Signal and malloc

About Signal handling

A signal is a software interrupt. It empowers a programmer to handle an asynchronous event. To send a signal to a process, the kernel sets a bit in the signal field of the process table entry, corresponding to the type of signal received.

When a signal that is being caught is handled by a process, the normal sequence of instructions being executed by the process is temporarily interrupted by the signal handler. The process then continues executing, but the instructions in the signal handler are now executed. If the signal handler returns, the process continues executing the normal sequence of instructions it was executing when the signal was caught.

About malloc

On most systems, malloc and free are not reentrant, because they use a static data structure which records what memory blocks are free. As a result, no library functions that allocate or free memory are reentrant. This includes functions that allocate space to store a result.

Malloc is not safe to use in signal handler

In the signal handler you can't tell what the process was executing when the signal was caught. What if the process was in the middle of allocating additional memory on its heap using malloc, and you call malloc from the signal handler? Or, you call some function that was in the middle of the manipulation of the global data structure and you call the same function from the signal handler. In the case of malloc, havoc can result for the process, because malloc usually maintains a linked list of all its allocated area and it may have been in the middle of changing this list.

What to do if signal handler needs memory

The best way to avoid the need to allocate memory in a handler is to allocate in advance space for signal handlers to use.

The best way to avoid freeing memory in a handler is to flag or record the objects to be freed, and have the program check from time to time whether anything is waiting to be freed. But this must be done with care, because placing an object on a chain is not atomic, and if it is interrupted by another signal handler that does the same thing, you could “lose” one of the objects.

Reference

http://www.ibm.com/developerworks/library/l-reent/

https://www.gnu.org/software/libc/manual/html_node/Nonreentrancy.html