Symbols at compile time

Question:

When compiling with only -c and viewing symbols with $nm ficheiro.o , what does symbol C mean and which section is assigned to it?

I have read that it means that the symbol is common and that it is a non-starting date, but in which cases is this symbol assigned and what is the difference to the B/b symbol?

Answer:

Let's see…

Briefly, simplistic and considering Linux , C , GCC , Assembly and compilation phases:

• Uninitialized global and static data initialized to 0 is placed in the .BSS section;
• Initialized data with non-zero values are allocated to section .DATA or .RODATA in the case of constants;
• When compiling to object (using -c in GCC), uninitialized data goes to the COMMON section – which exists in GCC for backwards compatibility . When linked ( linking ) to form the executable, they are dropped into the .BSS section padded with zeros as needed;

Probably other compilers and OSes behave similarly but with some behavior differences. For example, completely zero-filling uninitialized data or even doing so at runtime as needed.

Responding

Thus, given the basis above, we have the following:

• The b symbol is assigned to uninitialized local data, zero-filled by the developer or program initiator. They are placed in .BSS ;
• Symbol B is assigned to uninitialized global data, zero-filled by the developer or program initiator. They are placed in .BSS ;
• Symbol C is assigned to uninitialized data when skipping the 'linking' phase. They are placed in COMMON as they can contain other similar references;