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

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){
...

...

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.

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


Quzah.

Originally Posted by quzah

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

....edit

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.

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.

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.