Problem with char* pointers with GCC/Debian

BruceV · 04-01-2024, 08:48 AM

I have a console application with a main loop for reading a command string, interpreting it and doing stuff. In essence (irrelevant detail is omitted, this is pseudocode).

While (1)
{
printf (“Enter command.”) ;
getstring (cmdbuf) ;
do stuff.
}
A typical command string will consist of a number of abbreviations, and some numbers, like this:

Mov 23 45 shift 12

The spaces are just common whitespace.

The easiest way to separate the string elements into separate substrings seemed to be to do it in place, simply by finding the first whitespace character after each entry and changing it to \0, and storing the pointer address of each following entry in s char* pointer array. So I end up with an array of pointers to each entry, ie:

Char* Entry[0] is Mov
Char* Entry[1] is 23
Etc.

I ran into trouble once this was set up and I started doing things with my ‘substrings’. For example, the following statement causes a segmentation fault.

Num = atoi (Entry[1]) ;

I have confirmed that Entry 1 is “23”, with a \0 terminator. The Entry[] array is correctly declared. The command string memory is reserved with malloc(). My application worked fine until I implemented this 'in place' command string partitioning.

Can anyone see what's wrong?

jmgibson1981 · 04-01-2024, 04:00 PM

I can't picture what you are doing the way you describe it. Seems over complicated. strtok is a better choice. in the standard lib.

Here is my implementation of what I think you are describing. My stuff is BSD2.0 Licensed.

Code:

char *
string_field_search(const char * str,
                    const char * delimiter,
                    int field)
{
  assert(str); assert(delimiter);
  
  // declare & initialize
  char * retval = NULL;
  char * token = NULL;
  int counter = 0;
  char * buffer = strdup(str);

  // tokenize and iterate till field found
  do {
    token = strtok(buffer,
                   delimiter);
    if (!token) {
      break;
    }
    while (token) {

      // if proper value iterate until field
      if (counter == field) {
        retval = strdup(token);
        if (retval) {
          retval[strcspn(retval,
                       "\n")] = '\0';
        }
        token = NULL;
        break;
      } else {
        
        // else continue to next token until found
        counter++;
        token = strtok(NULL,
                       delimiter);
      }
    }
  } while (counter < field);

  // clean up and return
  free(buffer); buffer = NULL;
  return(retval);
}