struct, string, strcmp(), and "segmentation fault"

Roger · 11-07-2009, 08:38 PM

I'm getting a segmentation fault on the last line noted below. I'm comparing two strings.

Code:

struct node {
   char name[25];
   char address[65];
   float amount;
   struct node * next;
};
typedef struct node item;
item * curr, * head, * tail, * new;


void insert(char *name_, char *address_, float amount_){
	
	new = (item *)malloc(sizeof(item));
	
	strcpy (new->name, name_ );
	strcpy (new->address, address_ );
    	new->amount = amount_;

........................

	 printf("%s, ", new->name); printf("%s\n, ", head->name);
         //the above print statements where put in as tests. They print two names.   
         //But, the very next line below produces an error: "segmentation fault"
	if (strcmp(new->name, head->name) < 0){
...

...

quzah · 11-07-2009, 08:56 PM

Make a small working example.

Code:

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
struct foo
{
    char name[BUFSIZ];
};
int main( void )
{
    struct foo *a, *b;
    a = malloc( sizeof *a );
    b = malloc( sizeof *b );
    if( a && b )
    {
        strcpy( a->name, "hello" );
        strcpy( b->name, "world" );

        if( strcmp( a->name, b->name ) == 0 )
        {
            printf( "\'%s\' is the same as \'%s\'\n", a->name, b->name );
        }
        else
        {
            printf( "\'%s\' is not the same as \'%s\'\n", a->name, b->name );
        }
    }
    free( a );
    free( b );

    return 0;
}

Quzah.

nadroj · 11-07-2009, 08:59 PM

Code:

item * curr, * head, * tail, * new;
// ...
	 printf("%s, ", new->name); printf("%s\n, ", head->name);
         //the above print statements where put in as tests. They print two names.   
         //But, the very next line below produces an error: "segmentation fault"
	if (strcmp(new->name, head->name) < 0){

Do you assign anything to "head"? From your code snippet, "head" is never set to anything, so youll get a problem when you access "head->name". Does it print "(null)" for the name of "head->name"?

Also, use "strncpy" instead of "strcpy" as to prevent overflows: strncpy - C++ Reference

Roger · 11-07-2009, 09:07 PM

nadroj, head is NOT null ("John" and "Paul" is printed in my example), but I still get a segmentation fault on the next line.

quzah · 11-07-2009, 09:08 PM

Show the whole function, not just the parts you feel like showing.

Quzah.

Roger · 11-07-2009, 09:12 PM

Quote:

Originally Posted by quzah

Show the whole function, not just the parts you feel like showing.

....edit

quzah · 11-07-2009, 09:22 PM

You aren't assigning new->prev to anything. You need to always be assigning all pointers for a structure. Your first if doesn't assign prev.

Quzah.

Roger · 11-07-2009, 09:33 PM

Quzah,
I don't understand why this works (which I put in just for testing):

Code:

printf("%d\n", strcmp(new->name, head->name));

and this very next line prints a "sementation fault" error:

Code:

if (strcmp(new->name, head->name) < 0){..

.

quzah · 11-07-2009, 09:50 PM

It doesn't really matter why one works and another doesn't. You're not setting your pointers up right, so if it "works", it's just you getting lucky.

Code:

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

struct node {
   char name[30];
   char address[65];
   float amount;
   struct node * next;
   struct node * prev;
};
typedef struct node item;
item * curr, * head, * tail, * new;

void insert(char *name_, char *address_, float amount_){
    if ( (new = malloc( sizeof *new )) == NULL )
        return;
    strcpy( new->name, name_ );
    strcpy( new->address, address_ );
    new->amount = amount_;
    new->next = new->prev = NULL;

    if( head == NULL )
    {
        new-> prev = new->next = new;  /* I assume you want this circular. */
        head = tail = new;
    }
    else
    if( strcmp( new->name, head->name ) < 0 )
    {
        new->prev = head->prev;
        head->prev = new;
        new->next = head;
    }
    else
    if( strcmp( new->name, head->name ) > 0 )
    {
        new->next = head->next;
        new->prev = head;
        head->next = new;
    }
    else
    {
        free( new );
    }
}


int main() {
	insert("John", "12 main" , 112.2);
	insert("Paul", "14 oak", 200.5);
	
	curr = head;
	while(curr) {
		printf("%s, ", curr->name);
		printf("%s, ", curr->address);
		printf("%f\n", curr->amount);
		curr = curr->next;
   }

   return 0;
}

Quzah.

