Quick Pointer Issue (C)

psio15 · 03-29-2011, 06:20 PM

Here is the issue. I am reading in a outside text file and putting in the string on that line into a char array that is already allocated.

Code:

int main(int argc, char *argv[])
{
	FILE *fp;
	fp = fopen(argv[1], "r");
	int theString = atoi(argv[2]);
	if(fp == NULL)
	{
		printf("Cannot open %s\n", argv[1]);
	}
	/*
	/ Ptr is allocated 1000 "lines" 50 "spaces" wide, all the size of char
	*/
	char *ptr = (char *)malloc(1000*50*sizeof(char));
	char buffer[100];
	int dict[1000];
	int i = 0;
	
		//Get next line and put into ptr with length of 50 chars of the line
	while(fgets(buffer, sizeof buffer, fp) != NULL)
	{
		dict[i] = convertString(buffer);
		
		//Currently keeps resetting the current line to ptr not adding on... fail on my part!
		*ptr++ = buffer;
		i++;
	}
	
	printf("%c\n", ptr[0]);
	
	// THIS FREE CREATES AN ABORT?!?!?!
	//free(ptr);
	return 0;
}

I am more than likely doing something simple, very wrong. The buffer is the current char[] that holds the line's string and I want to add that to the ptr.

Basically to put it in an analogy I am copying page for page from one "book" to another. In this case the text file to the array. Thanks for any help!

Nominal Animal · 03-30-2011, 05:00 AM

Quote:

Originally Posted by psio15

Code:

	/*
	/ Ptr is allocated 1000 "lines" 50 "spaces" wide, all the size of char
	*/
	char *ptr = (char *)malloc(1000*50*sizeof(char));
	char buffer[100];
	int dict[1000];
        int i = 0;

So ptr is your data array, buffer is a temporary buffer you use to read the lines into, dict is a hash array of sorts (with convertString() being the hash function), and i being the line number (starting from zero). I'll omit the dict and convertString() bits below to make the issue clearer.

The next bit is the problem:

Quote:

Originally Posted by psio15

Code:

	while(fgets(buffer, sizeof buffer, fp) != NULL)
	{
		*ptr++ = buffer;
		i++;
	}

You do read the line (assuming it is less than 99 characters including the newline, otherwise it'll take more than one call to read the entire line -- so it'd be a good idea to set a bit bigger size to buffer, just in case). However, you don't check if you read too many lines, and you just copy the first char of buffer to the ptr, and increase ptr[I], making it very difficult to retrieve the actual start of ptr. Later on, you try to free a pointer within the allocated array, which is obviously an error. Do not modify ptr itself, use some kind of an offset (50*i perhaps).

I believe you should use strspn() and strcspn() to skip the leading and trailing newlines and control characters (and whitespace), and then memcpy() to copy the interesting bit (not exceeding the allocated space) to the ptr array -- or write your own "copy trimmed string" function. The details depend on whether you want the ptr array to be a two-dimensional character array (without end-of-string zero bytes), or an array of strings (each with an end-of-string zero byte).

If you want to use a dynamic array of strings, consider the following:

Code:

#include <stdlib.h>
#include <string.h>

int       lines_max = 0;
int       lines     = 0;
char    **line      = NULL;
/* line[i] exists, if (i >= 0 && i < lines). */

/** addline() - Add a new line to the line array
 * @string      Pointer to the string
 * @length      Length to copy
 * Return value will be the line number, or negative if out of memory.
*/
int add_line(char const *const string, size_t const length)
{
    /* Make sure line array has room for another line. */
    if (lines >= lines_max) {
        size_t const   max = (size_t)lines + (size_t)512;
        char         **tmp;

        tmp = realloc(line_ptr, sizeof(char *), max);
        if (!tmp)
            return -1;

        line  = tmp;
        lines = (int)max;
     }

     /* Allocate line buffer */
     line[lines] = malloc(length + (size_t)1);
     if (!line[lines])
        return -1;

     /* Copy string */
     memcpy(line[lines], string, length);
     line[lines][length] = 0;

     /* Increase lines, but return the prior value. */
     return lines++;
}