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Apr 19th, 2009

You'd think I would know how to use strings by now, but...

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I'm getting an error (unhandled exception writing address so and so) trying to modify a string. It gets weirder though. Let me show what i've got:

--Main.c--

C Syntax (Toggle Plain Text)
#include "stdafx.h"
#include "commands.h"
 
int main(int argc, char* argv[])
{
	char word[256];
	int id;
 
	strcpy(word, "move");
	strToUpper(word);
	printf("%s", word);
	id = getCommandId(word);
 
	if(id == -1)
		perror("Error");
	else if(id == 0)
		printf("No such command.");
	else
		printf("ID: %d\n", getCommandId("move"));
 
	getchar();
	return 0;
}
#include "stdafx.h"
#include "commands.h"

int main(int argc, char* argv[])
{
	char word[256];
	int id;

	strcpy(word, "move");
	strToUpper(word);
	printf("%s", word);
	id = getCommandId(word);

	if(id == -1)
		perror("Error");
	else if(id == 0)
		printf("No such command.");
	else
		printf("ID: %d\n", getCommandId("move"));

	getchar();
	return 0;
}

--commands.h--

C Syntax (Toggle Plain Text)
#pragma once
 
#define COMMAND_MOVE 1
#define COMMAND_TAKE 2
#define COMMAND_LOOK 3
 
int getCommandId(char *word);
void strToUpper(char *str);
#pragma once

#define COMMAND_MOVE 1
#define COMMAND_TAKE 2
#define COMMAND_LOOK 3

int getCommandId(char *word);
void strToUpper(char *str);

And --commands.c--

C Syntax (Toggle Plain Text)
#include "stdafx.h"
#include "commands.h"
 
void strToUpper(char *str)
{
	int i = 0;
	while(str[i] != '\0')
	{
		str[i++] &= 0xDF; //reset bit 5
	}
}
 
int getCommandId(char *word)
{
	FILE *fp;
	char buf[256];
	char *tok;
	int i = 0;
 
	fp = fopen("commands.dat", "r");
	if(fp == NULL)
		return -1;
 
	strToUpper(word);
 
	while(++i)
	{
		if(fgets(buf, 255, fp) == NULL)
			break;
 
		tok = strtok(buf, " ");
		while(tok != NULL)
		{
			if(strcmp(tok, word) == 0)
				return i;
 
			tok = strtok(NULL, " ");
		}
	}
 
	return 0;
}
#include "stdafx.h"
#include "commands.h"

void strToUpper(char *str)
{
	int i = 0;
	while(str[i] != '\0')
	{
		str[i++] &= 0xDF; //reset bit 5
	}
}

int getCommandId(char *word)
{
	FILE *fp;
	char buf[256];
	char *tok;
	int i = 0;

	fp = fopen("commands.dat", "r");
	if(fp == NULL)
		return -1;
	
	strToUpper(word);

	while(++i)
	{
		if(fgets(buf, 255, fp) == NULL)
			break;

		tok = strtok(buf, " ");
		while(tok != NULL)
		{
			if(strcmp(tok, word) == 0)
				return i;

			tok = strtok(NULL, " ");
		}
	}

	return 0;
}

Nothing real exciting happens with the call to strToUpper in main, other than the string being capitalized. Then getCommandId is called, and therefore strToUpper is too, with the same pointer word as before. Only this time, I get the nasty error described at the top of my post. Thoughts?

Last edited by death_oclock; Apr 19th, 2009 at 9:00 pm.

