Hello,
While studying the kernel_thread() function (2.4.8) for x86, which is written as inline assembly function, I came accross a question.
487 int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
488 {
489 long retval, d0;
490
491 __asm__ __volatile__(
492 "movl %%esp,%%esi\n\t"
493 "int $0x80\n\t" /* Linux/i386 system call */
494 "cmpl %%esp,%%esi\n\t" /* child or parent? */
495 "je 1f\n\t" /* parent - jump */
.... /* Lines 496 to 501 are not relevant to my question */
502 "movl %3,%0\n\t" /* exit */
503 "int $0x80\n"
504 "1:\t"
505 :"=&a" (retval), "=&S" (d0)
506 :"" (__NR_clone), "i" (__NR_exit),
507 "r" (arg), "r" (fn),
508 "b" (flags | CLONE_VM)
509 : "memory");
510 return retval;
511 }
In the above code listing, in line 493 I understand, that this is a programmed exception having vector 128 (0x80), which is used to trigger a system call, where the system
call is specified by the content of the eax register (from the context I guess that it should be the clone() system call, which is identified by __NR_clone).
What I don't understand is, how the correct system call number (__NR_clone(?)) gets into the eax register, before the programmed exception in line 493 - similar to lines
502, 503 I would have expected a "movl %2, %0\n\t" before line 493, to get __NR_clone into the eax register (by the way it is interesting to notice that __NR_clone is
specified as input operand %2, but that never used in the actual assembly code).
I am not very familiar with inline assembly, so I guess that that the answer to my question lies in the inline assembly syntax/features (e.g. I do not know what the ""
constraint for the output operand (__NR_clone) means).
thanks in advance for any help
with best regards
Martin
P.S. please put me on cc: in your reply, since I am not in the mailing list.
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