NOTE: this FAQ answer applies to the 2.4 kernel. 2.6 with 4kB stacks works differently and this FAQ should be updated.
How does get_current() work?
static inline struct task_struct * get_current(void)
{
struct task_struct *current;
__asm__("andl %%esp,%0; ":"=r" (current) : "0" (~8191UL));
return current;
}get_current() is a routine for getting access to the task_struct of the currently executing task. It uses the often confusing inline assembly features of GCC to perform this, as follows :
__asm__(
This signifies a piece of inline assembly that the compiler must insert into its output code. The __asm__ is the same as asm, but can't be disabled by command line flags.
"andl %%esp,%0
"%%" is a macro that expands to a "%". "%0" is a macro that expands to the first input/output specification.
So in this case, it takes the stack pointer (register %esp) and ANDs it into a register that contains 0xFFFFE000, leaving the result in that register.
Basically, the task's task_struct and a task's kernel stack occupy an 8KB block that is 8KB aligned, with the task_struct at the beginning and the stack growing from the end downwards. So you can find the task_struct by clearing the bottom 13 bits of the stack pointer value.
; "
The semicolon can be used to separate assembly statements, as can the newline character escape sequence ("\n").
:"=r" (current)
This specifies an output constraint (all of which occur after the first colon, but before the second). The '=' also specifies that this is an output. The 'r' indicates that a general purpose register should be allocated such that the instruction can place the output value into it. The bit inside the brackets - 'current' - is the intended destination of the output value (normally a local variable) once the C part is returned to.
: "0" (~8191UL));
This specifies an input constraint (all of which occur after the second colon, but before the third). The '0' references another constraint (in this case, the first output constraint), saying that the same register or memory location should be used for both. The '~8191UL' inside the brackets is a constant that should be loaded into the register allocated for the output value before using the instructions inside the asm block.
See also the gcc info pages, Topic "C Extensions", subtopic "Extended Asm".
(Mostly courtesy of David Howells of Redhat).