Reentrancy (computing) - Example

Example

This is an example of a swap function that fails to be reentrant (as well as failing to be thread-safe). As such, it should not have been used in the interrupt service routine isr:

int t; void swap(int *x, int *y) { t = *x; *x = *y; // hardware interrupt might invoke isr here! *y = t; } void isr { int x = 1, y = 2; swap(&x, &y); }

swap could be made thread-safe by making t thread-local. It still fails to be reentrant, and this will continue to cause problems if isr is called in the same context as a thread already executing swap.

The following (somewhat contrived) modification of the swap function, which is careful to leave the global data in a consistent state at the time it exits, is perfectly reentrant; however, it is not thread-safe since it does not ensure the global data is in a consistent state during execution:

int t; void swap(int *x, int *y) { int s; s = t; // save global variable t = *x; *x = *y; // hardware interrupt might invoke isr here! *y = t; t = s; // restore global variable } void isr { int x = 1, y = 2; swap(&x, &y); }