Segmentation Fault when using structs in pthreads - c

Question

I've been trying to split writing to a file with threads and to do so I'm trying to use structs to hold the start and end positions of the file. The code compiles, However, I've been getting a segmentation fault when the code tries to create multiple threads and doesn't execute the thread code. Am I using structs correctly?
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#define MAX_THREADS 100
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
struct position {
int start;
int end;
};
void *ThreadJob(void *id) //what the thread should do
{
pthread_mutex_lock(&mutex);
struct position *b;
printf("\nstart: %d \nend: %d\n", (*b).start, (*b).end);
double* arrayPtr = malloc( 100000* sizeof(double));
FILE *file;
FILE* nFile; // New file
double n;
nFile = fopen("newTriData1.txt","a");
char line[128]; //the lines of the txt file
file = fopen("TriData1.txt", "r");
long tid;
tid = (long)id;
int count = 0;
while (fgets(line, 128, file)) //gets the lines from the txt file - line by line
{
sscanf(line ," %lf", &arrayPtr[count]); //converts the value on the line into a double to manipulate
count++; //increment the count
}
free(arrayPtr);
while((*b).start<(*b).end){
double x = (sqrt(8*arrayPtr[(*b).start]+1) - 1) / 2; //equation to detect triangular numbers
if (x == floor(x)) //checks if the value has a remainder. The value should be a whole number
{
fprintf(nFile, "\nNumber %s: Triangular\n", line); //if true writes the value and triangular
}
else
{
fprintf(nFile, "\nNumber %s: Not Triangular\n", line);
}
(*b).start++;
}
(*b).start=(*b).end;
(*b).end = ((*b).end + (*b).end);
pthread_mutex_unlock(&mutex);
pthread_exit(NULL);
}
int main (void) //main
{
struct position a;
(a).start=0;
int line_count;
FILE *file;
double count_lines = 1.0;
char check;
double i = 4;
file = fopen("TriData1.txt", "r");
double divider;
check = getc(file);
while (check != EOF)
{
if (check == '\n')
{
count_lines = count_lines + 1;
}
check = getc(file); //take the next character from the file
}
printf("cl: %f", count_lines);
double vo = fmod(count_lines,4); //using fmod to find which number divides the line count into equal parts for the number of threads
if (fmod(count_lines,4) == 0) {
double value1 = count_lines/4;
double value2 = count_lines/4;
double value3 = count_lines/4;
double value4 = count_lines/4;
printf("v1: %f \n v2: %f \n v3: %f \n v4: %f", value1,value2,value3,value4);
divider =4;
line_count = count_lines/4;
(a).end=line_count;
}
else
{
while (fmod(count_lines, i) != 0) //if the value is not divisible by 4 then i will increment until a suitable divider is found
{
i++;
divider = i;
line_count = count_lines/i;
printf("divider: %f", divider);
}
(a).end=line_count;
}
fclose(file); //close file.
printf("There are %f lines in this file\n", count_lines);
printf("\nstart: %d \nend: %d\n", (a).start, (a).end);
pthread_t threads[MAX_THREADS];
int thread;
long threadNum ;
for(threadNum=0; threadNum<divider; threadNum++){
printf("Creating thread %ld\n", threadNum);
thread = pthread_create(&threads[threadNum], NULL, ThreadJob, (void *)threadNum);
if (thread){
printf("ERROR; %d\n", thread);
exit(-1);
}
}
pthread_exit(NULL);
return 0;
}

Answer 1

Almost every line of your code is wrong, but most of your mistakes will just make your program do the wrong thing, not crash entirely. Here's just the mistakes that are making your program crash:
struct position *b;
printf("\nstart: %d \nend: %d\n", (*b).start, (*b).end);
That will probably segfault because you can't dereference an uninitialized pointer.
free(arrayPtr);
while((*b).start<(*b).end){
double x = (sqrt(8*arrayPtr[(*b).start]+1) - 1) / 2; //equation to detect triangular numbers
That might segfault because you can't use memory after you free it. Also, the last line of it will probably segfault because you still never initialized b.
(*b).start++;
}
(*b).start=(*b).end;
(*b).end = ((*b).end + (*b).end);
All of those lines will probably segfault because b still isn't initialized.
Frankly, you should give up on advanced topics like threads for now and work on trying to understand the basics of C.