segfaulting!

**CMakesMeSad :(** · 07-08-2009

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

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

**cas** · 07-08-2009

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

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;
}

**tabstop** · 07-08-2009

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

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

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.

**CMakesMeSad :(** · 07-08-2009

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

**ಠ_ಠ** · 07-08-2009

Originally Posted by CMakesMeSad :(

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

fgets

**CMakesMeSad :(** · 07-08-2009

Originally Posted by ಠ_ಠ

fgets

Thanks, Ill definitely look into it!

**Sebastiani** · 07-08-2009

>> 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** · 07-08-2009

I would recommend starting with a fixed-size structure and no dynamic allocation first, until you have got everything else worked out.

**CMakesMeSad :(** · 07-08-2009

Hey everyone, Ive been working on this all day and have made some good progress, however Im stuck again. Im working on the option of listing the messages by priority. However, no matter what priority I enter, it will only list the first message i enter.

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX 100

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

typedef struct MSG message;


void add(struct MSG **n, int priority1, int destination1, char *message1);

int main ()
{
	struct MSG *temp;
	int choice;
	int num1, num2;
	char text[MAX];
 		
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
			if (node==NULL)
			{
				printf("No messages left!\n");
				break;
			}
			
			else
			{
				temp=node;
				while(temp != NULL)
				{
					printf("%d", temp -> priority);
					printf("%dn", temp -> destination);
					char *loc=(char *) malloc (strlen(temp -> message));
					strcpy(loc, temp -> message);
					printf("%s", loc);
					free(loc);
					temp=temp -> nextPtr;
				}
				printf("test=%d %d %s\n", t-> priority, t-> destination, t-> message);
			}


			break;
		case 3:
			//list messages by priority
				printf("Enter priority of the messages you would like to retrieve:\n");
				scanf("%d", &pnum);
				
				temp=node;
				while(temp != NULL)
				{
					//printf("%d", temp -> priority);
					//printf("%dn", temp -> destination);
					char *loc=(char *) malloc (strlen(temp -> message));
					strcpy(loc, temp -> message);
					//printf("%s", loc);
					free(loc);
					temp=temp -> nextPtr;
					if (t -> priority == pnum)
					{
						printf("%d %d %s\n", t -> priority, t -> destination, t -> message);
					}
				}


			break;
		case 4:
			//summarize message list
			break;
		case 5:
			//add new message
			printf("Enter priority, destination and message:");
			scanf("%d%d", &num1, &num2);
			fgets(text, MAX, stdin);
			add(&node, num1, num2, text);	
			break;
		default:
			printf("Invalid entry, please try again.\n");
			break;
	}
}while (choice <= 5);

	return 0;
}





void add(struct MSG **n, int priority1, int destination1, char *message1)
{
	struct MSG *temp, *node1;
	
	if(*n==NULL)
	{
		temp=(struct MSG *) malloc (sizeof(struct MSG));
		temp -> priority = priority1;
		temp -> destination=destination1;
		temp -> message=(char *) malloc (strlen(message1)+1);
		strcpy(temp -> message, message1);
		temp -> nextPtr=NULL;
		*n=temp;
		t=temp;
		printf("test=%d %d %s\n", t->priority, t->destination, t->message);
	}

	else
	{
		temp=*n;
		while(temp->nextPtr != NULL)
			temp=temp -> nextPtr;
		node1 = (struct MSG *) malloc(sizeof(struct MSG));
		node1 -> priority=priority1;
		node1 -> destination=destination1;
		node1 -> message=(char*) malloc(strlen(message1)+1);
		strcpy(node1 -> message, message1);
		node1 -> nextPtr = NULL;
		temp -> nextPtr = node1;
	}

}

**quzah** · 07-08-2009

You aren't printing anything other than the exact priority match. Is that what you want?

Quzah.

**CMakesMeSad :(** · 07-08-2009

Originally Posted by quzah

You aren't printing anything other than the exact priority match. Is that what you want?

Quzah.

Thats kind of what I want it do. Lets say input the following 4 messages:
1 5 message1
5 10 message2
1 6 message3
2 7 message4

The first number being the priority, I want to be able to list all the messages that fall under a priority of my choice. So if I pick priority 1, I want the program to ouput

1 5 message1
1 6 message3

This is what its doing right now:
*******************
input priority: 5 or 2
nothing happens

input priority 1
1 5 message1
1 5 message1
1 5 message1
1 5 message1
*******************
I hope that makes sense.

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