Segmentation error

This is a discussion on Segmentation error within the C Programming forums, part of the General Programming Boards category; So I run this in the terminal, give it two filenames and I get a segmentation error, basically it's supposed ...

  1. #1
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    Segmentation error

    So I run this in the terminal, give it two filenames and I get a segmentation error, basically it's supposed to open two files, and print a line from file1 then print a line from file2 until an EOF. Where am I getting a pointer problem?

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <ctype.h>
    #define MAX 1000
    
    int main(int argc, char *argv[])
    {
    	int i = 0, j = 0;
    	char ch, ct;
    //	char fname[MAX];
    	FILE *fo, *ft;
    	char line1[MAX];
    	char line2[MAX];
    	
    	if (argc != 3)
    		{
    		printf("Usage: %s filename \n", argv[0]);
    		exit(1);
    		}
    	
    	if ((fo = fopen(argv[1], "r")) == NULL)
    		{
    		printf("Can't open %s filename\n", argv[1]);
    		exit(1);
    		}
    		
    	if ((ft = fopen(argv[2], "r")) == NULL)
    		{
    		printf("Can't open %s filename\n", argv[1]);
    		exit(1);
    		}
    	
    	while ((ch = getc(fo)) != EOF && (ct = getc(ft)) != EOF)
    		{
    		i = j = 0;
    		
    		line1[i] = ch;
    		i++;
    		while ((ch = getc(fo)) != '\r')
    			{
    			line1[i] = ch;
    			i++;
    			}
    		line1[i] = '\0';
    		puts(line1);
    		
    		line2[j] = ct;
    		j++;
    		while ((ct = getc(ft)) != '\r')
    			{
    			line2[j] = ct;
    			j++;
    			}
    		line2[j] = '\0';
    		puts(line2);
    		}
    	
    	fclose(fo);
    	fclose(ft);
    	
    	return 0;
    }

  2. #2
    Deathray Engineer MacGyver's Avatar
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    3,211
    If you read a line greater than 1000 characters, then what?

    Are you sure you should be checking for '\r' and not '\n'?

    BTW, consider using fgets() for this.

  3. #3
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    my C book says macintosh saves text files with \r at the end of lines (granted the book is old so maybe in OS X they may have changed it) and the 1000 MAX shouldn't be a problem since both files merely say
    Code:
    test test test
    one two three
    though I just changed it to \n and it prints the first line but then gives a segmentation error again before printing the "one two three" lines

  4. #4
    Captain Crash brewbuck's Avatar
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    Location
    Portland, OR
    Posts
    7,249
    There's at least one problem, in the loops which look like this:

    Code:
    while ((ch = getc(fo)) != '\r')
    {
        line1[i] = ch;
        i++;
    }
    You're not checking for EOF. If you hit EOF before you see a '\r', this loop becomes infinite, and you overflow the array line1[]. Same for the other loop.

    Even if you fixed that, you could still overflow, because you never check if i or j has reached MAX. You really want a loop like this:

    Code:
    /* Stop after MAX - 1 chars, so we have a char remaining for the null byte */
    while((ch = getc(fo)) != EOF && ch != '\r' && i < MAX - 1)
    {
        line1[i] = ch;
        i++;
    }
    line1[i] = '\0';
    if(i == MAX - 1 && ch != EOF)
    {
        /* The line is longer than the buffer. You need to discard characters until you reach
         * the end of the current line, otherwise you'll get out of sync
         */
    }

  5. #5
    Registered User
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    Jul 2007
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    39
    that fixed it thanks, I figured that an EOF would always come right after a \n or \0 etc. bad assumption I guess. Thanks again guys

  6. #6
    Kernel hacker
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    Location
    Farncombe, Surrey, England
    Posts
    15,677
    Moral: Never ASSUME that a file is correctly formed - always check for "stupid" things like this. It's bitten me before now.

    --
    Mats

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