Hi all LKD (Robert Love) says that fork is implemented via clone() system call. But in linux kernel 2.6, there are 3 sys_calls sys_fork, sys_clone and sys_vfork (file: arch/i386/kernel/process.c) (line 570 onwards) http://lxr.linux.no/source/arch/i386/kernel/process.c?v=2.6.5 doesn't fork uses sys_fork directly instead of clone. Also there is comment above sys_vfork which suggest the sys_call is because of register pressure and some stuff related to call-clobbered registers. I have pasted the comment here: /* * This is trivial, and on the face of it looks like it * could equally well be done in user mode. * * Not so, for quite unobvious reasons - register pressure. * In user mode vfork() cannot have a stack frame, and if * done by calling the "clone()" system call directly, you * do not have enough call-clobbered registers to hold all * the information you need. */ Please help me understand what exactly is register pressure and what are call-clobbered registers. TIA sanjayk
