multithreading - Array Offset C++ pthreads return -

i running following code in c++.

it runs expected, however, first result null in second loop...however, every ensuing result correct.

    pthread_t pthread_t_array[thread_count];     int ireturn[thread_count];     float chunk_elements[thread_count];      (int = 0; < thread_count; i++)     {         float vector_chunk[3] = {2.0,4.0,3.0};          pthread_t x;         pthread_t_array[i] = x;          ireturn[i] = pthread_create(&x,null,worker,(void *) vector_chunk);     }      (int = 0; < thread_count; i++)     {         void * result;         ireturn[i] = pthread_join(pthread_t_array[i],&result);           // lines below: @ = 0, result = null         // however, @ every other value of i, 14 returned expected.           if (result != null){             chunk_elements[i] = *(float *) result;         }         free(result);     } 

the full code here.

i not sure why there array offset (at line testing if result null--at = 0, result null, every other i, not). seems going correctly.

correction: not offset, rather, first value null, there 1 less value.

if, seems likely, worker() thread, code not shown, attempts read vector_chunk parameter thread receives, undefined behavior, , bug.

this because vector_chunk array has automatic scope, , exists until end of for-loop iteration. pthread_create() returns, , return value gets put array, loop iteration ends, , vector_chunk array goes out of scope, , destroyed.

there no guarantee thread execute , read vector_chunk before parent thread reaches end of loop iteration, when vector_chunk goes out of scope. subsequent vector_chunk going out of scope, attempt worker() access vector_chunk parameter undefined behavior, , bug.

the solution dynamically allocate parameter thread, initialize, , start thread, passing dynamically-allocated parameter, thread parameter data. thread responsible retrieving parameter data, , deallocated parameter.