Search the symbol table of a child process

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# 1  
Old 04-23-2019
Search the symbol table of a child process


I am a newbie in Linux land, and I have a question about programming parent/child process interaction:

How do I search the value of a symbol in the child process? Is it possible?

I am doing a fork() and execve() to spawn any child possible, and I need something on the parent side to give me the value of any given symbol in the child, ie. "main".

In AmigaOS (my home country) this is very simple. Maybe there is a reason for it not to be so simple in UNIX land?

Best /Alpha
# 2  
Old 04-23-2019
You have to use an IPC (interprocess communication) mechanism, some listed here:
1. use a file
2. use pipes
3. use shared memory
4. semaphores / mutexes link for IPC
6 Linux Interprocess Communications Documentation

Unless you are using shared memory, there is normally no direct access to memory-only objects by the parent into a child process memory. Debuggers may use kernel mode memory access to get around this restriction -- a security issue otherwise.

Last edited by jim mcnamara; 04-23-2019 at 01:39 PM..
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# 3  
Old 04-23-2019

I still don't understand, though: If my child executes something like this:

int pid = fork();
if(pid == 0) {
    //where exactly do I set up the IPC? Before this point, I do not know any symbol values of 'processName', and after this point, I can no longer do any child specific symbol manipulation.
    execve(processName, 0, 0); //processName can be any process. Like 'ls'.
} else {
    //what to do here?

... where is the space, from where the child should conduct its part of the IPC? Can I somehow set up a handler function on the child side, or what exactly am I supposed to do?

Sorry, this is so alien to me, and probably I am trying something, that is not very 'Unix', because of my background in another operating system. Any help is very much appreciated.
# 4  
Old 04-23-2019
Where you set up IPC depends on which one you select. If you were to read the link, you would see a pipe example like this (from

// C program to demonstrate use of fork() and pipe() 
int main() 
    // We use two pipes 
    // First pipe to send input string from parent 
    // Second pipe to send concatenated string from child 
    int fd1[2];  // Used to store two ends of first pipe 
    int fd2[2];  // Used to store two ends of second pipe 
    char fixed_str[] = ""; 
    char input_str[100]; 
    pid_t p; 
    if (pipe(fd1)==-1) 
        fprintf(stderr, "Pipe Failed" ); 
        return 1; 
    if (pipe(fd2)==-1) 
        fprintf(stderr, "Pipe Failed" ); 
        return 1; 
    scanf("%s", input_str); 
    p = fork(); 
    if (p < 0) 
        fprintf(stderr, "fork Failed" ); 
        return 1; 
    // Parent process 
    else if (p > 0) 
        char concat_str[100]; 
        close(fd1[0]);  // Close reading end of first pipe 
        // Write input string and close writing end of first 
        // pipe. 
        write(fd1[1], input_str, strlen(input_str)+1); 
        // Wait for child to send a string 
        close(fd2[1]); // Close writing end of second pipe 
        // Read string from child, print it and close 
        // reading end. 
        read(fd2[0], concat_str, 100); 
        printf("Concatenated string %s\n", concat_str); 
    // child process 
        close(fd1[1]);  // Close writing end of first pipe 
        // Read a string using first pipe 
        char concat_str[100]; 
        read(fd1[0], concat_str, 100); 
        // Concatenate a fixed string with it 
        int k = strlen(concat_str); 
        int i; 
        for (i=0; i<strlen(fixed_str); i++) 
            concat_str[k++] = fixed_str[i]; 
        concat_str[k] = '\0';   // string ends with '\0' 
        // Close both reading ends 
        // Write concatenated string and close writing end 
        write(fd2[1], concat_str, strlen(concat_str)+1); 

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# 5  
Old 04-23-2019
Piping enables me to read the console output of a process. I did a fair bit of piping, back when I ported CMake to the Amiga platform.

Now, I simply do not get: How does that give me access to the symbols of the execve process? AFAICS there is no way, that a random process like 'ls' is going to come up with a generalized form of submitting a symbol value (ie. "main = 0xbeefdeaf") to either the stdin, stdout or stderr. I guess, if I *could* interrerupt the child process and execute something like

// ** child code
//int parentPipeFd = something sensible
void interrupt_handler() {
    void *main__symbol = dlsym(RTLD_DEFAULT, "main");
    fprintf(parentPipeFd, "0x%x", main__symbol);

But then how do I actually run a piece of code on the child side without having to write it in the actual code of the execve process?

Sorry, I am being really thick headed on this point, I can see. Please bear with me, I am so new to all of this Smilie.

EDIT: Could this be what I am looking for? EDIT EDIT: I am too new to post links. I think, I figured it out, thanks for the help Smilie.

Last edited by alphakili; 04-23-2019 at 03:54 PM..
# 6  
Old 04-23-2019
Well, in fact it seems, that I was wrong - I haven't solved the problem. What I was trying to do was to setup a signal handler, that would drive the interchange of information between the parent and the child. According to an answer to a similar question on another forum (I still cannot post links due to my status as initiate), this is indeed not possible at all. So I am about to give the answer to my own question, that what I am trying to do is actually impossible. I would love nothing more than to be proven wrong Smilie.
# 7  
Old 04-24-2019
Originally Posted by alphakili
Now, I simply do not get: How does that give me access to the symbols of the execve process?
It doesn't. The child is supposed to print its own symbols.

Either that, or you go whole-hog and attach a debugger. Probably much easier to use gdb than build it yourself.
This User Gave Thanks to Corona688 For This Post:
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