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README.md
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README.md

Exit Handlers

In system programming, an exit handler is a mechanism used to register functions that are automatically executed when a program terminates or exits. These exit handlers are part of the process control mechanisms provided by the operating system. They offer a way for developers to perform cleanup tasks, release resources, or handle finalization tasks before a program terminates, regardless of whether the termination is normal or due to an error.

How a C Porgram Starts & Terminates

Program

How a C Porgram Terminates

Normal termination

  • The main function's return statement.
  • Any function calling exit() library call.
  • Any function calling _exit() system call.

Abnormal Termination

  • Calling abort() function.
  • Terminated by a signal.

atexit()

The C library function int *atexit(void (func)(void)) causes the specified function func to be called when the program terminates. You can register your termination function anywhere you like, but it will be called at the time of the program termination.

int atexit(void (*func)(void))

This function returns a zero value if the function is registered successfully, otherwise a non-zero value is returned if it is failed.

man atexit

Limitations

  • An exit handler doesn't know what exit status was passed to exit(); which may be useful. e.g., we may like to perform different actions depending on whether the process is exiting successfully or unsuccessfully.

  • We can't specify an argument to exit handler when called; which may be useful to define an exit handler that perform different actions depending on its argument.

Library Call on_exit()

  • The on_exit() is also used to register exit handlers like atexit(), but is a more powerful than atexit()

  • It accepts two arguments, a function pointer and a void pointer

  • The func is a function pointer that is passed two arguments (an integer and a void)*

  • The first argument to func is the integer value passed to exit(), and the second argument is the pointer to on_exit().

man on_exit

Process Resource limits

  • Every process has as set of resource limits that can be used to restrict the amounts of various systems resources that the process may consume.

  • We can set the resource limits of the shell (terminal) using the ulimit built-in command. These limits are inherited by the processes that the shell creates to execute user commands.

  • Since kernel 2.6.24, the Linux-specific /proc/PID/limits file can be used to view all of the resource limits of any process.

man ulimit
man getrlimit
man getslimit

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