How to assign a separate segment to the bss of a C file in the link file?

woody_chen

How to assign a separate segment to the bss of a C file in the link file? [Copy link]

 

I want to put the bbs of a C file in a separate segment. In this way, when the program is running, all the RAM in this segment will be cleared to restore a module to its initial state. The compiler I use is GCC. I modified the link script and added the following segment.

.keyscan_bss :
{
            . = (((. + 3) / 4)*4);
            PROVIDE(_keyscan_bss_start_ = .);
            ./proj/drivers/keyboard.o(.sbss)
            ./proj/drivers/keyboard.o(.sbss.*)
            ./proj/drivers/keyboard.o(.bss)
            ./proj/drivers/keyboard.o(.bss.*)
}
PROVIDE(_keyscan_bss_end_ = .);

But I found that only static variables are put in this segment. Global variables are still in the original BSS segment. Don't global variables belong to keyboard.o? Please give me some advice?

兰博

@辛昕[/url] @chenzhufly[/url] , both of them are experts in this field.

freebsder

It's pattern matching. Check if there is a problem with the order of keyboard.o and the * wildcard.

woody_chen

freebsder posted on 2020-3-3 22:45 Pattern matching, check if there is a problem with the order of keyboard.o and the * wildcard.

I copied the original BSS segment like this.

.bss :
{
	    . = (((. + 3) / 4)*4);
	    PROVIDE(_start_bss_ = .);
        *(.sbss)
        *(.sbss.*)
        *(.bss)
        *(.bss.*)
}
PROVIDE(_end_bss_ = .);

I will replace * with keyboard.o

freebsder

.keyscan_bss :
{
            . = (((. + 3) / 4)*4);
            PROVIDE(_keyscan_bss_start_ = .);
            ./proj/drivers/keyboard.o(.sbss)
            ./proj/drivers/keyboard.o(.sbss.*)
            ./proj/drivers/keyboard.o(.bss)
            ./proj/drivers/keyboard.o(.bss.*)
}
PROVIDE(_keyscan_bss_end_ = .);

.bss :
{
	    . = (((. + 3) / 4)*4);
	    PROVIDE(_start_bss_ = .);
        *(.sbss)
        *(.sbss.*)
        *(.bss)
        *(.bss.*)
}
PROVIDE(_end_bss_ = .);

Try it.

woody_chen

freebsder published on 2020-3-4 10:18 .keyscan_bss : { . = (((. + 3) / 4)*4); PROVIDE(_keyscan_bss_start_ = . ...

I wrote it like this. But the global variables in keyboard.o are allocated to the bss below. Only the static variables are in keyscan_bss.

freebsder

Then provide the complete script file. I don’t have the energy to guess how to write it.

hotsauce1861

You are using the gcc compiler, use the __attribute__ (section) attribute directly in the code, see the gcc manual for details,

woody_chen

hotsauce1861 posted on 2020-3-4 22:13 Your compiler is gcc, use the __attribute__ (section) attribute directly in the code, please refer to the gcc manual for details,

This is a little more troublesome. You need to add this attribute before each variable.

成风

1. Reason: By default, for GCC, static variables without static modifiers belong to the common section, not the BSS section. GCC does this to make some optimizations (for specific optimizations, see the description of -fno-common in the GCC documentation). On the other hand, most link files will put the common section in the BSS section.

2. A solution: add -fno-common to the compilation option

woody_chen

Chengfeng published on 2020-4-10 17:49 1. Reason: By default, for GCC, static variables without static modifiers belong to the common section, not the BSS section. GCC...

Great! Thank you for your explanation. I will try it.

1. Static variables without static modifier belong to the common section

2. Most linker files will put the common section in the BSS section

Isn't this just like taking off your pants to fart, a waste of time?

成风

woody_chen posted on 2020-4-13 10:42 Great! Thank you for your explanation. I will try it. 1. Static variables without static modifier belong to common section 2. Most links...

1: Global variables are just a type of static variables, but for the sake of convenience, "global variables" are used instead of "static variables" below.

Second: Sorry, the description of "static variables without static modifier belong to common section" is wrong. It should be "global variables without static modifier and uninitialized (or initialized to 0) belong to common section".

1. int a;

2. int b = 0;

The following do not belong to the common section

1. static int c;

2. int d = 1;

3. static int e = 0;

4. static int f = 2;

3: Placing global variables in the common section is for optimization, and placing the common section in the bss section is to clear the common section.

1. Assuming that those global variables are not placed in the common section but directly in the bss section, then they are likely to be scattered in the bss section and cannot be optimized.

2. Assuming that the common section is not placed in the bss section, do we need to first write a code to clear the bss section, and then write another code to clear the common section? Wouldn't it be better to put the common section in the bss section and clear them together?

成风

If -fno-common doesn't work, try using the KEEP keyword in the link file.

woody_chen

Chengfeng posted on 2020-4-13 21:45 woody_chen posted on 2020-4-13 10:42 Great God! Thank you for your explanation. I will try it. 1. Static variables without static modifier belong to co ...

Pro-tested and available.

After adding -fno-common, duplicate definition of variables will cause errors. Without this option, no errors will be reported.

终南愚公

It may have been optimized, and the KEEP command should be added.