CODEWITHSUNDEEP
linuxassemblykerneli386irq
Adambynes
Adambynes

Reputation: 207

What's the meaning for this inline assembly (:"0" (THREAD_SIZE - 1)) in linux kernel 2.6.11 for i386

in do_IRQ you can find the following code!

#ifdef CONFIG_DEBUG_STACKOVERFLOW
   /* Debugging check for stack overflow: is there less than 1KB free? */
    {
       long esp;

        __asm__ __volatile__("andl %%esp,%0" :
                "=r" (esp) : "0" (THREAD_SIZE - 1));
       if (unlikely(esp < (sizeof(struct thread_info) + STACK_WARN))) {
           printk("do_IRQ: stack overflow: %ld\n",
                esp - sizeof(struct thread_info));
            dump_stack();
        }
    }
#endif

i did't understand the meaning of this asm assembly
asm _volatile_("andl %%esp,%0" : "=r" (esp) : "0" (THREAD_SIZE - 1)); THREAD_SIZE - 1 means what? I remeber the symbol in the parenthesis should be the C variable like the esp in the output part， but in the input part it looks like a integer but not a C symbol， can some noe help

Upvotes: 4

Views: 234

Answers (1)

Benny
Benny

Reputation: 4321

The "0" constraint means: use the same constraints as the 0th operands (http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#ss6.1, and 6.1.3 Matching(Digit) constraints).

Basically, this snippet takes THREAD_SIZE - 1 as an input register, and output an anded value in the same register. This register is referenced as the esp variable in the source code.

Upvotes: 3

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