Problems splitting and searching array segments with pthreads - c

Question

I'm working on a personal project to fill an array with random numbers, split into a number of user defined segments using pthread (POSIX), search for a target in each segment, and return the number of times the target was found. I'm having bugs and issues. For more than one thread, issues like the target not being held in the struct member and a thread not being created and other things happen. I'm sure my logic is off and my code and it's output reflect this, but I'm stumped. How would you split an array into threads? What logic am I messing up?
HEADER FILE...
#ifndef COUNT_ARRAY_H
#define COUNT_ARRAY_H
// structure declarations
typedef struct
{
int threadNum;
int *array;
int first;
int last;
int target;
int numFound;
} ThreadInfo;
// function prototypes
void* ThreadFunc(void *vptr);
#endif // COUNT_ARRAY_H
MAIN FILE....
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include "count_array.h"
int main(void)
{
int numSegs;
int numSegElems;
int maxRand;
int target;
int totalElems;
int totalFound = 0;
ThreadInfo *infoPtr;
pthread_t *threadHandles;
int index = 0;
int first;
int last;
int threadNum = 0;
//get primary info from user...
printf(" Please enter the total number of elements? ");
scanf("%d", &totalElems);
printf(" Please enter the maximum random value: ");
scanf("%d", &maxRand);
printf(" Please enter the number of segments (1 to 15857): ");
scanf("%d", &numSegs);
if(numSegs > 15857)
{
puts(" Too many segments for machine!");
exit(EXIT_FAILURE);
}
numSegElems = totalElems/numSegs;
// configure the array to work with
// declare array here...
int myArray[totalElems];
//and fill array here
for(; index < totalElems; index++)
{
// % rand() and maxRand to get good range and
//not go beyond users max number
myArray[index] = (rand() % maxRand);
//test printf...ignore if still here
printf(" %d \n", myArray[index]);
}
// get the target value to look for
printf(" Please enter the target value: ");
scanf("%d",&target);
// display initial information
printf("*** Begin search: target = %d, # elements = %d, # segments = %d, "
"# segment elements = %d\n"
, target
, totalElems
, numSegs
, numSegElems);
// initialize the array first/last indexes into the integer array
if(numSegs == 1)
{
first = totalElems;
last = 0;
}
else
{
first = totalElems - numSegElems;
last = (first - numSegElems);
}
// allocate an array to store the thread handles
int size; //size of segment
if(numSegs > 1)
{
size = numSegElems;
}
else
{
size = totalElems;
}
//test printf...please ignore if still here
//printf(" size %d \n", size);
int segA[size];//not sure if I need this
// loop and create threads (# of segments)
index = 0;
for(; threadNum < numSegs; index++)
{
// allocate a thread info structure from the heap
threadHandles = calloc(totalElems, sizeof(pthread_t));
infoPtr = calloc(totalElems, sizeof(ThreadInfo));
// store the information in the allocated structure
infoPtr[index].threadNum = threadNum;
infoPtr->target = target;
infoPtr->first = first;
infoPtr->last = last;
infoPtr->array = myArray;
// create the secondary thread, passing the thread info
pthread_create(&threadHandles[index], NULL, ThreadFunc, &infoPtr[index]);
// update the first/last array indexes for the next thread
first = last;
last = first-numSegs;
++threadNum;
}
// loop and join the threads to fetch each thread's results
for(index = 0; index < numSegs; index++)
{
// join with the next thread
pthread_join(threadHandles[index], NULL);
// get the total number of matches from the thread's infoPtr
// and display a message
printf(" *** pthread_join returned: threadNum = %d, numFound = %d\n",
infoPtr[index].threadNum, infoPtr->numFound);
}
// release the infoPtr structure back to the heap
free(infoPtr);
// display the final results
// release heap memory
free(threadHandles);
return 0;
} // end of "main"
void* ThreadFunc(void *vptr)
{
//declare and set vars
ThreadInfo *ptr = vptr;
ptr->numFound = 0;
int index = ptr->first-1;
//test printf...ignore if still here
printf(" Targ %d \n", ptr->target);
//start search
for(; index >= ptr->last; --index)
{
printf(" %d \n", ptr->array[index]);
//if target found
if(ptr->target == ptr->array[index])
{
puts(" Target found! ");
//increment numFound
++ptr->numFound;
}
}
//drop out and display message
}

Answer 1

You've got multiple errors in your allocation of threadHandles and infoPtr. First, you don't really want to be allocating totalElems of them -- you only need numSegs. Second, and more crucially, you're reallocating them and changing the values of the pointers infoPtr and threadHandles every time through the thread invocation loop. Third, you've mixed treating infoPtr as an array of ThreadInfo structures here:
infoPtr[index].threadNum = threadNum;
with treating it as a pointer to a changing ThreadInfo structure here:
infoPtr->target = target;
infoPtr->first = first;
infoPtr->last = last;
infoPtr->array = myArray;
so every time through the loop, you're setting these parameters on the first thread only.
To fix this, edit and move the allocations before the loop and treat infoPtr consistently as an array:
threadHandles = calloc(numSegs, sizeof(pthread_t));
infoPtr = calloc(numSegs, sizeof(ThreadInfo));
for(; threadNum < numSegs; index++)
{
infoPtr[index].threadNum = threadNum;
infoPtr[index].target = target;
infoPtr[index].first = first;
infoPtr[index].last = last;
infoPtr[index].array = myArray;
and also fix up the second use of infoPtr in this printf a little further down:
printf(" *** pthread_join returned: threadNum = %d, numFound = %d\n",
infoPtr[index].threadNum, infoPtr[index].numFound);
and things will work a little better.
There are still more bugs lingering in your setting of first and last. I suggest you print out their values and make sure they are coming out the way you intend. I was able to get them to become negative (and start searching random memory) pretty easily.