c - Function using a local static variable thread safe/reentrant -

i have function generate unique random number(basically increment previous) different threads invoking it.

is thread safe or reentrant.assume used static variable number.

i have seen in forum static variables cant used reentrant/thread safe.

does applies local/global static.

or implementation defined.

changing "ordinary" object shared between threads never thread safe, unless take special care of it. (and statically declared variable falls in category). there 2 standard ways of dealing that

• use mutex or other lock structure protect shared object inside "critical section"
• use atomic operations access object, new c standard, c11 has interfaces this

being reentrant asks if execution (even without threads) can modify part of state, examples recursion, signal handlers or jumps goto or longjmp. think of sharing variable "yourself". statically allocated variables make same problems here, if modify them different places of program.