在BSS 28额外的字节 [英] 28 extra bytes in bss

问题描述

struct abc
{
   char cc[32];
} mystruct;

int  main()
{
}

当我运行上面的程序，.bss段有64个字节。我期待它是36个字节。 32字节MYSTRUCT并采取其他图书馆4个字节。

When I run the above program, the .bss section has 64 bytes. I was expecting it to be 36 bytes . 32 bytes for mystruct and 4 bytes which is taken by other libraries.

如果我更改字符CC [32] 到字符CC [31] 然后我得到了36个字节。 BSS。

If I change char cc[32] to char cc[31] then I get 36 bytes in .bss.

-bash-3.00$ g++ bssSize.cc

-bash-3.00$ readelf --sections ./a.out | grep bss
  [23] .bss              NOBITS          08049580 000578 000040 00  WA  0   0 32
-bash-3.00$ 

64是十六进制000040

64 is 000040 in hex

为什么.bss中这些额外的28个字节呢？

Why are these extra 28 bytes there in .bss ?

推荐答案

的 readelf 输出的最后一列是对齐，显示的数值确实是32。所以，BSS的尺寸被向上舍入到32字节的最接近倍数

The last column of the readelf output is alignment, and the value shown is indeed "32". So, the BSS size is being rounded up to the nearest multiple of 32 bytes.

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