Thread access static variable cause segmentation error [duplicate] - c

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initializing a static variable in header
(4 answers)
Closed 5 years ago.
I'm trying to compare performance of methods about adding two matrix.
one method is just to add.
the other method is to use threads.
but I have a trouble about segmentation error when a thread access a static variable!
Here is my code.
main.c
#include "matrixProcessor.h"
void main()
{
time_t s0, e0;
int i;
int status;
inputVec1 = (int*)malloc(sizeof(int)*(SIZE*SIZE));
inputVec2 = (int*)malloc(sizeof(int)*(SIZE*SIZE));
outputVec = (int*)malloc(sizeof(int)*(SIZE*SIZE));
srand(time(NULL));
initializeVector(inputVec1);
initializeVector(inputVec2);
//printf("=== INPUT VECTOR 1 ===\n");
//printVector(inputVec1);
//printf("=== INPUT VECTOR 1===\n");
//printVector(inputVec2);
//s0 = clock();
//addVec(inputVec1, inputVec2, outputVec);
//e0 = clock();
//printf("Basic Method Time : %f (s)\n",(double)(e0-s0)/CLOCKS_PER_SEC);
s0 = clock();
for(i = 0; i < NUM_THREAD; i++)
{
printf("%d-Thread Call\n",i);
pthread_create(&tid[i], NULL, &addProc, (void*)&i);
sleep(1);
}
e0 = clock();
printf("=== OUTPUT VECTOR===\n");
printVector(outputVec);
printf("Thread Method Time : %f (s)\n",(double)(e0-s0)/CLOCKS_PER_SEC);
free(inputVec1);
free(inputVec2);
free(outputVec);
}
matrixProcessor.c
#include "matrixProcessor.h"
void initializeVector(int* matPtr)
{
int i = 0;
for(i = 0; i < SIZE*SIZE ; i++)
{
matPtr[i] = rand()%100;
}
}
void addVec(int* inputVec1, int* inputVec2, int* outputVec)
{
int i = 0;
for(i = 0; i < SIZE * SIZE; i++)
{
outputVec[i] = inputVec1[i] + inputVec2[i];
}
}
void* addProc(void *p)
{
int* idx = (int*)p;
int count = (SIZE * SIZE) / NUM_THREAD;
int i;
printf("idx value : %d\n",*idx);
printf("Test : %d ", inputVec1[0]);
for(i = (*idx) * count ; i < (*idx)*count + count; i++)
{
outputVec[i] = inputVec1[i] + inputVec2[i];
}
return NULL;
}
void printVector(int* vec)
{
int i = 0;
int j = 0;
for(i = 0; i < SIZE ; i++)
{
for(j = 0; j < SIZE; j++)
{
printf("%d\t", vec[SIZE * i + j]);
}
printf("\n");
}
}
matrixProcessor.h
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <pthread.h>
#define SIZE 10
#define NUM_THREAD 10
pthread_t tid[NUM_THREAD];
static int* inputVec1;
static int* inputVec2;
static int* outputVec;
void initializeVector(int* matPtr);
void printVector(int* vec);
void addVec(int* inputVec1, int* inputVec2, int* outputVec);
void* addProc(void *p);
when compiling, i use -static - lpthread options.
I'm sure that accessing static variable cause segmentation error,
because this program prints some messages before reaching a code line which access static variable.
Here is result.
0-Thread Call
idx value : 0
Segmentation Error! ./main
Please someone help me..!

Answer 1

There are multiple problems in your code. The two that jump right out to me are listed below.
First, this one:
pthread_create(&tid[i], NULL, &addProc, (void*)&i);
What's the value of i when the child thread accesses it? It can be anything, because it changes as the main thread spawns more threads and continues to run.
Second, what do you think these free() statements do when they're executed while the child threads are still running:
free(inputVec1);
free(inputVec2);
free(outputVec);
When your code makes those calls, the child threads are still running since you don't call pthread_join() to make sure they've all completed.
You likely get the segmentation violation because your threads are accessing free()'d memory.