Reentrancy (computing) - Further Examples

Further Examples

In the following piece of C code, neither f nor g functions are reentrant.

int g_var = 1; int f { g_var = g_var + 2; return g_var; } int g { return f + 2; }

In the above, f depends on a non-constant global variable g_var; thus, if two threads execute it and access g_var concurrently, then the result varies depending on the timing of the execution. Hence, f is not reentrant. Neither is g; it calls f, which is not reentrant.

These slightly altered versions are reentrant:

int f(int i) { return i + 2; } int g(int i) { return f(i) + 2; }

In the following piece of C code, the function is thread-safe, but not reentrant.

int function { mutex_lock; ... function body ... mutex_unlock; }

In the above, function can be called by different threads without any problem. But if the function is used in a reentrant interrupt handler and a second interrupt arises inside the function, the second routine will hang forever. As interrupt servicing can disable other interrupts, the whole system could suffer.