Value in shared memory different after shared memory access C++ - c++

Question

I am trying to create shared memory, but whenever I access it from a child process its value is different than what it should be. I think that I am using shmget() correctly. I have tried a lot of stuff that I have found online, but I can't find anyone with the same problem I am having. No matter what I enter num as, whenever I try to get l->returnLicense it outputs 0. I'm really at a loss as to what to try next.
#include "license.h"
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#include <string>
#include <sys/wait.h>
#include <iostream>
#include <unistd.h>
using namespace std;
int validateArguments (int num) {
if (num == -69) {
//no arg
return 10;
}
if (num < 1 || num > 20) {
//warning use 20 as num
return 10;
}
return num;
}
int initSharedMemory (License *l) {
key_t key = ftok("/tmp", 'J');
cout << "key: " << key << endl;
int shmid = shmget(key, sizeof(l), 0666|IPC_CREAT);
if (shmid == -1) {
perror("Shared memory");
return -1;
}
l = (License*)shmat(shmid, (void*)0, 0);
if (l == (void*) -1) {
perror("Shared memory attach");
return -1;
}
return shmid;
}
void detachSharedMemory (License *l) {
shmdt(l);
}
void destroySharedMemory (int shmid) {
shmctl(shmid, IPC_RMID, NULL);
}
void spawn (int shmid) {
pid_t c_pid = fork();
if (c_pid == -1) {
perror("fork");
} else if (c_pid > 0) {
cout << "parent" << shmid << endl;
c_pid = wait(NULL);
} else {
cout << "child" << endl;
License *l;
key_t key = ftok("/tmp", 'J');
cout << "key: " << key << endl;
int shmid = shmget(key, sizeof(l), 0666);
cout << shmid;
l = (License*) shmat(shmid,0,0);
if(l == (void*) -1) {
perror("memory attach");
exit(0);
}
int num = l->returnLicense();
cout << num << "num\n";
shmdt(l);
char* args[] = {"./testChild", NULL};
execvp(args[0],args);
exit(0);
}
}
int main (int argv, char *argc[]) {
int num;
if (argv == 2) {
num = atoi(argc[1]);
} else {
num = -69;
}
num = validateArguments (num);
License *l;
int shmid = initSharedMemory (l);
License *tmp = l;
tmp->initLicense(num);
spawn(shmid);
cout << l->returnLicense() << endl;
detachSharedMemory(l);
destroySharedMemory(shmid);
return 0;
}
I'm not including the entirety of my code, but I think this is enough to illustrate my problem. I copied code from the testChild that I exec from this process so that you can see the problem I'm facing all in one file.

Answer 1

License *l; // uninitialized
int shmid = initSharedMemory (l); // pass l by value, UB!
License *tmp = l; // copy uninitialized pointer, UB!
tmp->initLicense(num); // call member function through uninitialized pointer, BOOM!
spawn(shmid);
cout << l->returnLicense() << endl; // call member function through uninitialized pointer, BOOM!
Probably you meant for your initSharedMemory() function to have a reference-typed parameter, so that it would affect the License * l; variable in main().