Thread: what am i missing now...

I was trying to write a brief example for scarlet00014 ("How do I read this in as a string?") using strtok in a method I've used dozens of times before.

Code:

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

int main (int argc, char *argv[]) {
        short int i=0;
        size_t len;
        char *line=NULL, *tok, DL[]=":\n";
        FILE *fstRO=fopen("/root/test/test.txt", "r");
        struct {char *name;
                int age;
        } *example;

        while((getline(&line,&len,fstRO)) != -1) {
                tok=strtok(line,DL);
                printf("%d: %s=%d\n", i,tok,strlen(tok));
                example[i].name=malloc(strlen(tok)+1);
                strcpy(example[i].name,tok);
                puts(example[i].name);
                tok=strtok(NULL,DL);
                example[i].age=atoi(tok);    
                free(line);
                i++;
        }   
}

except now it causes this:

0: Jamie lynn=10
Segmentation fault

which gdb says is caused by example[i].name=malloc(strlen(tok)+1);
As I was saying, the example, as far as I can tell, is EXACTLY like other routines I've already written that work fine.

Where did I get confused?

Originally Posted by rags_to_riches

You haven't allocated any memory for example[i].

Bang on

> The segmetation fault is caused by trying to free memory (line) that was never allocated.
No it's not. line was allocated by GNU getline()

Even so, free(NULL) is defined to do nothing.

Originally Posted by zacs7

Even so, free(NULL) is defined to do nothing.

I thought that only applies to "delete 0". AFAIK bad things happen when you free a NULL pointer...

QuantumPete

Originally Posted by zacs7

> The segmetation fault is caused by trying to free memory (line) that was never allocated.
No it's not. line was allocated by GNU getline()

Even so, free(NULL) is defined to do nothing.

Yes, free(line) should be after the while block, not in it. But that doesn't cause a segfault for me -- it causes a "double free" error.

The segfault was definately because, as rags_to_riches pointed out, I didn't malloc and realloc memory for an array of structs.

A debugged and functional version of this was posted to scarlet00014's question.

> I thought that only applies to "delete 0". AFAIK bad things happen when you free a NULL pointer...
Not according to the standard, free() checks the pointer is not NULL before it frees. Thus

Code:

if(x != NULL)
    free(x);

Is redundant and silly.

> Yes, free(line) should be after the while block, not in it.
Depends on what effect you want. If you pass a pointer pointing to nothing (ie NULL) getline() will allocate memory for you. Otherwise it will use whatever you give it. However if you wanted to do the former over and over, you'd want to free the line and set it to NULL. So you keep passing in a pointer pointing at NULL.

Well, since in the example execution will never get to that point, it is all kind of theoretical, but I'd still like to know how line ended up as NULL*.

Actually I think it's strange about the double free and having to move the line out of the block...perhaps something to do with the action of strtok.

*b/t/w this is not a question, I'm being facetious