fgets problems


 
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# 1  
Old 02-17-2010
fgets problems

I've been having trouble with reading past the end-of-file in C. Can anyone find my stupid mistake?

This is the minimal code needed to cause the error for me:

Code:
FILE *f = fopen(name, "r");
if (!f)
	return;
pari_sp ltop = avma;
char line[1100];
while(fgets(line, 1100, f) != NULL)
	printf(".");
fclose(f);

So name is a C-string pointing to a valid filename. The FILE f opens without trouble, and the fgets loop runs through each line in the file. (The actual processing code, which I deleted, parses those lines without trouble; here I replaced it with a line which shows how many lines it reads.) But the last read through fgets does *not* return NULL but causes an application-ending error. fclose is never run.

What am I doing wrong?
# 2  
Old 02-17-2010
That is not the minimal code to crash it... it works absolutely fine.

The crash is probably in some destructor somewhere. Or it may be that you've already corrupted the heap by the time you've started calling fread, maybe with that pari_sp object? Try getting it down to a really minimal program that still crashes.
# 3  
Old 02-17-2010
Quote:
Originally Posted by Corona688
That is not the minimal code to crash it... it works absolutely fine.
I was afraid of that. This is part of a huge project, so I tried to extract what I could to make it fail without involving too much complexity. In particular, the actual code is using the Pari library.

Quote:
Originally Posted by Corona688
The crash is probably in some destructor somewhere. Or it may be that you've already corrupted the heap by the time you've started calling fread, maybe with that pari_sp object? Try getting it down to a really minimal program that still crashes.
Working on it. The pari_sp isn't actually doing anything at the moment (it was going to be a part of the garbage collection process) so I know that's not it.
# 4  
Old 02-17-2010
You might try valgrind to help hunt down double-frees and memory leaks and such. And gdb, to see what code it's actually crashing inside.
# 5  
Old 02-17-2010
Good news! I managed to fix it. Unfortunately I don't know what I did -- I was actually just focusing on making it easier to find the error. Smilie

Thanks for your help.
# 6  
Old 02-18-2010
What Corona was saying - you probably have corrupted stack or heap memory somewhere earlier than the fgets call. This means there may still exist serious data corruption. You are just no longer trashing it to the point it crashes.

Does your output - even though code appears to work - seem odd? Pari is a bignum Math package, are your results reasonable? I would be inclined to seriously question the validity of the final value(s); can you verify independently? Please tell me you are not computing orbital data for earth crossing asteroids... Smilie
# 7  
Old 02-18-2010
Quote:
Originally Posted by jim mcnamara
What Corona was saying - you probably have corrupted stack or heap memory somewhere earlier than the fgets call. This means there may still exist serious data corruption. You are just no longer trashing it to the point it crashes.
Right, that's why it's good I rewrote the earlier portions.

Quote:
Originally Posted by jim mcnamara
Does your output - even though code appears to work - seem odd?
This is fortunately only an I/O routine; no calculations to speak of. And after my rewrite it's quite short:
Code:
#define MAX_VECLEN 10000
#define MAX_LINELEN 1100
GEN
bfilein(char* name)
{
	FILE *f = fopen(name, "r");
	if (!f)
		pari_err(openfiler, "input", name);
	
	GEN v = vectrunc_init(MAX_VECLEN + 1);
	char line[MAX_LINELEN];
	int i = 0;
	while(fgets(line, MAX_LINELEN, f) != NULL) {
		char* kept = getBValue(line);
		GEN value = strtoi(kept);
		if (value == NULL)
			continue;
		if (++i > MAX_VECLEN) {
			pari_warn(warner, "only %d terms used; b-file has unread terms", MAX_VECLEN);
			break;
		}
		vectrunc_append(v, value);
	}
	fclose(f);
	return v;
}

with one helper function:
Code:
char*
getBValue(char* line)
{
	int start = 0;
	while (line[start] == ' ' || line[start] == '\t')
		start++;
	if (line[start] == '#')
		return NULL;	// Comment
	if (line[start] == '-')
		start++;
	while (line[start] >= '0' && line[start] <= '9')
		start++;
	while (line[start] == ' ' || line[start] == '\t')
		start++;
	int end = start;
	if (line[end] == '-')
		end++;
	while (line[end] >= '0' && line[end] <= '9')
		end++;
	if (start == end)
		return NULL;	// Blank line, or no numbers found
	line[end] = '\0';
	
	return line + start;
}

The parser is pretty boneheaded, but this isn't performance-limiting -- usually < 100 kB input.
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