Simple C file I/O question

vargadanis · 07-28-2007, 11:53 AM

Hi!

I am having trouble opening a file and then reading its content and in the end displaying it. Here is the code:

Code:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
	FILE *fp;
	fp = fopen("/boot/grub/menu.lst", "r");
	char *line;
	line = malloc(128);
	int i;
	if(fp == NULL)
	{
		printf("Could not open the file!\n");
	}else
	{
		printf("File opened for input");
		while(!feof(fp))
		{
			fgets(line, 128, fp);
			for(i = 0; i < 128; i++ )
				printf("%c", line[i]);
		}
	}
	fclose(fp);
	return 0;
}

The output is a complete mess:

Quote:

itrd /boot/initrd.img-2.6.20-15-generic
=UUID=2b154dc2-73a3-4df2-817a-90e112bc04cd ro single
lash
title Ubuntu, memtest86+
6.20-15-generic
=UUID=2b154dc2-73a3-4df2-817a-90e112bc04cd ro single
lash
root (hd0,0)
memtest86+
6.20-15-generic
=UUID=2b154dc2-73a3-4df2-817a-90e112bc04cd ro single
lash
kernel /boot/memtest86+.bin
0-15-generic
=UUID=2b154dc2-73a3-4df2-817a-90e112bc04cd ro single
lash
quiet
/boot/memtest86+.bin
0-15-generic
=UUID=2b154dc2-73a3-4df2-817a-90e112bc04cd ro single
lash

And this is just a part of the output. As you can see, parts of the things are repeated. How could I prevent this? And what could go wrong?

Thanx:
Dan

Dan04 · 07-28-2007, 12:06 PM

Why are you printfing the contents of your buffer one char at at time? More importantly, why loop from 0 to 128 when lines are usually not exactly 128 characters long? That's probably why you're printing so much garbage.

vargadanis · 07-28-2007, 12:13 PM

Yes... I noticed that after NULLing the value of line[i] the garbage is not gonne be printed out. The loop is 128 because a line is not gonna be longer than 128chars/bytes. I thought that this must be enough.
Is there a more dynamic way to allocate the buffer or something? How can I print out the entire line at once?

Dan04 · 07-28-2007, 12:15 PM

To print the entire line at once, use:

Code:

printf("%s\n", line);

or

Code:

puts(line);

vargadanis · 07-28-2007, 12:18 PM

printf("%s", line);
This one worked like a charm. Awesome... I told you it was easy

Thanx

vargadanis · 07-28-2007, 12:30 PM

back to the more dynamic memory handling thing that I mentioned above.
Is there any other way to handle the memory more dynamically? To use only that is needed?

binutils · 07-28-2007, 12:53 PM

Code:

	char *line;
	line = malloc(128);

above code is

Code:

#define MAXINPUT 128
char line[MAXINPUT]={0};

try to use malloc() between 3 and 4

Code:

1	while(!feof(fp))
2	{
3		fgets(line, 128, fp);
4		for(i = 0; i < 128; i++ )
5			printf("%c", line[i]);
6	}

vargadanis · 07-28-2007, 12:59 PM

I thought that I had to predefine all the memory reservations in the beginning of each function. I don't know why. That works too. Thanks