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Passing Arguments to Threads

The pthread_create() routine permits the programmer to pass one argument to the thread start routine. For cases where multiple arguments must be passed, this limitation is easily overcome by creating a structure which contains all of the arguments, and then passing a pointer to that structure in the pthread_create() routine.

All arguments must be passed by reference and cast to (void *).

Question: How can you safely pass data to newly created threads, given their non-deterministic start-up and scheduling?

Answer (Click to view.) *Make sure that all passed data is thread safe - that it can not be changed by other threads. The three examples that follow demonstrate what not and what to do.*

Example 1 - Thread Argument Passing

This code fragment demonstrates how to pass a simple integer to each thread. The calling thread uses a unique data structure for each thread, insuring that each thread’s argument remains intact throughout the program.

long taskids[NUM_THREADS];

for(t=0; t<NUM_THREADS; t++)
{
   taskids[t] = t;
   printf("Creating thread %ld\n", t);
   rc = pthread_create(&threads[t], NULL, PrintHello, (void *) taskids[t]);
   ...
}

See the source code.

Output:

Creating thread 0
Creating thread 1
Creating thread 2
Creating thread 3
Creating thread 4
Creating thread 5
Creating thread 6
Creating thread 7
Thread 0: English: Hello World!
Thread 1: French: Bonjour, le monde!
Thread 2: Spanish: Hola al mundo
Thread 3: Klingon: Nuq neH!
Thread 4: German: Guten Tag, Welt!
Thread 5: Russian: Zdravstvytye, mir!
Thread 6: Japan: Sekai e konnichiwa!
Thread 7: Latin: Orbis, te saluto!

Example 2 - Thread Argument Passing

This example shows how to setup/pass multiple arguments via a structure. Each thread receives a unique instance of the structure.

struct thread_data{
   int  thread_id;
   int  sum;
   char *message;
};

struct thread_data thread_data_array[NUM_THREADS];

void *PrintHello(void *threadarg)
{
   struct thread_data *my_data;
   ...
   my_data = (struct thread_data *) threadarg;
   taskid = my_data->thread_id;
   sum = my_data->sum;
   hello_msg = my_data->message;
   ...
}

int main (int argc, char *argv[])
{
   ...
   thread_data_array[t].thread_id = t;
   thread_data_array[t].sum = sum;
   thread_data_array[t].message = messages[t];
   rc = pthread_create(&threads[t], NULL, PrintHello, 
        (void *) &thread_data_array[t]);
   ...
}

See the source code.

Output:

Creating thread 0
Creating thread 1
Creating thread 2
Creating thread 3
Creating thread 4
Creating thread 5
Creating thread 6
Creating thread 7
Thread 0: English: Hello World!  Sum=0
Thread 1: French: Bonjour, le monde!  Sum=1
Thread 2: Spanish: Hola al mundo  Sum=3
Thread 3: Klingon: Nuq neH!  Sum=6
Thread 4: German: Guten Tag, Welt!  Sum=10
Thread 5: Russian: Zdravstvytye, mir!  Sum=15
Thread 6: Japan: Sekai e konnichiwa!  Sum=21
Thread 7: Latin: Orbis, te saluto!  Sum=28

Example 3 - Thread Argument Passing (Incorrect)

This example performs argument passing incorrectly. It passes the address of variable t, which is shared memory space and visible to all threads. As the loop iterates, the value of this memory location changes, possibly before the created threads can access it.

int rc;
long t;

for(t=0; t<NUM_THREADS; t++) 
{
   printf("Creating thread %ld\n", t);
   rc = pthread_create(&threads[t], NULL, PrintHello, (void *) &t);
   ...
}

See the source code.

Output:

Creating thread 0
Creating thread 1
Creating thread 2
Creating thread 3
Creating thread 4
Creating thread 5
Creating thread 6
Creating thread 7
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392
Hello from thread 140737488348392