The getpid system call returns the process id of the invoking process.
How does the kernel figure out which process is invoking the system call ?
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3 Answers
The kernel does job scheduling and provides system calls.
When a process is running, the kernel schedules its runtime - especially it assigns a PID to it - such information is stored inside the kernel address space, in data structures (e.g. inside a task struct).
Thus, when a process calls the getpid() system call, the kernel just has to look in the task structure of the calling (i.e. currently running) process.
answered Aug 30, 2015 at 19:35
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1But I think the question is how the kernel knows which task structure is the current one. E.g., is it stored at a fixed location on the kernel stack, computable by masking the stack pointer? Or does the kernel get the current CPU number and looking it up in a table indexed by CPU number? Or what other mechanism is used. Commented Aug 30, 2015 at 21:53
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Actually
getpid()is probably not a system call but it's stored in the thread local information and is just looked up there.– JustSidCommented Aug 30, 2015 at 23:01 -
@JustSid just click on the link in my answer - it directs you to the definition of the system call ... Commented Aug 31, 2015 at 6:58
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@user3188445, well, the question is quite short - if you think that it implies all that you should put your comment under the question and ask the author to clarify. In any case, I included a link that points you to all the details ... Commented Aug 31, 2015 at 7:00
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1@schily: Not necessarily. Linux could theoretically store the pid in the vdso segment. However, as of kernel 4.1.6, the vdso only seems to implement clock_gettime, getcpu, gettimeofday, and time. Commented Aug 31, 2015 at 10:52
When a system call is executed, there is a privilege switch, i.e. the executed code is allowed to execute more instructions and access data forbidden to userland code.
There is however no process context switch so the kernel code is still running in the calling process context. That means the kernel does not need to search which process is calling it, it already knows it. The getpid system call code simply retrieve the process id from a pointer to an internal structure that contains its own process/thread specific information. This structure is operating system implementation dependent.
For example with Illumos (OpenSolaris), the structure is named proc: http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/sys/proc.h#131
answered Aug 30, 2015 at 22:58
In a single CPU system, there is a global variable that points to the proc structure of the running process or the current thread. The proc structure contains the process id.
In a multi CPU system, there is either a similar pointer for every CPU or the MMU context is used to set up such a global variable for the syscall.
int64_t
getpid(void)
{
rval_t r;
proc_t *p;
p = ttoproc(curthread);
r.r_val1 = p->p_pid;
In this example, curthread is the global variable that is used.
