Why do I need to use the sigemptyset() function before the sigaddset()?

Question

I was told that every time I want to use sigset_t block_mask , I also need to use the sigemptyset() function before I use sigaddset() . For example:

sigset_t block_mask;
sigemptyset(&block_mask);
sigaddset(&block_mask, SIGSEGV);

Why do I need to do it every time?

Answer 1

sigaddset changes a single signal in the set, but leaves the information about all other signals unaffected. Without sigemptyset , that other information would not have been initialized.

Think of a signal set as a bitmask, something like this:

sigset_t block_mask;                int block_mask;
sigemptyset(&block_mask);           block_mask = 0;
sigaddset(&block_mask, SIGSEGV);    block_mask |= 1 << SIGSEGV;