I have the following problem:

for an assignment I am supposed to read a txt file line by line, make every line an element in a linked list and then print them out in the correct order.

If I understand the debugger correctly, I am already achieving this but my output looks like a mess. I suspected the nulltermination to be wrong, but that doesn't seem to be the case here (see further down). Picture of the output

For readability I haven't written out the whole code, just what I think is the important part of the program.

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

typedef struct node{
    struct node *p;
    char* string;
}node;

char *readline(FILE *stream)
//returns reference to string, returns NULL if EOF is reached before any char was read.
//If a line is EOF terminated it will return the string to the point where EOF was reached.
{
    char *string = malloc(sizeof(char));
    char c;
    int len = 0;
    if((c = getc(stream)) == EOF) return NULL;
    do{
        *(string   len) = c;
        len  ;
        realloc(string, (sizeof(char)*(len) 1));
        if(c == '\n') break;
    }while((c = getc(stream)) != EOF);
    string[len] = '\0'; //make it nullterminated
    return string;
}

node *new_node(FILE* stream)
// creates a new node from a line read by the readline() function.
{
    node *new = malloc(sizeof(node));
    new->string = readline(stream);
    new->p = NULL;
    return new;
}

void tail(node *newnode, node *head)
{
    node *tmp = head;
    while(tmp->p != NULL)
    {
        tmp = tmp->p;
    }
    tmp->p = newnode;
}

int main()
{
    FILE *moby = fopen("testtxt.txt", "r");
    if(moby == NULL) perror("FILE ERROR");
    node *head = new_node(moby);
    node *tmp;

    //constructing the list
    while((tmp = new_node(moby))->string != NULL)
    {
        tail(tmp, head);
    }

    //printing the list
    while(1){
        printf("%s", head->string);
        if(head->p == NULL) break; //end of linked list is reached
        head = head->p;
    }

    fclose(moby);
    return 0;
}

Especially the following part seems to have an issue with the way I constructed my 'strings':

while(1){
        printf("%s", head->string);
        if(head->p == NULL) break; //end of linked list is reached
        head = head->p;
    }

My understanding is that in order for printf to work with the %s modifier, the string must be nullterminated. I think I achieved that with the string[len] = '\0'; and from what the debugger is showing me, the way I implemented it is working. Screenshot of the debug. Also I let the debugger interpret it as an array so that I could see if the '\0' character is in the position I expect it to be, which is the case. Screenshot of the debug.

Now I am kind of at a loss since I can't find other examples of this problem and the debugger seems to show me that I am producing the right data.

I hope I just missed a minor detail that someone is able to spot.

TIA

(I am using eclipse and the output is consistent between win 10 console and eclipse console)

CodePudding user response：

There may be other problems but your use of realloc is wrong.

Here

realloc(string, (sizeof(char)*(len) 1));

you need to save the return value because that's where the allocated memory is.

Like

char* temp = realloc(string, (sizeof(char)*(len) 1));
if (temp == NULL)
{
    // add error handling
}
string = temp;

Unrelated to the described problem:

When you want to add new elements to the end of the list all the time, it's typically a good idea to have a tail pointer that points to the last element. That will make inserts much faster as you don't need the while(tmp->p != NULL) loop.