👀 Views: 69 💬 Answers: 1 📅 Created: 2025-06-03

linux pthreads shared-memory C

I'm trying to debug I'm learning this framework and I'm relatively new to this, so bear with me... I've searched everywhere and can't find a clear answer. I'm experiencing a segmentation fault when using pthreads alongside shared memory in my application running on Ubuntu 22.04. I have a producer-consumer model where multiple threads are supposed to read from a shared memory segment and process the data concurrently. Here’s a simplified version of my code: ```c #include <stdio.h> #include <stdlib.h> #include <pthread.h> #include <sys/ipc.h> #include <sys/shm.h> #include <string.h> #define SHM_SIZE 1024 void *reader(void *arg) { char *shared_memory = (char *)arg; printf("Reader thread is running. Reading data: %s\n", shared_memory); return NULL; } int main() { int shm_id; char *shm_ptr; pthread_t thread; shm_id = shmget(IPC_PRIVATE, SHM_SIZE, IPC_CREAT | 0666); if (shm_id < 0) { perror("shmget failed"); exit(1); } shm_ptr = shmat(shm_id, NULL, 0); if (shm_ptr == (char *)(-1)) { perror("shmat failed"); exit(1); } strcpy(shm_ptr, "Hello from shared memory!"); if (pthread_create(&thread, NULL, reader, shm_ptr)) { fprintf(stderr, "behavior creating thread\n"); return 1; } pthread_join(thread, NULL); if (shmdt(shm_ptr) < 0) { perror("shmdt failed"); exit(1); } shmctl(shm_id, IPC_RMID, NULL); return 0; } ``` When I run this, I receive a segmentation fault at the line `printf("Reader thread is running. Reading data: %s\n", shared_memory);`. I have verified that the shared memory segment is created successfully and the string is copied without issues. I tried using Valgrind to debug the memory usage, and I received the following output: ``` ==12345== Invalid read of size 4 ==12345== at 0x4005F2: reader (main.c:8) ==12345== by 0x40065E: main (main.c:22) ==12345== Address 0x5a0a25c is 0 bytes after a block of size 1024 alloc'd ``` I suspect this might be related to the lifetime of the shared memory segment or how the thread accesses the memory. I've tried locking the shared memory with mutexes but it doesn’t seem to resolve the scenario. What could be going wrong here? Thanks in advance! I'm working on a service that needs to handle this. I've been using C for about a year now. Am I missing something obvious? This is my first time working with C latest. I'd be grateful for any help.