reentrant - What is really re-entrant function? -


i have read may threads on re-entrant subjects on as on http://en.wikipedia.org/wiki/reentrancy_(computing).

i might ideas of re-entrant functions. when read example on wiki site, confused.

the first example:

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); }

as site explained: " it still fails reentrant, , continue cause problems if isr() called in same context thread executing swap()." => kinds of prolems might happen here? swapping result not correct? or value of t variable modified?

and second example, improved first one:

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); }

how improves first one? mean variable t kept unchanged inside swap() function?

re-entrant means function can interrupted @ point, , able correctly finish executing after interruption, in cases when same function called 1 or more times in interrupted state.

the crucial part here invocation of function called in interrupted state must finish before original call state restored. main difference between re-entrancy , thread safety: in order thread-safe, function must able proceed if interrupting invocation unfinished before control gets original call.

that's why second version of swap re-entrant: leaves t in unchanged state upon exiting, entering , exiting swap in middle of interrupted call not alter global state seen interrupted invocation.


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