segfaulting!

This is a discussion on segfaulting! within the C Programming forums, part of the General Programming Boards category; Im getting a segmentation fault when I try to run the program and add new message. As soon as I ...

  1. #1
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    segfaulting!

    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;
    }

  2. #2
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    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.
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  3. #3
    cas
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    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.

  4. #4
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    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;
    }

  5. #5
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    Quote Originally Posted by cas View Post
    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

  6. #6
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by CMakesMeSad :( View Post
    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?

  7. #7
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    Quote Originally Posted by tabstop View Post
    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.

  8. #8
    and the Hat of Guessing tabstop's Avatar
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    Quote Originally Posted by CMakesMeSad :( View Post
    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.

  9. #9
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    Quote Originally Posted by CMakesMeSad :( View Post
    Im here to learn so if you have better alternatives id love to hear them
    fgets
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  10. #10
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    Quote Originally Posted by tabstop View Post
    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

  11. #11
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    Quote Originally Posted by ಠ_ಠ View Post
    fgets
    Thanks, Ill definitely look into it!

  12. #12
    Guest Sebastiani's Avatar
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    >> 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.
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  13. #13
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    Quote Originally Posted by CMakesMeSad :( View Post
    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
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  14. #14
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    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;
    }
    Last edited by CMakesMeSad :(; 07-08-2009 at 05:59 PM.

  15. #15
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    Quote Originally Posted by CMakesMeSad :( View Post
    sorry guys, heres what i have now

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