Btukfyl

This is one of my first questions so I hope I formulate it well. I checked some other similar questions on StackOverflow but I got no real answers from them. I have the following struct:

struct process {

int priority;

char* path;

char* parameters;

struct process *next;

};

I am reading each line from a file and adding the string tokens I get in my struct linked list using a while loop. Here is the adding method:

struct process * add(int prio,char* pat, char* par,  struct process *head) {

  struct process *new_node;

  new_node = ( struct process *) malloc(sizeof( struct process));

  new_node->priority = prio;

  new_node->path = pat;

  new_node->parameters = par;

  new_node->next= head;

  head = new_node;

  return head;

}

So the main algorithm gets the lines from a file using fgets in a while loop :

while (fgets(line, sizeof(line), file))

then I tokenize all the strings I need and use the add method to add them to my linked list. I convert the first string to int to respect the types.

This is my while loop and main algo :

        FILE *file = fopen(filename , "r");

    char line[124];

    // can be put outside and passed as argument.

    struct process *head = ( struct process *)malloc(sizeof(struct process));

    new_node->priority = prio;

    new_node->path = pat;

    new_node->parameters = par;

    new_node->next= head;

    char* priority,*path,*parameters;



    while (fgets(line, sizeof(line), file)) {

         priority=strtok(line," ");             

         // The fix here is the following :

         char *path_token = strtok(NULL, " ");

         path = malloc(strlen(path_token) + 1); 

         strcpy(path, path_token);

         char *par_token = strtok(NULL, "n");

         parameters = malloc(strlen(par_token) + 1);

         strcpy(parameters, par_token);

         // End of edit Thanks to the answers bellow

         char* junk;

         int p = strtol(priority,&junk,10);

         printf("prio : %d  ",p);

         head = add(p, path, parameters,head);

         printf("n");

         pront(head);

         printf("n");

    }

    \ free the mallocs

We notice here that I use the pront() method to print my linked list at every step. I also do it after my while loop. Here is the code for pront():

void pront(struct process *head) {

struct process *current_node = head;

while ( current_node != NULL) {

    printf("%d , %s , %s ", current_node->priority, current_node->path, current_node->parameters);

    printf("n");

    current_node = current_node->next;

}

}

the method prints nonsense :

prio : 2  

2 , ./printchars , a 12 b 



prio : 15  

15 , ./printchars , c 23 d 

2 ,  ,  



prio : 7  

7 , ./printchars , e 34 f 

15 , /printchars ,  34 f 

2 , ./printchars , e 34 f 

it is supposed to print :

7 , ./printchars , e 34 f 

15 , ./printchars , c 23 d 

2 , ./printchars , a 12 b 

I am sure that the problem comes from the while loop since when using the adding method outside of the loop and then printing, I get valid results. But as soon as it gets in the loop, the Head linked list doesn't store the values properly!
Thanks for the help

EDIT : The issue is fixed and the issue was my awfully wrong use of strtok and pointers to char's without malloc. Lesson learned !

Your linked list does not store the values at all.

The issue isn't the loop, but strtok - you are using it incorrectly: it does not allocate memory to save the tokens it parses, but instead uses a single internal static buffer.

So, to save the values, you must allocate memory for your strings (path and parameters) and use strcpy to copy the strings to the allocated memory.
Remember to free each item fields before freeing list items!

Example:

char *path_token = strtok(NULL, " ");

path = malloc(strlen(path_token) + 1); /* remember to allocate space for  terminator! */

strcpy(path, path_token));

Issue is with below code.

new_node->priority = prio;

new_node->path = pat;

new_node->parameters = par;

Your structure has a character pointer for path and parameters

What you are doing here is just assigning the pointer in the structure to the pointer in the function, passed. Later on when pat or par changes value or has some garbage value, the structure element will also have garbage value.

What you need to do is to allocate memory for each element and strcpy the data

Additionally, as pointed out by @Ruks -

struct process *head = ( struct process *)malloc(sizeof(struct process)); 

head = NULL;

is wrong. You are loosing the pointer returned by malloc. If you want to ensure that the linked list is empty initially, you should initialise all elements of head.

head-> priority = 0; 

head-> path = NULL;

head-> parameters = NULL;

head-> next = NULL;