Understanding race condition

Question

I'd like to know if my program could make a race condition or not? If so, please give me an example, because I can't see anyone.

#define STRING_SIZE 1024
char *string; 
int main(int argc,char**argv){
 int length; 
 if(argc != 2) return ;
 length = strlen(string);
 strncpy(string+length,argv[1],STRING_SIZE,STRING_SIZE-length);
return 0;

}

what if i make a lock ? does this correct the problem ?

#define STRING_SIZE 1024
int lock;
char *string; 
int main(int argc,char**argv){
 int length; 
 while(lock != 0){}
 lock = 1;
 if(argc != 2) return ;
 length = strlen(string);
 strncpy(string+length,argv[1],STRING_SIZE,STRING_SIZE-length);
 lock = 0;
 return 0;
}

for race condition you must have 2 thread or process access 1 object at same time and at least one of them wants to change that object. can you specify what is your process or threads and what is your object ? — Parham Alvani, Apr 20 '15 at 18:55
well the variable *string is in shared memory so i guess that if i launch the program twice it would make a race condition no ? — spleef troner, Apr 20 '15 at 18:58
giving `int` variable name `lock`, does not make it a real lock — Dabo, Apr 20 '15 at 18:58
In order to have shared memory between two process you must use functions like `shm_open()` in linux. — Parham Alvani, Apr 20 '15 at 19:00
No you never have race conditions in programs like that (programs without threads and IPC (Inter Process Communication) use) — Parham Alvani, Apr 20 '15 at 19:01
@spleeftroner No, there will never be. See [this](https://stackoverflow.com/questions/34510/what-is-a-race-condition?rq=1) for clarification. — edmz, Apr 20 '15 at 19:02

Prabhu · Accepted Answer · 2015-04-20T19:08:51.183

Race conditions occur if there are more more than one thread of execution in a process and they all tend to(have access to) operate (read/change) a shared variable.

In your case there is only thread in execution - main though the char *string is declared global.Hence no race condition.

To see race conditions in actions, create threads using pthread. In the thread function access/change the shared variables randomly ( the `char* string in your example). Print the values during the execution. You will see the impact.

Note: In your example, there isn't memory allocated for char *string. Doing strlen on that is incorrect.

i see and what if that program i run it with two threads ? – spleef troner Apr 20 '15 at 19:07 — spleef troner, Apr 20 '15 at 19:07

Understanding race condition

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