Thread: String trouble, VERY new C programmer

Ok. I'm getting very confused. To be honest, C's pointers aren't totally clear to me (despite my reading EVERYTHING out there).

I am trying to write a "shell" basically for hw (so hints please so i can actually learn this stuff).
I have a global

Code:

char *dirs[MAXPATHS];

at the top to keep the paths of the machine around. I'm not exactly sure what the difference b/t this and char dirs[MAXPATHS] would be....

Well, i can get the paths info in there ok. Here is where i'm stuck.

Code:

char *lookupPath(char *name)
{
char *pName; //this was suggested by instructor, but i don't know what to use it for

char*workingName = NULL;
int counter = 0;

while(dirs[counter] != NULL)
  {
     if (name != NULL && name[0] == '/')
       {
       }
     else
       {
       //PROBLEM CODE!!!!
       strcat(workingName, "");
       strcat(workingName, dirs[counter]);
       strcat(workingName, "/");
       strcat(workingName, name);
       if (access(workingName, X_OK) == 0)
           return workingName;
        }
    char *workingName = NULL;
    counter++;
   }
printf("Error %s was not found.\n", name);
return NULL;
}

how do i get workingName to be the dir[counter] + / + name of the file AND get it reset after each search?


Thanks so much for your time


-Zany

Originally Posted by Dave_Sinkula

Have you tried something like this?

Code:

sprintf(workingName, "%s/%s", dirs[counter], name);

In addition to allocating memory for workingName?

how would i allocate memory for workingName?
i tried char *workignName;
then your suggestion and got a segfault, just like i do when i run the strcat's

Originally Posted by Dave_Sinkula

Code:

char workingName[BUFSIZ];

i get some warnings, but it runs thanks so much!!

how would i allocate memory for workingName?

By using malloc() to allocate the memory and free() to deallocate the memory:

Code:

char* workingName = NULL;
...
workingName = malloc( strlen( dirs[counter] ) + strlen( name ) + 1 );
...
/* Then when you no longer need workingName. */
free( workingName );

Originally Posted by Salem

> i get some warnings, but it runs thanks so much!!
Bad idea, if you've got warnings, you've still got problems. The fact that it runs is as much down to dumb luck as it is to your skill.

If you're confused about what a particular warning means, then feel free to post it and the relevant section of code it related to, then we should be able to tell you what it means.

Code:

/* This function is passed in a name of a executable file
 *	or is given an absolute file path name and then 
 *  checks to see if it exists.
 * Returns the path name if found, or null if not 
 *  found(an error is printed too). 
 */
char *lookupPath(char *name)
{
  char *pName;
  char *workingName[BUFSIZ];
  int counter = 0;

  while (dirs[counter] != NULL)
    {    
      //printf("Working on %s trying to find %s\n"
      //  , dirs[counter], name);
  
  if(name != NULL && name[0]  == '/')
    {
      if( access(name, X_OK) == 0 )
	return name;
    }
  else 
    {	
      //This way didn't work so well...
	  //strcat(workingName, "");
      //strcat(workingName, dirs[counter]);
      //strcat(workingName, "/");
      //strcat(workingName, name);
      
      sprintf(workingName, "%s/%s", dirs[counter], name);
      
      if( access(workingName, X_OK) == 0 )
	return workingName;
      
      
    }
  counter++;
}
  printf("Error %s was not found.", name);
return NULL;
}

I get several warnings that i totally don't understand when i compile that. Here is the entire code so you may understand better....

Code:

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#define MAXPATHS 64

char *dirs[MAXPATHS];

/* This function will store the $PATH's found in the
 * global var *dirs[MAXPATHS].  Uses strtok, and is finished.
 */
int parsePath()
{
  //char *strtok(char *str1, const char *str2); This is the prototype
  char *path = getenv("PATH");
  char *delims = ":"; //TODO why does char delims[] = ":" also work here?
  char *result = NULL;
  result = strtok(path, delims);
  
  int counter = 0;
  while(result != NULL)
    {
      //printf("result is \"%s\"\n", result);
      dirs[counter] = result;
      counter++;

      /* Subisquent calls are made with 
       * NULL according to man page.*/

      result = strtok(NULL, delims); 
                                     
    }
  //www.elook.org/programming/c/strtok.html

  return 1;
}

/* This function is passed in a name of a executable file
 *	or is given an absolute file path name and then 
 *  checks to see if it exists.
 * Returns the path name if found, or null if not 
 *  found(an error is printed too). 
 */
char *lookupPath(char *name)
{
  char *pName;
  char *workingName[BUFSIZ];
  int counter = 0;

  while (dirs[counter] != NULL)
    {    
      //printf("Working on %s trying to find %s\n"
      //  , dirs[counter], name);
  
  if(name != NULL && name[0]  == '/')
    {
      if( access(name, X_OK) == 0 )
	return name;
    }
  else 
    {	
      //This way didn't work so well...
	  //strcat(workingName, "");
      //strcat(workingName, dirs[counter]);
      //strcat(workingName, "/");
      //strcat(workingName, name);
      
      sprintf(workingName, "%s/%s", dirs[counter], name);
      
      if( access(workingName, X_OK) == 0 )
	return workingName;
      
      
    }
  counter++;
}
  printf("Error %s was not found.", name);
return NULL;
}

main(int argc, char *argv[])
{
  int argc_dup = argc;
  parsePath();
  while (argc--)
    {
      //printf("%i", argc_dup - argc);
      if (NULL != argv[argc_dup - argc])
	printf(lookupPath(argv[argc_dup - argc]));
	  printf("\n");
    }
  printf("I'm done\n");
  exit(EXIT_SUCCESS);
}

Originally Posted by Salem

> return workingName;
This is a really bad idea (did you get something like returning address of a local?)
When the function returns, all the local variables go out of scope, and any pointer to a local becomes invalid.

If you really need to return the working pathname as a result, then supply an additional parameter indicating where you want the result stored (see strcpy for example).

Thank you for your comments. This one i don't quite understand. So workingName here is a pointer? I though i was returning a string here...