Why do I need the-s flag in gcc?

The compiler puts in the executable file not only what is needed to run (that is, code and data), but also additional (and generally unnecessary) information (for example, the names of all variables and functions, including those that were declared with the static specifier).

This additional information is only needed at the build stage, but the compiler leaves it in the final executable file, perhaps for the convenience of further debugging. The flag is responsible for cutting it out -s.

Take, for example, a program consisting of two source code files:

1. main.cpp

   #include <iostream>
   #include <string>
   
   std::string partOne("Hello, "); // Глобальная переменная
   
   std::string getString(); // Прототип функции, объявленной в другом файле
   
   int main()
   {
       std::cout << getString() << std::endl;
       return 0;
   }
   

2. other.cpp

   #include <string>
   
   extern std::string partOne;
   static std::string partTwo("World!");
   
   std::string getString()
   {
       return partOne + partTwo;
   }
   

3. CMake.txt

   add_executable(example1 main.cpp other.cpp)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3 -flto") 
   

   Flag -flto I inserted it to demonstrate that even enabling inter-module optimizations does not allow the compiler to remove any names.

Let's build the project in the Release mode (-DCMAKE_BUILD_TYPE=Release) and let's study the symbol table with the utility nm:

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Doesn't it seem like this is somehow too much for a program that doesn't export anything in the spirit of a DLL?

At the same time, we will remember the program size: 52,105 bytes.

Now let's change the CMakeList.txt:

add_executable(example2 main.cpp other.cpp)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O3 -flto -s")

We added the -s flag and changed the name of the executable file. Now after building the program, nm outputs the following:

nm: example2.exe: no symbols

That is, the program does not need any of the 1,001 above-mentioned names to work!

What with volume? 16,896 bytes. That is, we were able to painlessly reduce the size of the program by three times! And this is for two simple object files. When the project becomes more complex, the share of names from the size of the executable file will be even greater.

By the way, have you noticed that most of the characters are taken up by the names of the sections? So, this is a consequence of the difference in the formats of object (ELF) and executable (PE EXE) files when building under MinGW, which does not allow you to combine these names and use them for their intended purpose. The name table also contains a large number of characters from the standard library object files.

When writing this answer, the following software versions were used:

• CMake 3.4.0,
• GCC 4.9.2 (i686-posix-dwarf-rev1, Built by MinGW-W64 project),
• GNU nm (GNU Binutils) 2.24.