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BSS Vs COMMON

Posted by michaeln32 on January 2, 2018

Hi In the map file I get BSS section and COMMON section.

Can anybody please tell me what is the difference between those two ?

Thank you

Michael


BSS Vs COMMON

Posted by richard_damon on January 2, 2018

This is really a tools question, and not a FreeRTOS question (so telling us what tool you are using could be helpful), but in general, the BSS section is used for variables which are not initialized, so the whole area can just be zeroed, while many tools, for historical reasons, call the region that is initialized as 'COMMON', so that area will have initialization information.


BSS Vs COMMON

Posted by davidbrown on January 2, 2018

On 02/01/18 11:43, Michael Nesher wrote: > Hi > In the map file I get BSS section and COMMON section. > > Can anybody please tell me what is the difference between those two ? >

As Richard says, this is a tools question. But I can give some information here, which might be of interest to people here. It has nothing to do with FreeRTOS, but we all use C !

It comes from the way objects (variables) are declared in different C files. (The explanation below is for common practice - tools can, in theory, do something different.)

The standard way to define global (file scope, non-static) variables is to have a single definition ("int x = 1;" or "int x;") in one C file, and to have as many external declarations ("extern int x;") as you want - usually by putting the declaration in a header.

When you have an initialised definition ("int x = 1;"), the variable gets allocated in the ".data" section. If you try to have more than one initialised definition in two C files, you will get a linker error. So far, so good.

When you use an uninitialised definition ("int x;"), it is called a "tentative definition" and you can have repeated "int x;" definitions in the same file, or at most one initialised definition. If there are no other initialisations, this is treated much like "int x = 0;", except that the compiler will put the allocation in the ".bss" section to make the "initialisation to 0" more efficient.

If you have uninitialised definitions ("int x;") in more than one C file, this is not allowed in standard C. However, it is something that used to be common in older C code, and was accepted by compilers. The way this was handled was the same as for Fortran (where it is allowed, AFAIK). On seeing "int x;" without an initialiser, the compiler puts "x" in a ".common" section. Each definition of "x" from different translation units is merged, and the C startup code will clear it to 0.

As long as you are consistent, this works out okay. A single "int x;" in one C file is fine. Having "int x;" in other files will work too, as long as none of them have an initialiser. The problem comes if you are inconsistent - like having a 4-byte "int x;" in one file and an 8-byte "double x;" in another. The linker will allocate both "x" at the same address in ".common", and it will be either 4 bytes or 8 bytes (there is no general way to specify which - a toolchain may pick the first size it meets, the last size, the maximum, or whatever). Clearly the code has a bug, and clearly it may go seriously wrong.

For that reason, you should disable "common" support if your toolchain supports it. For gcc, it is the "-fno-common" flag. It will give you better error checking by complaining if the same identifier is used in more than one definition in the program. If you can't disable "common" support in your toolchain, then still be careful to avoid duplicate definitions. The clue here is that every file-scope definition should be either "static", or have a matching "extern" declaration in the header - /never/ put something like "int x;" in a header.


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