Multiple threads to find prime factors of integers, segmentation fault - c

Question

I can't figure out what I am doing wrong with my pointers. It is causing a segmentation fault. I am convinced the problem is rooted in my use of the array of pointers I have and the pthread_join I am using.
The goal is to read multiple integers into a gcc compiler, then print out the integer with all its factors, like this, 12: 2 2 3
I created a struct containing an int array to store the factors of each integer as the factor function pulls it apart and a counter(numfact) to store how many factors there are stored in the array.
I commented out the section at the bottom that prints out the factors.
I think the problem is how I am trying to store the output from the pthread_join in the pointer array, ptr[]. Whenever I comment it out, it does not get the segmentation error.
Either I have my pointers screwed up in a way I don't understand or I can't use an array of pointers. Either way, after many hours, I am stuck.
Please help.
#include <stdio.h>
#include <pthread.h>
#include <math.h>
#include <stdlib.h>
struct intfact
{
long int factors[100];
int numfact;
};
struct intfact *factor(long int y)
{
struct intfact threadfact;
threadfact.numfact = 0;
// Store in struct the number of 2s that divide y
while (y % 2 == 0)
{
threadfact.factors[threadfact.numfact] = 2;
threadfact.numfact++;
y = y/2;
}
// Store in struct the odds that divide y
for (int i = 3; i <= floor(sqrt(y)); i = i+2)
{
while (y % i == 0)
{
threadfact.factors[threadfact.numfact] = i;
threadfact.numfact++;
y = y/i;
}
}
// Store in struct the primes > 2
if (y > 2)
{
threadfact.factors[threadfact.numfact] = y;
threadfact.numfact++;
}
struct intfact *rtnthred = &threadfact;
return rtnthred;
}
/* Trial Division Function */
void *divde(void *n)
{
long int *num = (long int *) n;
struct intfact *temp = factor(*num);
return temp;
}
/* Main Function */
int main(int argc, char *argv[])
{
pthread_t threads[argc-1];
void *ptr[argc-1];
/* loop to create all threads */
for(int i=0; i < argc; i++)
{
long temp = atol(argv[i+1]);
pthread_create(&threads[i], NULL, divde, (void *) temp);
}
/* loop to join all threads */
for(int i=0; i < argc; i++)
{
pthread_join(threads[i],(void *) ptr[i]); //THIS POINTER IS THE PROBLEM
}
/* loops to print results of each thread using pointer array*/
//for(int i = 0; i < argc; i++)
//{
// printf("%s: ", argv[i+1]); /* print out initial integer */
// struct intfact *temp = (struct intfact *) ptr[i]; //cast void pointer ptr as struct intfact pointer
// printf("%d", temp->numfact);
//for(int j = 0; j < temp->numfact; j++) /*(pull the numfact(count of factors) from the struct intfact pointer??)*/
//{
// printf("%d ", temp->factors[j]); /* print out each factor from thread struct */
//}
}
}
In my Linux) terminal this code is stored in p3.c
"./p3 12" should yeild "12: 2 2 3"