Roger · 11-07-2009, 10:29 PM

I appreciate your help, but it's not supposed to be circular,,, the reason for the "tail" and "previous" is to run backwards through the list starting at the tail; i.e., the list can be printed forwards from the head or backwards from the tail.

I'm trying to follow your code, but 1) I'm still a newb, and 2) getting tired and so I got confused with this line too:

Code:

if ( (new = malloc( sizeof *new )) == NULL )
        return;

quzah · 11-07-2009, 10:45 PM

It allocates the size of the structure, and if malloc fails, returns from the function. If you don't want it to be circular, remove the line that mentions it being circular.

Quzah.

11-07-2009, 08:59 PM	#3
nadroj Registered User Join Date: Oct 2006 Location: Canada Posts: 848	Code: item * curr, * head, * tail, * new; // ... printf("%s, ", new->name); printf("%s\n, ", head->name); //the above print statements where put in as tests. They print two names. //But, the very next line below produces an error: "segmentation fault" if (strcmp(new->name, head->name) < 0){ Do you assign anything to "head"? From your code snippet, "head" is never set to anything, so youll get a problem when you access "head->name". Does it print "(null)" for the name of "head->name"? Also, use "strncpy" instead of "strcpy" as to prevent overflows: strncpy - C++ Reference
nadroj is offline

11-07-2009, 09:07 PM	#4
Roger Registered User Join Date: Oct 2009 Posts: 22	nadroj, head is NOT null ("John" and "Paul" is printed in my example), but I still get a segmentation fault on the next line.
Roger is offline

11-07-2009, 09:08 PM	#5
quzah +++ OK NO CARRIER Join Date: Oct 2001 Posts: 10,262	Show the whole function, not just the parts you feel like showing. Quzah. __________________ Hundreds of thousands of dipshits can't be wrong. Are you up for the suck?
quzah is offline

11-07-2009, 09:22 PM	#7
quzah +++ OK NO CARRIER Join Date: Oct 2001 Posts: 10,262	You aren't assigning new->prev to anything. You need to always be assigning all pointers for a structure. Your first if doesn't assign prev. Quzah. __________________ Hundreds of thousands of dipshits can't be wrong. Are you up for the suck?
quzah is offline

11-07-2009, 09:33 PM	#8
Roger Registered User Join Date: Oct 2009 Posts: 22	Quzah, I don't understand why this works (which I put in just for testing): Code: printf("%d\n", strcmp(new->name, head->name)); and this very next line prints a "sementation fault" error: Code: if (strcmp(new->name, head->name) < 0){.. .
Roger is offline

11-07-2009, 10:29 PM	#10
Roger Registered User Join Date: Oct 2009 Posts: 22	I appreciate your help, but it's not supposed to be circular,,, the reason for the "tail" and "previous" is to run backwards through the list starting at the tail; i.e., the list can be printed forwards from the head or backwards from the tail. I'm trying to follow your code, but 1) I'm still a newb, and 2) getting tired and so I got confused with this line too: Code: if ( (new = malloc( sizeof *new )) == NULL ) return;
Roger is offline

11-07-2009, 10:45 PM	#11
quzah +++ OK NO CARRIER Join Date: Oct 2001 Posts: 10,262	It allocates the size of the structure, and if malloc fails, returns from the function. If you don't want it to be circular, remove the line that mentions it being circular. Quzah. __________________ Hundreds of thousands of dipshits can't be wrong. Are you up for the suck?
quzah is offline

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