Accessing struct pointer as pthread arg causes seg fault? - c

Question

So here is a small portion of my code. Essentially I am starting another thread from a method, but when I pass the integer to the pthread, I can't access the struct members like I could only two lines before I called the thread. What has occurred since I passed this argument and run a new thread?
Note that without fail, my program always crashes immediately after printf("1\n");, I discovered this error when I realised FD_ZERO didn't work.
Struct definition (in global area):
typedef struct {
fd_set read_set, write_set;
unsigned int room_id;
char room_name[16];
} room;
Caller method:
void create_new_room(int cli_index, char buffer[]) {
pthread_mutex_lock(&mutex);
char room_name[16], password[16];
int capacity, r_index;
room *new_room = malloc(sizeof(room));
pthread_t tid;
FILE *room_file = NULL;
if((room_file = fopen("room-info.txt", "a")) == NULL) {
perror("Failed to open file.");
exit(-1);
}
// Split command data into separate strings ready to assign as room struct members
strtok(buffer, " ");
strcpy(room_name, strtok(NULL, " "));
capacity = atoi(strtok(NULL, " "));
strcpy(password, strtok(NULL, " "));
// Initialise room struct
// --Zero write set
FD_ZERO(&(new_room->write_set));
// --Set room name
strcpy(new_room->room_name, room_name);
// --Set room id
new_room->room_id = ++natural_id;
// --Add room to room_list[] and get its index in the array
for(r_index = 0; r_index < 10000; ++r_index) {
if(!room_list[r_index]) {
room_list[r_index] = new_room;
break;
}
}
// Write data to file
fprintf(room_file, "%s\n", room_name);
fprintf(room_file, "id=%u\n", natural_id);
fprintf(room_file, "owner=%s\n", client_list[cli_index]->name);
fprintf(room_file, "capacity=%d\n", capacity);
fclose(room_file);
printf("about to test\n");
printf("Testing room struct:\n");
printf("--room name = %s\n", room_list[r_index]->room_name);
printf("--room id = %u\n", room_list[r_index]->room_id);
printf("post-test.....................\n");
// Run new thread as active room
printf("Starting new room: %s\n", room_name);
pthread_create(&tid, NULL, (void *) &active_room, &r_index);
pthread_mutex_unlock(&mutex);
}
Start of new pthread method:
void active_room(void *index) {
char storage_buffer[BSIZE], output_buffer[BSIZE+32];
struct timeval timeout;
int r_index = *(int *) index;
while(1) {
// debugging lines.
printf("1\n");
printf("room name: %s\n", room_list[r_index]->room_name);
printf("room id: %u\n", room_list[r_index]->room_id);
FD_ZERO(&(room_list[r_index]->read_set));
read_set = write_set;
timeout.tv_sec = 5;
printf("2\n");

Answer 1

The r_index variable in create_new_room is local to the function. You then pass its address to pthread_create so the thread can use it, but then create_new_room exits, causing the lifetime of r_index to end. Then when the thread tried to dereference its argument, it is pointing to memory that is no longer valid, triggering undefined behavior.
You'll need to dynamically allocate memory for the thread's argument so it survives after create_new_room exits:
int *r_index_arg = malloc(sizeof *r_index_arg);
*r_index_arg = r_index;
pthread_create(&tid, NULL, (void *) &active_room, r_index_arg);
Then you need to make sure you free the memory in the thread:
int r_index = *(int *) index;
free(index);