New answers tagged architecture
The login binary is pretty straightforward (in principle). It's just a program that runs as root user (started, indirectly through getty or an X display manager, from init, the first user-space process). It performs authentication of the logging-in user, and if that is successful, changes user (using one of the setuid() family of system calls), sets ...
This site seems like a really good place to start. It has a discussion of /etc/passwd as well as PAM. I think you are really looking for a description of PAM. If you are looking to implement something for a specific use, you can edit your question to give us an idea of what you are trying to accomplish.
Because of how waitpid works. On a POSIX system, a signal (SIGCHLD) is delivered to a parent process when one of its child processes dies. At a high level, all waitpid is doing is blocking until a SIGCHLD signal is delivered for the process (or one of the processes) specified. You can't wait on arbitrary processes, because the SIGCHLD signal would never ...
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