[ILUG] C string concat question revisited

[Dave Neary]

On Tue, Jul 03, 2001 at 10:51:16AM +0100, Dave Neary wrote:
> Conor Daly wrote:
> > Therin lies the rub.  I'm trying to be general here so I don't know the
> > length of the destination char *array* 's'.  I can find out the length of the
> > string *within* 's' using strlen() but I don't know of a function that will
> > return the amount of memory *allocated* to 's'.
> 
> If you haven't explicitly allocated memory to s then it's not going to
> have any memory allocated to it. If you're sure that there is some
> memory allocated to s then realloc() can be used to change the size to
> something else (assuming it succeeds). If it's uninitialised then
> there's no issue - just allocate whatever you need to it and away you
> go.

Having thought about what you wanted to do, I reckon this is the
best way if you don't mind calling malloc/realloc a lot - if you
do, just keep a track of how big a buffer is allocated for my_buf
as you go, and when you get near the end of it, realloc another
big chunk.

Cheers,
Dave.

char *my_buf = NULL;
char *next_string;

/* This is just to set up next_string to get some strings, and
 * append them to my_buf, which automatically expands to fit */
next_string = malloc(80);
while (fgets(stdin, next_string, 80))
{
  if (my_buf)
  {
    char *tmp = realloc(my_buf, strlen(my_buf) + 
                        strlen(next_string) + 1);
    if(tmp == NULL)
    {
      fprintf(stderr, "realloc failed - not modifying my_buf\n");
    }
    else
      tmp = my_buf;
   
    strncat(my_buf, next_string, strlen(next_string)+1);
  }
  else
  {
    my_buf = malloc(strlen(next_string)+1);
    if(my_buf == NULL)
    {
      fprintf(stderr, "Failed to allocate space for my_buf");
      return -1;
      /* Consider setting errno too */
    }
   
    strncpy(my_buf, next_string,
    strlen(next_string)+1);
  }
  printf("%s", my_buf); /* Check what's in there */
}

free(next_string);
free(my_buf);
-- 
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