[SOLVED] A compiled C program copied using a self-written program, failed to work

trainee · 06-26-2013, 09:06 PM

Hi

I have a program like this:

Code:

#include <stdio.h>

/* print Fahrenheir-Celcius table
 * for fahr = 0, 20, ..., 300 */

main()
{
	int fahr, celsius;
	int lower, upper, step;

	lower = 0;	/* lower limit of temperature scale */
	upper = 300;	/* upper limit of temperature scale */
	step = 20;	/* step size */

	fahr = lower;
	while (fahr <= upper) {
		celsius = 5*(fahr - 32) / 9;
		printf("%d\t%d\n", fahr, celsius);
		fahr = fahr + step;
	}
}

I compiled it using the command
gcc ./convert.c -o convert

and ran the file and everthing was OK.

Next I wrote my own mycopy.c program as below

Code:

#include <stdio.h>


int main(int argc, char *argv[])
{

	if (argc != 3) 
	{
		printf("The program requires exactly 2 file names.");
		return 0;
	}

	FILE *old_file, *new_file;

	old_file = fopen(argv[1], "r");
	new_file = fopen(argv[2], "w");

	int i;

	char c;

	
	c = fgetc(old_file);


	while (c != EOF)
	{
		putc(c,new_file);
		c = fgetc(old_file);
	}

	fclose(new_file);

	return 1;
}

I compiled this file, output to a filenamed mycopy. I tested and it worked fine with text file. But if I used it to copy the file convert above, using the command
mycopy convert newconvert
chmod +x newconvert
Then the file newconvert fail to run with the error message:
Segmentation failed.

Would you please let me know why?
Thanks.

ta0kira · 06-26-2013, 09:15 PM

You can't use text-mode file buffers to copy a file because it will skip certain binary characters. Try ls -l convert newconvert to see the size difference. You need to open the files with open, then use read and write.

Kevin Barry

NevemTeve · 06-27-2013, 02:58 AM

As a start, try these commands

Code:

ls -l <original_executable> <copied_executable>
cmp <original_executable> <copied_executable>

H_TeXMeX_H · 06-27-2013, 03:20 AM

I agree with ta0kira.

Use fopen to open the file for read binary "rb" and the other for "wb", and then use fread and fwrite to copy the file.

NevemTeve · 06-27-2013, 03:38 AM

I'd suggest that also if it weren't unix:

Quote:

The mode string can also include the letter 'b' either as a last char-
acter or as a character between the characters in any of the two-char-
acter strings described above. This is strictly for compatibility with
C89 and has no effect; the 'b' is ignored on all POSIX conforming sys-
tems, including Linux. (Other systems may treat text files and binary
files differently, and adding the 'b' may be a good idea if you do I/O
to a binary file and expect that your program may be ported to non-Unix
environments.)

NevemTeve · 06-27-2013, 03:40 AM

Anyway, this will stop at the first \xff (0FFH) character:

Code:

    char c;
    c = fgetc(old_file);

fixed:

Code:

    int c;
    c = fgetc(old_file);

ta0kira · 06-27-2013, 09:35 AM

Quote:

Originally Posted by NevemTeve

I'd suggest that also if it weren't unix:

Same here; that's why I suggested using the system calls directly. It's possible that fread and fwrite would work, but there's no sense using buffers if no parsing or formatting needs to be done.

Kevin Barry

NevemTeve · 06-27-2013, 09:40 AM

Well, there is a reason why fgetc is defined as

Code:

int fgetc(FILE *stream);

It's so that you could tell apart EOF (=-1) and normal characters (0..255)

trainee · 06-27-2013, 07:53 PM

I tried the suggestion

Code:

int fgetc(FILE *stream);

And things work. I have not tried the fread, fwrite or the open, read, write suggestion. I'll try it and report the result later. Thanks everybody.

EDIT: the open, read, write works too.