need help building a linked list

**supern00b** · 12-10-2009

i'm trying to build a linked list from an input txt file.

example input file (note: the actual file has more parameters):

NAME: John
NUMBER: 3

NAME: Fred
NUMBER: 4

etc...

i wrote a bunch of code, but i kept getting bus error. i edited the heck out of it and settled for trying to just get the name to read, and i still got a bus error.

any ideas?

Code:

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

typedef struct people_node{
    char *first_name;
}people;


int main(int argc,char *argv[])
{
    FILE *community;
    people *current;
    char line[100], *token;

    community = fopen(argv[1], "r");
    current = malloc(sizeof(people));
    while(fgets(line,100,community)!=NULL){
        token = strtok(line," ");
        token = strtok(NULL,"\n");
        strcpy(current->first_name, token);
    }
    printf("%s\n", current->first_name);
    
    return 0;
}

the argc, argv thing is because i have another input file for other purposes...

**strange** · 12-10-2009

Technically, a linked list should probably have at least a next pointer, so like . . .

Code:

struct linked_list_node_t {
    /* data elements . . . */
    struct linked_list_node_t *next;
}

Does that make sense? Otherwise, you're just using a straight structure to store data.

NOTE: I may be misunderstanding you here. I apologize if I am.

**cas** · 12-10-2009

What you're doing is the equivalent of this:

Code:

char *first_name;
strcpy(first_name, "bob");

See the problem? You're not allocating any memory for the string, so you wind up copying to some random address.

“Ah hah,” you say, “but I used malloc().”

Correct. But you only allocated memory for the struct. One part of the struct is a pointer, so you allocated memory for the pointer; but not what the pointer points to. You either need to allocate memory for current->first_name, or you could make current->first_name an array of char. For now an array of 100 bytes would work fine because your fgets() will never read more than that; but do be aware of problems with writing more to an array (or, for that matter, malloc()'d memory) than you have space for.

When you get a bus error (or something similar like a segfault), your first step should be to use a debugger. I recommend Valgrind if it supports your platform. gdb is useful, too.

**supern00b** · 12-10-2009

so like this?

Code:

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

typedef struct people_node{
    char *first_name;
}people;


int main(int argc,char *argv[])
{
    FILE *community;
    people *current;
    char line[100], *token;

    community = fopen(argv[1], "r");
    current = malloc(sizeof(people));
    while(fgets(line,100,community)!=NULL){
        token = strtok(line," ");
        token = strtok(NULL,"\n");

        current->first_name = (char *)malloc((strlen(token)+1)*sizeof(char)); 

        strcpy(current->first_name, token);
    }
    printf("%s\n", current->first_name);
    
    return 0;
}

i had something like this in another iteration of the code, and i still got a bus error. i am using code::blocks right now, but i don't know for the life of me how to debug with it, but that's for another time.

P.S. yeah the thread name is a little misleading. sorry about that. during my edits, i forgot that i had taken the pointer out. eventually i want include a linked list to link entries together.

**quzah** · 12-10-2009

Better. You keep allocating new memory in your loop, but you're not keeping track of the old memory, so you're ending up with a memory leak. Plus you never free anything at the end either.

Quzah.

**supern00b** · 12-11-2009

so if i put the linked list part back in, like fill in first_name for one link, then go to the next one, would that fix it? then free the space after everything else?

**iMalc** · 12-11-2009

Originally Posted by sufoode

#include <list>

That's absurd! For one this is the C forum, not the C++ one, and secondly it is obvious that this is some kind of learning exercize.

supern00b: You would be better off using some linked-list tutorial at first. There are plenty of them around. Cprogramming.com Tutorial: Linked Lists
If you get stuck while going through that, then ask your questions here.
Otherwise we'd only be duplicating here what is already written down.

**laserlight** · 12-11-2009

Originally Posted by sufoode

Why don't you upgrade from that archaic language?

Stop trolling. This is the C programming forum.

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