Dinesh Ahuja wrote: > Thanks for your replying. Still I am not clear in a > below output, that If we are loading shared object > librt in a memory map, then why are we calling unmmap > before we make a call to the functions exposed by > litrt e.x rt_sigaction. The call to unmap() may simply be to free unused heap memory. Without reading the source code to the dynamic linker or the C runtime library, I can't say for sure, but you can be certain that no shared library is being unmapped before it is used. > Could you explain at what time a context switch takes > place from user space to a kernel space. A context switch from user-space to kernel-space happens when an interrupt happens. There are two kinds of interrupts: software interrupts (used for system calls) and hardware interrupts. A common hardware interrupt is the clock interrupt, which happens many times per second and (among other things) gives the kernel a chance to preempt a process that is hogging the CPU. > The function > call gettimeofday is being executed in a user space as > it is not a kernel sys call It is a system call. Type "man gettimeofday" and notice the "(2)" following the function name. That means it is a system call. > so at what point a switch > takes place. How is responsible for switching from > user to kernel and vice versa and I would like to see > that code. All system calls have corresponding functions in the C runtime library. When your program calls one, such as read(), the function in the C library (named "read") loads the corresponding system call number into a register (along with other arguments it received from the user program) and then executes a machine instruction that causes a software interrupt. The CPU responds to a software interrupt by switching context to the kernel. On an Intel x86 CPU, the software interrupt instruction is: INT 0x80. For for info, see: http://www.tldp.org/LDP/khg/HyperNews/get/syscall/syscall86.html Hope this helps. -- Francis Litterio franl <at> world . std . com -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
