segfaulting!

CMakesMeSad :( · 07-08-2009, 03:56 PM

Im getting a segmentation fault when I try to run the program and add new message. As soon as I input the 3 values it segfaults. Any help?

Code:

#include <stdio.h>
#include <string.h>

struct MSG {
	int priority;
	int destination;
	char *message;
	int length;
	struct MSG *nextPtr;
};

typedef struct MSG Message;

int main ()
{
	struct MSG {
		int priority;
		int destination;
		char *message;
		int length;
		struct MSG *nextPtr;
	};

	struct MSG std[100];
	int I;
	int n;
	
	
	
	int choice = 0;


do
{
	printf("1.) Quit\n");
	printf("2.) Transmit next message\n");
	printf("3.) List messages by priority\n");
	printf("4.) Summarize message list\n");
	printf("5.) Add new message\n");
	printf(":");
	scanf("%d", &choice);
	switch(choice)
	{
		case 1:
			return 0;
		case 2:
			//transmit next message
			break;
		case 3:
			//list messages by priority
			break;
		case 4:
			//summarize message list
			break;
		case 5:
			//add new message
		        printf("How many messages would you like to enter?\n");
			scanf("%d", &n);
			printf("Enter a priority, destination, and message:");
			for (I=0; I < n; I++)
			{
				scanf("%d%d%s", &std[i].priority, &std[i].destination, &std[i].message);
			}
			printf("\ntest info:");
			for (I=0; I < n; I++)
			{
				printf("%d%d%s\n", std[i].priority, std[i].destination, std[i].message);
			}
			break;
		default:
			printf("Invalid entry, please try again.\n");
			break;
	}
}while (choice <= 5);

	return 0;
}

ಠ_ಠ · 07-08-2009, 04:11 PM

your message pointer needs to be initialized before stuff can be stored in it

cas · 07-08-2009, 04:15 PM

Your program is essentially doing this:

Code:

char *s;
scanf("%s", &s);

There are two problems there. First: you're trying to write to a location that has no memory allocated. s points nowhere (or rather, its value is indeterminate, but that's not important). You have to allocate space. You can either make s an array (which allocates the chars for you), or use malloc() to set aside some memory.

Second, you're passing &s, which has type "pointer to pointer to char", but %s expects "pointer to char". Though you've been conditioned to use & with scanf(), it's not for use with strings. Instead:

Code:

/* Either of these allocates space */
char *s = malloc(1024); /* should make sure s is not NULL now */
char a[1024];
/* Both of these are now correct */
scanf("%s", s);
scanf("%s", a);

scanf() isn't safe being used like this, but I presume you'd rather learn about that at a later date. scanf() is actually a pain to use properly for most applications.

CMakesMeSad :( · 07-08-2009, 04:24 PM

Thanks for the help guys, I have this code now which freezes after I input the values:

Code:

#include <stdio.h>
#include <string.h>


int main ()
{
	struct MSG {
		int priority;
		int destination;
		char *message;
		int length;
		struct MSG *nextPtr;
	};

	struct MSG std[100];
        char message[100];
	int I;
	int n;
	
	
	
	int choice = 0;


do
{
	printf("1.) Quit\n");
	printf("2.) Transmit next message\n");
	printf("3.) List messages by priority\n");
	printf("4.) Summarize message list\n");
	printf("5.) Add new message\n");
	printf(":");
	scanf("%d", &choice);
	switch(choice)
	{
		case 1:
			return 0;
		case 2:
			//transmit next message
			break;
		case 3:
			//list messages by priority
			break;
		case 4:
			//summarize message list
			break;
		case 5:
			//add new message
		        printf("How many messages would you like to enter?\n");
			scanf("%d", &n);
			printf("Enter a priority, destination, and message:");
			for (I=0; I < n; I++)
			{
				scanf("%d%d%s", &std[i].priority, &std[i].destination, &std[i].message);
			}
			printf("\ntest info:");
			for (I=0; I < n; I++)
			{
				printf("%d%d%s\n", std[i].priority, std[i].destination, std[i].message);
			}
			break;
		default:
			printf("Invalid entry, please try again.\n");
			break;
	}
}while (choice <= 5);

	return 0;
}

CMakesMeSad :( · 07-08-2009, 04:27 PM

Quote:

Originally Posted by cas

scanf() isn't safe being used like this, but I presume you'd rather learn about that at a later date. scanf() is actually a pain to use properly for most applications.

Im here to learn so if you have better alternatives id love to hear them

tabstop · 07-08-2009, 04:41 PM

Quote:

Originally Posted by CMakesMeSad :(

Thanks for the help guys, I have this code now which freezes after I input the values:

Code:

#include <stdio.h>
#include <string.h>


int main ()
{
	struct MSG {
		int priority;
		int destination;
		char *message;
		int length;
		struct MSG *nextPtr;
	};

	struct MSG std[100];
        char message[100];
	int I;
	int n;
	
	
	
	int choice = 0;


do
{
	printf("1.) Quit\n");
	printf("2.) Transmit next message\n");
	printf("3.) List messages by priority\n");
	printf("4.) Summarize message list\n");
	printf("5.) Add new message\n");
	printf(":");
	scanf("%d", &choice);
	switch(choice)
	{
		case 1:
			return 0;
		case 2:
			//transmit next message
			break;
		case 3:
			//list messages by priority
			break;
		case 4:
			//summarize message list
			break;
		case 5:
			//add new message
		        printf("How many messages would you like to enter?\n");
			scanf("%d", &n);
			printf("Enter a priority, destination, and message:");
			for (I=0; I < n; I++)
			{
				scanf("%d%d%s", &std[i].priority, &std[i].destination, &std[i].message);
			}
			printf("\ntest info:");
			for (I=0; I < n; I++)
			{
				printf("%d%d%s\n", std[i].priority, std[i].destination, std[i].message);
			}
			break;
		default:
			printf("Invalid entry, please try again.\n");
			break;
	}
}while (choice <= 5);

	return 0;
}

I and i are not the same thing, although when I quote the code it magically has fixed itself.

