reading and writing int from/to shared memory in c - c

Question

I'm trying to make a program where a thread writes an integer into a shared memory location and then the other thread reads and prints that integer. the problem I'm facing is that the second thread keeps reading the integer as -1.
here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>
#include <string.h>
struct args {
void* memptr;
sem_t* semptr;
};
void *p1(void *vargp)
{
void* memory = ((struct args*)vargp)->memptr;
sem_t* semaphore = ((struct args*)vargp)->semptr;
//sem_wait(semaphore);
//sleep(0.5);
for(int i=0; i<=10; i++)
{
if (!sem_wait(semaphore)) {
printf("got in if p1\n");
sprintf(memory, "%d", i);
sem_post(semaphore);
sleep(1);
}
}
if (!sem_wait(semaphore)) {
sprintf(memory, "%d", 0);
sem_post(semaphore);
sleep(1);
}
sleep(0.1);
}
void *p2(void *vargp)
{
void* memory = ((struct args*)vargp)->memptr;
sem_t* semaphore = ((struct args*)vargp)->semptr;
sleep(0.1);
while(1)
{
if (!sem_wait(semaphore)) {
printf("got in if p2\n");
if((int)memory == 0){
break;
}
printf("%d\n", (int)memory);
sem_post(semaphore);
sleep(1);
}
}
}
const int ByteSize = 4;
const char* SharedName = "memNameTest";
const char* SemaphoreName = "semNameTest";
int main()
{
int fd = shm_open(SharedName, O_RDWR, 0644);
ftruncate(fd, ByteSize);
void* memptr = mmap(0, ByteSize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
sem_t* semptr = sem_open(SemaphoreName, O_CREAT, 0644, 0);
sem_post(semptr);
struct args *Share = (struct args *)malloc(sizeof(struct args));
Share->memptr = memptr;
Share->semptr = semptr;
pthread_t thread1, thread2;
printf("Before Thread\n");
pthread_create(&thread1, NULL, p1, (void*)Share);
pthread_create(&thread2, NULL, p2, (void*)Share);
pthread_join(thread1, NULL);
pthread_join(thread2, NULL);
printf("After Thread\n");
munmap(memptr, ByteSize);
close(fd);
sem_close(semptr);
unlink(SharedName);
return 0;
exit(0);
}
I have tried changing (int)memory into *((int*)memory) but that resulted in a segmentation error.
(edit)
as suggested I tried this in a single-threaded program and got it to work as follows:
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
int main()
{
/* the size (in bytes) of shared memory object */
const int SIZE = 4;
/* name of the shared memory object */
const char* SharedName = "memoryInt";
/* create the shared memory object */
int shm_fd = shm_open(SharedName, O_CREAT | O_RDWR, 0644);
/* configure the size of the shared memory object */
ftruncate(shm_fd, SIZE);
/* memory map the shared memory object */
void* memptr = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);
for(int i=1; i<=10; i++){
/* write to the shared memory object */
//sprintf(memptr, "%d", i);
memcpy(memptr, &i, sizeof(int));
printf("%d\n", *((int*)memptr));
sleep(1);
}
return 0;
}
though this still doesn't work in a multi-threaded program as i get a segmentation fault.
this is the output:
Before Thread
got in if p1
Segmentation fault

Answer 1

First, you have to show what happen on your terminal when you compile your program.
Secondly, the function sprintf has the declaration:
sprintf(char *str, const char *format, ...);
That means the p1 will write the null terminated string of character. In your code, i dont understand why you use the void pointer memory instead of using char pointer as the description. You should verify the read/write function by using single-threaded before applying to the multi-thread.