wired paths when using strcat

km603 · 02-02-2008, 05:55 PM

Hi,

the following are my codes:

Code:

char* getCommandPath(char* command) {

	FILE *fpin;

  	char p[1024];
	char **pts;

 	// char *temp2 = (char *)calloc(1024, sizeof(char));
	char *path;
	path = getenv("PATH");

	char *temp2 = malloc(strlen(path) + 1);

  	strcpy ( p, path );

  	path = strtok_r ( p, ":" , pts);   // Find the first PATH

  	while ( path != NULL )
	{
		strcpy ( temp2, path );  // Save a copy of substring
		strcat ( temp2, "/" );     // Form a path and file name
		strcat (temp2, command);
		printf(" \tpath is %s, temp2 is %s \n", path, temp2); //check outputs

    		if ( ( fpin = fopen ( temp2, "r" ) ) == NULL )
		{
			path = strtok_r ( NULL, ":", pts );  // Try next path
			printf(" \tpath is %s, temp2 is %s \n", path, temp2); //check outputs again
    		}
 
		else 
		{

      			break;  // Must have a good fpin
		}
  	}

  	if ( fpin == NULL )
    		return NULL;

  	else 
    		return temp2;
}

when i made command to be "xeyes", the paths are so weird.

Code:

path is /usr/local/sbin, temp2 is /usr/local/sbin/TÒ�s
        path is /usr/local/bin, temp2 is /usr/local/sbin/TÒ�s
        path is /usr/local/bin, temp2 is /usr/local/bin/cÒ�s
        path is /usr/sbin, temp2 is /usr/local/bin/cÒ�s

Can you tell me where i did wrong?

thanks

rubadub · 02-02-2008, 07:13 PM

you allocate how much memory you need for your string here:

Code:

char *path;
	path = getenv("PATH");

	char *temp2 = malloc(strlen(path) + 1);

then you do other stuff to it (like this)

Code:

strcpy ( temp2, path );  // Save a copy of substring
		strcat ( temp2, "/" );     // Form a path and file name
		strcat (temp2, command);

therefore it's probably longer than how much memory you've reserved for it.

Use a intermediary string and resize, or keep resizing as you go along...

km603 · 02-02-2008, 07:26 PM

thanks for your reply.

even i allocate the memory to very big, the issue still exists.

Code:

char *temp2 = (char *)calloc(10240, sizeof(char));

I think it is big enough to hold the data.

rubadub · 02-02-2008, 10:56 PM

Not that i've covered this before, but from what I can tell, what is returned from getenv() is a pointer and I think your getting into trouble when your copying it and then trying to reuse the variable you've set as a pointer as a string (or something like that (don't you just love pointers?)).

Anyway, you were right, when setting the size of 'temp2' to the size of 'path' it should be generally be big enough to hold the new string, but imagine if your system only had the '/bin' path and then you added '/xeyes', it'd be too short. Therefore there's a variable called MAXPATH set in 'limits.h' which you can add with the length of the command, the path separator (+1)... anyway here's what i've got working... Hope it helps!

Code:

char* getCommandPath(char* command)
{
	FILE *fpin;

	char *path;
	path = getenv("PATH");
	
	//char *temp2 = (char*)malloc(strlen(path) + 1);
	char *temp2 = (char*)malloc(PATH_MAX+strlen(command)+2);
	
	char *tok;
	tok = strtok(path, ":");
	
	while (tok = strtok(NULL, ":"))
	{
		strcpy(temp2, tok);
		strcat(temp2, "/");
		strcat(temp2, command);
		if ( ( fpin = fopen ( temp2, "r" ) ) != NULL )
		{
			break;
		}
	}
	
	if ( fpin == NULL )
	{
		return "NULL";
	}
	else 
	{
		close(fpin);
		return temp2;
	}
}

paulsm4 · 02-03-2008, 01:46 AM

Hi -

There are lots of problems with the code, including:
1. I wasn't even able to compile it as coded
2. You open the file...but never close it
3. You alloc memory ... but never free it
4. The same thing (you're basically just re-implementing the "whereis"
function) could be accomplished with a lot less code

But...

I think the biggest problem was your use of "pts" in "strtok_r()". The whole purpose of this argument is:
1. You allocate a buffer yourself (the "classic" strtok() uses an internal, static buffer - which makes it non-reentrant)
2. You declare a pointer to this buffer (e.g. "pts")
3. You pass a pointer to the pointer (so that when strtok_r updates the buffer, your function receives the updated pointer).

For example:

Code:

  char p[1024];
  char *pts = p;
  char *path = getenv("PATH");
  char *temp2 = malloc(strlen(path) + 1);
  strcpy ( p, path );
  path = strtok_r ( p, ":" , &pts);   // Find the first PATH
  ...

Here's the output I got (in my test, I used "sh" instead of "xeyes"):

Quote:

path is /usr/bin, temp2 is /usr/bin/sh
path is /bin, temp2 is /usr/bin/sh
path is /bin, temp2 is /bin/sh

'Hope that helps .. PSM