Skip to content

Exercise: The getpid System Call

Tools: GCC, strace

Goal

Build your own getpid() on top of the raw syscall instruction, with no libc in between. Afterwards you will be able to look up a system call number for x86-64 and write the wrapper that invokes it.

Background

In the write demo you saw the three-layer pattern:

  high-level helper   -->   syscall wrapper   -->   my_syscall()   -->   kernel

getpid(2) returns the Process ID of the calling process. It is the simplest system call there is: no arguments, no output parameters, and no way for it to fail.

Every system call has a number, and that number is architecture-specific — the same call has a different number on 32-bit x86 and on AArch64. This is why the table lives in an asm/ header rather than a portable one.

Your Task

Open main.c and complete the two TODOs. my_syscall() is already there and needs no changes.

  1. SYS_getpid — define the correct x86-64 syscall number for getpid. Find it yourself:

    grep '__NR_getpid ' /usr/include/x86_64-linux-gnu/asm/unistd_64.h

    On some distributions the header is at /usr/include/asm/unistd_64.h instead.

  2. my_getpid() — the syscall wrapper. getpid takes no arguments, so pass 0 for every argument slot, and return what my_syscall() gives back.

Build & Run

make
./getpid

Check Your Work

  • The program should print a plausible PID: a positive number, typically in the thousands or higher. A -1 means the wrapper is still returning its placeholder value; a very large number close to 2^64 means a negative kernel error code is being printed as unsigned.
  • Run it twice. The number must change between runs, and you should be able to say why.

  • Confirm the number really came from the kernel rather than from somewhere else:

    strace -e trace=getpid ./getpid

    There should be exactly one getpid() line, and its return value must equal the number your program printed. If strace shows no getpid() at all, your wrapper is not reaching the kernel.

  • Show the strace output to the teaching assistant and explain which register carried the syscall number and which one carried the result.