XV6 Describe the life of getpid globl getpid getpid movl 11

XV6

Describe the life of getpid()

.globl getpid;
getpid:
movl $11, %eax;

int $64;

ret

where the return value is coming from for the getpid, so I can print the ppid in the following code:

void

mem(void)

int ppid;

ppid=getpid();

printf(1,\"mem=%d\ \",ppid);

}

int

main (int argc, char *argv [])

{

printf (1, \"mem starting \ \");

mem ();

exit ();

}

Solution

getpid() returns the process id of calling process. Every process is managed by a kernel, which means the ketnel is aware of running, scheduled or invoked process. It stores process information in kernel address space in data structures (task struct). So a process calling getpid() means the kernel has to look at the process\'s task struct to get that information. The kernel is always running. For a process invoked in userspace, the kernel is running and scheduling in that process\'s context. Hence it knows the pid to answer for this call. Also a call to getpid() is always successsul, because there will always be a process that called it or else it would not be invoked!