Answer 1

For starters:
Here
long temp = atol(argv[i+1]);
pthread_create(&threads[i], NULL, divde, (void *) temp);
you define a long int and pass it as argument to the thread. For example 12
Inside the thread function then
void *divde(void *n)
{
long int *num = (long int *) n;
you treat the long int passed in as pointer to long int.
And then here dereference it
... = factor(*num);
So this *num for example would become *12. That is referencing memory address 12 to read out its content and pass it to factor). Aside the fact that this mostly likely is an invalid address, there would be nothing relevant store, at least nothing your code defined.
To (more or less fix) this do
void *divde(void *n)
{
long int num = (long int) n;
... = factor(num);
The second issues is mentioned in the comment: Multiple threads to find prime factors of integers, segmentation fault
The problem you are trying to solve is a special case of parallel programming, namely that the tasks to be run in parallel are completely independent. In such cases it makes sense to give each task its own context. Here such a context would include the
thread-id,
the thread specific input
as well as its specific output.
In C grouping variables can be done using structures, as your implementation already comes up with for the output of the tasks:
struct intfact
{
long int factors[100];
int numfact;
};
So what is missing is thread-id and input. Just add those for example like this.
/* group input and output: */
struct inout
{
long int input;
struct intfact output;
};
/* group input/output with thread-id */
struct context
{
pthread_t thread_id;
struct inout io;
};
Now before kicking off the threads define as many contexts as needed:
int main(int argc, char *argv[])
{
size_t num_to_process = argv - 1;
struct context ctx[num_to_process];
then create the threads passing in what is needed, that is input along with space/memory for the output:
for (size_t i = 0; i < num_to_process ; i++)
{
ctx[i].io.input = atol(argv[i]);
pthread_create(&ctx[i].thread_id, NULL, divide, &ctx[i].io);
}
Inside the thread function convert the void-pointer received back to its real type:
void *divide(void * pv)
{
struct inout * pio = pv; /* No cast needed in C. */
Define the processing function to take a pointer to the context specific input/output variables:
void factor(struct inout * pio) /* No need to return any thing */
{
/* Initialise the output: */
pio->output.numfact = 0;
/* set local copy of input: */
long int y = pio->input; /* One could also just use pio->input directly. */
Replace all other occurrences of threadfact by pio->output.
Use
return;
}
to leave the processing function.
Then inside the thread function call the processing function:
factor(pio);
Use
return NULL;
}
to leave the thread function.
In main() join without expecting any result from the threads:
/* loop to join all threads */
for (size_t i = 0; i < num_to_process; i++)
{
pthread_join(ctx[i].thread_id, NULL);
}
Putting this all together:
#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>
#include <math.h>
struct intfact
{
long int factors[100];
size_t numfact;
};
/* group input and output: */
struct inout
{
long int input;
struct intfact output;
};
/* group input/output with thread-id */
struct context
{
pthread_t thread_id;
struct inout io;
};
void factor(struct inout * pio)
{
/* Initialise the output: */
pio->output.numfact = 0;
/* set local copy of input: */
long int y = pio->input; /* One could also just use pinout->input directly. */
if (0 == y)
{
return; /* Nothing to do! */
}
// Store in struct the number of 2s that divide y
while (y % 2 == 0)
{
pio->output.factors[pio->output.numfact] = 2;
pio->output.numfact++;
y = y/2;
}
// Store in struct the odds that divide y
for (int i = 3; i <= floor(sqrt(y)); i = i+2)
{
while (y % i == 0)
{
pio->output.factors[pio->output.numfact] = i;
pio->output.numfact++;
y = y/i;
}
}
// Store in struct the primes > 2
if (y > 2)
{
pio->output.factors[pio->output.numfact] = y;
pio->output.numfact++;
}
return;
}
void *divide(void * pv)
{
struct inout * pio = pv; /* No cast needed in C. */
factor(pio);
return NULL;
}
int main(int argc, char *argv[])
{
size_t num_to_process = argc - 1;
struct context ctx[num_to_process];
for (size_t i = 0; i < num_to_process; i++)
{
ctx[i].io.input = atol(argv[i+1]);
if (!ctx[i].io.input)
{
fprintf(stderr, "COnversion to integer failed or 0 for '%s'\n", argv[i]);
}
pthread_create(&ctx[i].thread_id, NULL, divide, &ctx[i].io);
}
/* loop to join all threads */
for (size_t i=0; i < num_to_process; i++)
{
pthread_join(ctx[i].thread_id, NULL);
}
/* loops to print results of each thread using pointer array*/
for(size_t i = 0; i < num_to_process; i++)
{
printf("%ld: ", ctx[i].io.input); /* print out initial integer */
printf("%zu factors --> ", ctx[i].io.output.numfact);
for(size_t j = 0; j < ctx[i].io.output.numfact; j++)
{
printf("%ld ", ctx[i].io.output.factors[j]); /* print out each factor from thread struct */
}
putc('\n', stdout);
}
}