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Why is C++ initial allocation so much larger than C's? Ask Question

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Why is C++ initial allocation so much larger than C's? Ask Question

When using the same code, simply changing the compiler (from a C compiler to a C++ compiler) will change how much memory is allocated. I'm not quite sure why this is and would like to understand it more. So far the best response I've gotten is "probably the I/O streams", which isn't very descriptive and makes me wonder about the "you don't pay for what you don't use" aspect of C++.

I'm using the Clang and GCC compilers, versions 7.0.1-8 and 8.3.0-6 respectively. My system is running on Debian 10 (Buster), latest. The benchmarks are done via Valgrind Massif.

#include <stdio.h>

int main() {
    printf("Hello, world!\n");
    return 0;
}

The code used does not change, but whether I compile as C or as C++, it changes the results of the Valgrind benchmark. The values remain consistent across compilers, however. The runtime allocations (peak) for the program go as follows:

  • GCC (C): 1,032 bytes (1 KB)
  • G++ (C++): 73,744 bytes, (~74 KB)
  • Clang (C): 1,032 bytes (1 KB)
  • Clang++ (C++): 73,744 bytes (~74 KB)

For compiling, I use the following commands:

clang -O3 -o c-clang ./main.c
gcc -O3 -o c-gcc ./main.c

clang++ -O3 -o cpp-clang ./main.cpp
g++ -O3 -o cpp-gcc ./main.cpp

For Valgrind, I run valgrind --tool=massif --massif-out-file=m_compiler_lang ./compiler-lang on each compiler and language, then ms_print for displaying the peaks.

Am I doing something wrong here?

ベストアンサー1

The heap usage comes from the C++ standard library. It allocates memory for internal library use on startup. If you don't link against it, there should be zero difference between the C and C++ version. With GCC and Clang, you can compile the file with:

g++ -Wl,--as-needed main.cpp

This will instruct the linker to not link against unused libraries. In your example code, the C++ library is not used, so it should not link against the C++ standard library.

You can also test this with the C file. If you compile with:

gcc main.c -lstdc++

The heap usage will reappear, even though you've built a C program.

The heap use is obviously dependant to the specific C++ library implementation you're using. In your case, that's the GNU C++ library, libstdc++他の実装では、同じ量のメモリを割り当てない、またはメモリをまったく割り当てない可能性があります（少なくとも起動時には割り当てない）。LLVM C++ライブラリ（libc++) は、少なくとも私の Linux マシンでは、起動時にヒープ割り当てを行いません。

clang++ -stdlib=libc++ main.cpp

ヒープの使用は、それに対してまったくリンクしない場合と同じです。

(コンパイルが失敗した場合は、libc++ がインストールされていない可能性があります。パッケージ名には通常、「libc++」または「libcxx」が含まれます。)

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