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death_oclock

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

>>str[i++] &= 0xDF; //reset bit 5
Programming is not about trying to be cute, but about writing code that you and everyone else in the world can easily comprehend.
str[i] = toupper(str[i]); ++i;

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

"Being cute" was not my intention. I was thinking of efficiency at the time. And while I appreciate the suggestion, it doesn't help my immediate problem.

death_oclock

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

Do you still get the error if you remove:
* the file stuff? (replace it with a single buf load to tokenize)
* the token stuff?
Post a single file minimal version that recreates the error.

nucleon

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

I put your program all in one file and here is the result (using VC ++ 2008 Express)

C Syntax (Toggle Plain Text)
#include <ctype.h>
#pragma warning(disable: 4996)
 
 
void strToUpper(char *str)
{
	int i = 0;
	while(str[i] != '\0')
	{
		str[i] = toupper(str[i]); //reset bit 5
        ++i;
	}
}
 
int getCommandId(char *word)
{
	FILE *fp;
	char buf[256];
	char *tok;
	int i = 0;
 
	fp = fopen("commands.txt", "r");
	if(fp == NULL)
		return -1;
 
	strToUpper(word);
    i = 0;
	while(fgets(buf, 255, fp) != NULL)
	{
        strToUpper(buf);
		tok = strtok(buf, " ");
        ++i;
		while(tok != NULL)
		{
			if(strcmp(tok, word) == 0)
				return i;
 
			tok = strtok(NULL, " ");
		}
	}
 
	return 0;
}
 
int main(int argc, char* argv[])
{
	char word[256];
	int id;
 
	strcpy(word, "move");
	strToUpper(word);
	printf("%s", word);
	id = getCommandId(word);
 
	if(id == -1)
		perror("Error");
	else if(id == 0)
		printf("No such command.");
	else
		printf("ID: %d\n", getCommandId("move"));
 
	getchar();
	return 0;
}
#include <ctype.h>
#pragma warning(disable: 4996)


void strToUpper(char *str)
{
	int i = 0;
	while(str[i] != '\0')
	{
		str[i] = toupper(str[i]); //reset bit 5
        ++i;
	}
}

int getCommandId(char *word)
{
	FILE *fp;
	char buf[256];
	char *tok;
	int i = 0;

	fp = fopen("commands.txt", "r");
	if(fp == NULL)
		return -1;
	
	strToUpper(word);
    i = 0;
	while(fgets(buf, 255, fp) != NULL)
	{
        strToUpper(buf);
		tok = strtok(buf, " ");
        ++i;
		while(tok != NULL)
		{
			if(strcmp(tok, word) == 0)
				return i;

			tok = strtok(NULL, " ");
		}
	}

	return 0;
}

int main(int argc, char* argv[])
{
	char word[256];
	int id;

	strcpy(word, "move");
	strToUpper(word);
	printf("%s", word);
	id = getCommandId(word);

	if(id == -1)
		perror("Error");
	else if(id == 0)
		printf("No such command.");
	else
		printf("ID: %d\n", getCommandId("move"));

	getchar();
	return 0;
}

The main problem with the program is that main() calls getComandId() twice. In the printf() line, instead of calling getCommandId() again (line 60 of the code I posted) just use the return value from the previous call.

getCommandId() also needs to close the file before exiting, which I did NOT correct.

Last edited by Ancient Dragon; Apr 19th, 2009 at 10:16 pm.

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

Nice one AD!

I did not spot that call tucked away in there.
getCommandId is being called with a string literal (constant),
which it tries to modify in strToUpper:
getCommandId("move"));

nucleon

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

The real reason you're having problems with your code is you're trying to modify a string literal:
>printf("ID: %d\n", getCommandId("move"));

You never allocated space for "move", so who knows what strtok() and your strToUpper() function are trying to do with your computer's memory.

John A

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Apr 19th, 2009

Re: You'd think I would know how to use strings by now, but...

as an aside....

the plan to "reset bit 5" as a method to convert a string to uppercase will make for some fun times if the string ever contains a numeral or other non-alphabetical character.

It is indeed rather "cute".

Last edited by jephthah; Apr 19th, 2009 at 11:48 pm.

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Apr 20th, 2009

Re: You'd think I would know how to use strings by now, but...

Oh wow, I forgot about the second call myself. Jepthah: point taken.

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