Unix foundations

fork Optimization

• It is wasteful to make an actual copy of the address space of the parent
• Copy-on-write (SV) : only the pages that are modified must be copied
• vfork (BSD): The parent loans the address space and blocks until the child returns to it.
• dangerous!
• csh: exploits it.

Invoking a New Program

• Process address space
• Text: code
• Initialized data:
• Uninitialized data(bss):
• Shared memory(SV):
• Shared libraries:
• Heap: dynamic space
• User stack: space allocated by the kernel

exec System Call

• Parse & access
• Verify the permission
• Read the header and check if valid executable
• If the file has SUID or SGID bits set in its mode, change the caller’s effective UID or GID to that of the owner
• Copy the arguments to exec and the env. variables into kernel.
• Allocate swap space for the data and stack region
• Set up the new address space.
• Copy the arguments and env. variables back onto the new user stack.
• Reset all signal handlers to default actions.
• Initialize the hardware context.

Process Termination (exit())

• Turns off all signals.
• Closes all open files.
• Releases the text file and other resources such as the current directory.
• Writes to the accounting log.
• Saves resource usage statistics and exit status in the proc structure.
• Changes state to SZOMB, and puts the proc on the zombie process list.
• Makes the init process inherit any live children of the exiting process.
• Releases the address space, u area, ATM, and swap space.
• Notifies the parent by sending it a SIGCHLD signal.
• Wakes up the parent if it is asleep.
• Calls swtch() to schedule a new process to run.

Awaiting Process Termination

• wait(stat_loc);/* SV, BSD & POSIX*/
• wait3(statusp, options, rusagep); /*BSD*/
• waitpid(pid, stat_loc, options);/*POSIX*/
• waitid(idtype, id, infop, options);/*SVR4*/