GCC compiler error: “redefinition…previously defined”

Question

I\'m getting a lot of \" redefinition of x....x previously defined here\". Please what does this error means?

Answer 1

You need to limit each file from being included only once. You can do this in 2 ways.

1) At the top of your header files put:

#pragma once

Or 2) if your compiler doesn't support that, put at the top/end of your header files:

#ifndef _MYFILE_H_
#define _MYFILE_H_
...
#endif

Replace MYFILE with the name of your file, and replace ... with the contents of the header file.

Answer 2

You are probably including a header file twice. Make sure your header files are surrounded by #ifndef statements.

http://www.fredosaurus.com/notes-cpp/preprocessor/ifdef.html

Answer 3

The same thing just happened to me and it was because I accidentally included the .c/.cpp file (within it) instead of the header file.

That would definitely get you a lot of that error.

Answer 4

The error means that there is a symbol that has been defined in one place and an alternate definition has been made in another place.

This can occur if in cases like:

• if you define two functions with the same name
• if there is a mismatch between a function and it's prototype
• you call a non-trivial function before it has been defined, and without a prototype

In this last case there will be a mismatch between the real function and the "implicit declaration" that the compiler assumes when it doesn't have a prototype to use.

These situations can be avoided by:

• Ensuring that function prototypes are only declared once
• Ensuring that all functions have unique names within their scope (ie. within a file if they are static, or unique if they are used between object files)
• Be careful if using extern statements in source files to declare prototypes. Better to use a prototype from the appropriate header file.
• Ensure that all functions have prototypes - either within the source file in the case of static functions, or in a header file if they are to be used by other object files.
• Ensure that all header files can only be included once for each source file, by using either of the constructs suggested by Mehrdad and Brian R. Bondy