What do you do to cause the error, and how far do you get?

CMakesMeSad :( · 07-08-2009, 04:44 PM

Quote:

Originally Posted by tabstop

I and i are not the same thing, although when I quote the code it magically has fixed itself.

What do you do to cause the error, and how far do you get?

I noticed that, in my code they're both "I" but when I post it hear they convert to "i"s for some reason. I get to the point where I add a new message, tell it how many messages I want to add, then as soon as I hit enter after inputing the first message it freezes.

tabstop · 07-08-2009, 04:57 PM

Quote:

Originally Posted by CMakesMeSad :(

I noticed that, in my code they're both "I" but when I post it hear they convert to "i"s for some reason. I get to the point where I add a new message, tell it how many messages I want to add, then as soon as I hit enter after inputing the first message it freezes.

That's not frozen. You said you wanted to input <n> messages; you've got to type <n> messages.

ಠ_ಠ · 07-08-2009, 04:57 PM

Quote:

Originally Posted by CMakesMeSad :(

Im here to learn so if you have better alternatives id love to hear them

fgets

CMakesMeSad :( · 07-08-2009, 05:38 PM

Quote:

Originally Posted by tabstop

That's not frozen. You said you wanted to input <n> messages; you've got to type <n> messages.

I removed everything and just put this:

Code:

printf("Enter priority:\n");
scanf("%d", &std[i].priority);

I still get a segmentation fault as soon as I enter a number

CMakesMeSad :( · 07-08-2009, 05:39 PM

Quote:

Originally Posted by ಠ_ಠ

fgets

Thanks, Ill definitely look into it!

Sebastiani · 07-08-2009, 05:46 PM

>> I still get a segmentation fault as soon as I enter a number

You should post the current version of the code, otherwise we'd just be making wild guesses.

ಠ_ಠ · 07-08-2009, 05:48 PM

Quote:

Originally Posted by CMakesMeSad :(

I removed everything and just put this:

Code:

printf("Enter priority:\n");
scanf("%d", &std[i].priority);

I still get a segmentation fault as soon as I enter a number

If you removed the for loop then you never initialized I

CMakesMeSad :( · 07-08-2009, 05:51 PM

sorry guys, heres what i have now

Code:

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

struct MSG {
	int priority;
	int destination;
	char *message[100];
	int length;
	int is_valid;
};


int main ()
{

	int *my_list, *temp;
	int list_size, choice, position= -1, i;
	list_size=10;
	my_list = malloc(list_size *sizeof(int));
	struct MSG *messages;
	messages = malloc(list_size *sizeof(struct MSG));
	
	//struct MSG std[100]={0};

do
{
	printf("1.) Quit\n");
	printf("2.) Transmit next message\n");
	printf("3.) List messages by priority\n");
	printf("4.) Summarize message list\n");
	printf("5.) Add new message\n");
	printf(":");
	scanf("%d", &choice);
	switch(choice)
	{
		case 1:
			return 0;
		case 2:
			//transmit next message
			break;
		case 3:
			//list messages by priority
			break;
		case 4:
			//summarize message list
			break;
		case 5:
			//add new message
			printf("Enter a priority:\n");
			scanf("%d", &messages[position].priority);
			break;
		default:
			printf("Invalid entry, please try again.\n");
			break;
	}
}while (choice <= 5);

	return 0;
}

ಠ_ಠ · 07-08-2009, 05:54 PM

Quote:

Originally Posted by CMakesMeSad :(

sorry guys, heres what i have now

Code:

position= -1;
printf("Enter a priority:\n");
scanf("%s", &messages[position].message);

... what are you doing

07-08-2009, 04:11 PM	#2
ಠ_ಠ Banned Join Date: Mar 2009 Posts: 533	your message pointer needs to be initialized before stuff can be stored in it __________________ ╔╗╔══╦╗ ║║║╔╗║║ ║╚╣╚╝║╚╗ ╚═╩══╩═╝ Last edited by ಠ_ಠ; 07-08-2009 at 04:14 PM.
ಠ_ಠ is offline

07-08-2009, 04:15 PM	#3
cas Registered User Join Date: Sep 2007 Posts: 343	Your program is essentially doing this: Code: char s; scanf("%s", &s); There are two problems there. First: you're trying to write to a location that has no memory allocated. s points nowhere (or rather, its value is indeterminate, but that's not important). You have to allocate space. You can either make s an array (which allocates the chars for you), or use malloc() to set aside some memory. Second, you're passing &s, which has type "pointer to pointer to char", but %s expects "pointer to char". Though you've been conditioned to use & with scanf(), it's not for use with strings. Instead: Code: / Either of these allocates space / char s = malloc(1024); /* should make sure s is not NULL now / char a[1024]; / Both of these are now correct */ scanf("%s", s); scanf("%s", a); scanf() isn't safe being used like this, but I presume you'd rather learn about that at a later date. scanf() is actually a pain to use properly for most applications.
cas is offline

07-08-2009, 05:46 PM	#12
Sebastiani Guest Join Date: Aug 2001 Posts: 4,895	>> I still get a segmentation fault as soon as I enter a number You should post the current version of the code, otherwise we'd just be making wild guesses.
Sebastiani is offline

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