Question

In gcc doc one reason is given for using section. This reason is to map to special hardware. But this seems to be not my case.

So I have given a task to modify a shared library that we use on our project. It is a Linux library. There is variable declarations in the library that puzzeles me. They look like this (roughly):

static int my_var_1 __attribute__((section("STACK"))) = 0;

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Update 1:
There are a dozen of variables defined in this way (__attribute__((section("STACK"))))

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Update 2:
my_var_1 is not a constant. my_var_1 might be changed in code during initialization:

my_var_1 = atoi(getenv("MY_VAR_1") ? getenv("MY_VAR_1") : "0");

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later in the library it is used like this:

inline void do_something() __attribute__((always_inline));
inline void do_something()
{
  if (my_var_1)
    do_something_else();
}

What might be the point in using __attribute__((section("STACK")))? I understand that section tells the compiler to put a variable in the particular section. However what might be the point in putting static int exactly in the “STACK” section?

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Update 3
These lines are excerpt from the output from readelf -t my_lib.so

  [23] .got.plt
       PROGBITS         00000000002103f0  00000000000103f0  0
       00000000000003a8 0000000000000008  0                 8
       [0000000000000003]: WRITE, ALLOC
  [24] .data
       PROGBITS         00000000002107a0  00000000000107a0  0
       00000000000000b0 0000000000000000  0                 16
       [0000000000000003]: WRITE, ALLOC
  [25] STACK
       PROGBITS         0000000000210860  0000000000010860  0
       00000000000860e0 0000000000000000  0                 32
       [0000000000000003]: WRITE, ALLOC
  [26] .bss
       NOBITS           0000000000296940  0000000000096940  0
       0000000000000580 0000000000000000  0                 32
       [0000000000000003]: WRITE, ALLOC

---

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Update 4
Managed to get information from the author of the shared library.
__attribute__((section("STACK"))) was added since he had not managed to build the library on Solaris. Then he found this workaround. Before the workaround the definition of the my_var_1 was like:

int my_var_1 = 0;

and everything was OK. Then he changed it since my_var_1 was in fact needed only in this translation unit:

static int my_var_1 = 0;

And after that change he did not manage to build the library on Solaris. So he added __attribute__((section("STACK"))) and it helped somehow.

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Answer 1

First the STACK section won’t be the stack of any running task.

Putting variables, functions in a specific Section allow to select a memory area for them (thanks to the linker script). On some (mostly embedded) architecture, you want put often accessed data in the faster memory.

Other solution, some development post-link script will set all the STACK section to 1: a development software will always do do_something_else(). And the released software may keep the default value of 0.

An other possibility, if there are other variables in the STACK section, the developer wants to keep them close in the memory. All Variable in the STACK section will be near each other. Maybe a cache optimization ?