Pointer passing between libraries

[Canadian0469]

Hi all,

I'm puzzled...
I've got a main function as follows:

...
int main()
{

    char* cmdv[20];
    int i;
	for(i=0;i<20;i++)
	cmdv[i] = NULL;
    int cmdc = 0;
	prompt_user(&cmdc, cmdv);

   	if(cmdc>0)
   	{
...

prompt_user is a function defined in a static library as follows:

#include <stdio.h> //for NULL
#include <unistd.h>
#include <string.h>
#define MAXSIZE 256

void prompt_user(int *cmdc, char *cmdv[])
{
                    char userInput[MAXSIZE];
	    int nread;

	    char *k;

	    if(write(STDOUT_FILENO,"user: ",6)<0){
		perror("write");
		exit(1);
	    }

	    nread = read(STDIN_FILENO, userInput, MAXSIZE-1);

	    if(nread<0){
		perror("read");
		exit(1);
	    }

	    userInput[nread] = '\0';

		(*cmdc) = 0;
		k = userInput;
		while((cmdv[*cmdc]=(char*)strtok(k, " \n"))){
		    k = NULL;
		    (*cmdc)++;
		}
}

The idea is that main calls prompt_user to fill in an array cmdv[].
The thing that confuses me is that if I print cmdv[0] in the code for prompt_user, then I see what I typed in; however, if I print cmdv[0] in main after returning from the call to prompt_user, I see junk.
main makes a call to itself in the end and oddly enough, the second time it runs it works properly (i.e. cmdv[0] has the expected value after returning from the call to prompt_user.

If you need further information please let me know, the code for the main function is quite long because it compares the value of cmdv[0] to different strings so I didn't post the whole thing. Also, I don't want to post all of the main function because this is an assignment so I'm not allowed to post the whole thing of course...

Is anybody able to see what's going on here?

Thanks!

[Elysia]

The thing that I can see is that strtok returns a pointer relative to your LOCAL variable k and k is pointing to your LOCAL variable userInput. Therefore, when the function ends, k aka your cmdv, points to invalid addresses.

[vart]

the user input is stored in the local variable userInput

cmdv pointers are set to point to different locations inside this buffer
after the prompt_user function exits - local variable does not exists, so cmdv pointers are pointing to garbage, as you see it

[Canadian0469]

Oh.. I see... Anybody have any ideas of how I can modify the array with valid values but still use the the static library for prompt_user?

[Elysia]

You can create your userInput buffer on the heap. Don't forget to release it after you're finished with it.
HOWEVER, a better solution is pointed out by matsp. I suggest you heed that instead.

[matsp]

Create your userInput buffer on the heap. Don't forget to release it after you're finished with it.

Or simply pass in a buffer to the function - if you can hold it on the stack in one place, then it can be held on the stack somewhere else - and it saves trying to keep track of the allocations.

--
Mats

[Elysia]

Might be a better solution, yes...
I change my mind Go for mtsp's solution, it's the easiest and safest!

[matsp]

Might be a better solution, yes...
I change my mind Go for mtsp's solution, it's the easiest and safest!

Of course, it also requires a second parameter of "maxsize". Otherwise it's definitely not safe.

--
Mats

[Canadian0469]

Sorry but I'm not sure exactly how to implement matsp's solution...
Do I do it by:

1. Cutting char userInput[MAXSIZE]; from prompt_user and pasting it at the top of main()
2. Changing prompt_user(&cmdc, cmdv); in main() to prompt_user(&cmdc, cmdv, userInput);
3. Changing the definition of the prompt_user function to void prompt_user(int *cmdc, char *cmdv[], char userInput[])

?

I tried this but I get a Segmentation fault... I have a feeling I'm making a pointer mistake because I still don't have a good understanding of pointers.

Thanks for all the help so far!

[Canadian0469]

Forget what I said about the segmentation fault, it was just a silly logic mistake somewhere else in the code.

Thanks so much everyone!!!!

[Elysia]

Don't forget to pass the size of your array and check when getting your user input that you don't exceed the buffer size. That will be a security risk.
Or you could go with C++ and use string classes.