How to pass specific value to one thread? - c

Question

I'm working through a thread exercise in C, it's a typical thread scheduling code many schools teach, a basic one can be seen here, my code is basically the same except for my altered runner method
http://webhome.csc.uvic.ca/~wkui/Courses/CSC360/pthreadScheduling.c
What I'm doing is basically altering the runner part so my code prints an array with random numbers within a certain range, instead of just printing some words. my runner code is here:
void *runner(void *param) {
int i, j, total;
int threadarray[100];
for (i = 0; i < 100; i++)
threadarray[i] = rand() % ((199 + modifier*100) + 1 - (100 + modifier*100)) + (100 + modifier*100);
/* prints array and add to total */
for (j = 0; j < 100; j += 10) {
printf("%d,%d,%d,%d,%d,%d,%d,%d,%d,%d\n", threadarray[j], threadarray[j+1], threadarray[j+2], threadarray[j+3], threadarray[j+4], threadarray[j+5], threadarray[j+6], threadarray[j+7], threadarray[j+8], threadarray[j+9]);
total = total + threadarray[j] + threadarray[j+1] + threadarray[j+2] + threadarray[j+3] + threadarray[j+4] + threadarray[j+5] + threadarray[j+6] + threadarray[j+7] + threadarray[j+8] + threadarray[j+9];
}
printf("Thread %d finished running, total is: %d\n", pthread_self(), total);
pthread_exit(0);
}
My question lies in the first for loop where I'm assigning random numbers to my array, I want this modifier to change based on which thread it is, but I can't figure out how to do it, for example if its the first thread the range will be 100-199, 2nd will be 200-299, etc and so on. I have tried to assign i to an value before doing pthread_create and assigning that value to an int in runner to use as the modifier, but since there are 5 concurrent threads it ends up assigning this number to all 5 threads, and they end up having the same modifier.
So I'm looking for a method to approach this where it will work for all the individual threads instead of assigning it to all of them, I have tried to change the parameters to something like (void *param, int modifier) but when I do this I have no idea how to reference runner, since by default it's refrenced like pthread_create(&tid[i],&attr,runner,NULL);

Answer 1

You want to make param point to a data structure or variable who's lifetime will exist longer than the thread lifetime. And you cast the void* parameter to the actual data type it was allocated as.
Easy example:
struct thread_data
{
int thread_index;
int start;
int end;
}
struct thread_info;
{
struct thread_data data;
pthread_t thread;
}
struct thread_info threads[10];
for (int x = 0; x < 10; x++)
{
struct thread_data* pData = (struct thread_data*)malloc(sizeof(struct thread_data)); // never pass a stack variable as a thread parameter. Always allocate it from the heap.
pData->thread_index = x;
pData->start = 100 * x + 1;
pData->end = 100*(x+1) - 1;
pthread_create(&(threads[x].thread), NULL, runner, pData);
}
Then your runner:
void *runner(void *param)
{
struct thread_data* data = (struct thread_data*)param;
int modifier = data->thread_index;
int i, j, total;
int threadarray[100];
for (i = 0; i < 100; i++)
{
threadarray[i] = ...
}