diomidis Spinellis

Reputation: 19385

According to the C standard (ISO 9899:1999 section 7.1.3), "all external identifiers defined by the library are reserved in a hosted environment. This means, in effect, that no user-supplied external names may match library names."

Your problem can be easily solved by adding a unique prefix to all your identifiers, e.g. "mylang_".

As an alternative, you can avoid the problem by using the LLVM or GCC -ffreestanding flag, which will compile your code for a non-hosted environment. (The C standard specifies that the restriction only applies to a hosted environment.) In this case you can use all the names you want (apart from main, which is still your program's entry point), but you must make your own arrangements for your library. This is how operating system kernels can legally define their own versions of the C library functions.

Upvotes: 2

Saba
Saba

Reputation: 412

The reason is explained here and a relevant extract is given below. http://www.gnu.org/software/libc/manual/html_node/Reserved-Names.html I get an error in gcc as well.

The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:

  1. Other people reading your code could get very confused if you were using a function named exit to do something completely different from what the standard exit function does, for example. Preventing this situation helps to make your programs easier to understand and contributes to modularity and maintainability.
  2. It avoids the possibility of a user accidentally redefining a library function that is called by other library functions. If redefinition were allowed, those other functions would not work properly.
  3. It allows the compiler to do whatever special optimizations it pleases on calls to these functions, without the possibility that they may have been redefined by the user. Some library facilities, such as those for dealing with variadic arguments (see Variadic Functions) and non-local exits (see Non-Local Exits), actually require a considerable amount of cooperation on the part of the C compiler, and with respect to the implementation, it might be easier for the compiler to treat these as built-in parts of the language.

The page also describes other restricted names.

Upvotes: 1

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