Piping a valid pointer address created in a child process to a parent process

Question

I'm new to programming, and I made code that does the following:

create a pointer address in a fork() parent process that points to "int 2".
The parent process converts the pointer address to a long int, and then a string.
The parent process writes the string into a pipe.
wait(NULL); forces the parent process to wait for the child process to exit.
the child process proceeds to read the address into an empty string.
the child process converts the string back into a long int, then into a pointer address.
the child process attempts to print the value that the address points to "int 2".
the child process exits.
the parent process resumes and converts the string back into a long int, which then gets converted back into a pointer address.
the parent process attempts to print the value that the address points to "int 2"

The result is that the parent process receives the address that was converted from a string, and successfully prints "2". The child process receives exactly the same address before the parent process does, but the address contains junk because it prints "0". printf commands were peppered throughout to ensure that the original address remained intact throughout the process.

The following is the relevant code. The output is placed after the code. Note that WAIT(400) is just a loop that makes the child process wait for the parent process to finish writing the address into the pipe. I defined it elsewhere, and it doesn't interact with any of the other functions.

int main(void) {
    int s[2];
    pipe(s);
    int pid;
    long nbytes;
    char buffer[80];
    pid = fork();
    if (pid == -1) {
        perror("fork failed");
        exit(EXIT_FAILURE);
    }
    if (pid == 0) {
        close(s[1]);
        WAIT(200);
        nbytes = read(s[0], buffer, sizeof(buffer));
        printf("%s\n", buffer);
        int *q = stringToAdress(buffer);
        printf("%ld\n", q);
        printf("%d\n", *q);
    } else {
        close(s[0]);
        int *p = malloc(sizeof(int));
        *p = 2;
        printf("%ld\n", p);
        char *str = addressToString(p);
        printf("%s\n", str);
        write(s[1], str, (strlen(str) + 1));
        wait(NULL);
        int *q = stringToAdress(str);
        printf("%ld\n", q);
        printf("%d\n", *q);
        free(p);
        exit(EXIT_SUCCESS);
    }
}

The following is the output. I believe that everything is expected except for the 0. I don't know why it's not a 2.

Program ended with exit code: 0

See [this for how to share memory between parent and child for example by `mmap`](http://stackoverflow.com/questions/13274786/how-to-share-memory-between-process-fork) — Antti Haapala -- Слава Україні, Mar 05 '16 at 19:21
Addresses cannot be exchanged this way between processes. Furthermore, addresses may not fit in type `long int`, and indeed would not in Windows 64 bits. You should use `uintptr_t`. — chqrlie, Mar 05 '16 at 19:46

score 1 · Accepted Answer · answered Mar 05 '16 at 18:58

In short, you cannot do that. The pointer from the parent process is not usable in that way from the child because the two processes don't share the same address space. You have to use another method for communication from child to parent, such as some form of IPC or files.

Piping a valid pointer address created in a child process to a parent process

1 Answers1