From Forum Nokia Wiki

This article is worth for developers who are porting their c code to run on Symbian 3rd Edtion devices

Following are the major changes that have to be done :

1> Go to the following path

C:\Symbian\9.1\S60_3rd\Epoc32\tools\compilation_config

        OR

$(EPOCROOT)\tools\compilation_config

2> Open the gcce.mk file

search for the following lines

# For .c Source files
C_LANG_OPTION=-x c

Edit the above line as

# For .c Source files
C_LANG_OPTION=-x c++

The above change tells the compiler that the C files will be compiled by C++ compiler

Also Add the following line in the .MMP file

YourApp.mmp
// This tells the compiler to  supress typecasting errors
OPTION          CW -lang c++   
OPTION          ARMCC --cpp
//Informing the compiler that the source is C++ 
OPTION          GCC     -x c++

Also Visit this link for more information on C Code porting [Mixing and Compiling C Code on Symbian Phones]