Segmentation fault while referencing and adding values to a structure

Question

Okay, so the problem concerns adding values through function to structure. Honestly, I couldn't solve the problem (spent a lot of time trying), so I am asking for your help. While executing the program, I get a segmentation fault. It occurs while using the variables from stack stos.

typedef struct e {
    int zaglebienie[100];
    char *nazwa_funkcji[100];
    int poz;
} *stack;

void put_on_fun_stack(int par_level, char *funame, stack stos) {
    int i = stos->poz;
    stos->zaglebienie[i] = par_level;
    char *funkcja = strdup(funame);
    stos->nazwa_funkcji[i] = funkcja;
    stos->poz++;
}

int main() {
    char *p = "makro";
    stack stos;
    stos->zaglebienie[0] = 0;
    put_on_fun_stack(1, p, stos);
    return 0;
}

*Never ever typedef pointers! You eventually run into trouble sooner or later. In your case it was sooner. — too honest for this site
– too honest for this site, Commented Dec 8, 2016 at 15:40
Translation to the above: stos is an non-allocated pointer. Yes, the typedefed pointer have confused me too. — Eugene Sh.
– Eugene Sh., Commented Dec 8, 2016 at 15:42
@Olaf what do you mean exactly? Same problem occures while typedefing ...}stack and changing all the "->" to ".". I dont really get the point of your answers. — chrislacorunna
– chrislacorunna, Commented Dec 8, 2016 at 15:44
It's not the "same problem" for sure. Right now you are dereferencing some uninitialized and unallocated pointer. — Eugene Sh.
– Eugene Sh., Commented Dec 8, 2016 at 15:47

yLaguardia · Accepted Answer · 2016-12-08 17:55:43Z

1

You're declaring a pointer to stack but you're not allocating any memory to it.

And as already mentioned in the comments, using typedef with with a pointer will unnecessarily complicate your life.

So I suggest you create the struct stack and then in main declare a pointer to stack and allocate memory for it, somewhat like this:

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

typedef struct e {

    int zaglebienie[100];
    char *nazwa_funkcji[100];
    int poz;

} stack;


void put_on_fun_stack(int par_level, char *funame, stack *stos)
{
    int i = stos->poz;
    stos->zaglebienie[i] = par_level;
    char *funkcja = strdup(funame);
    stos->nazwa_funkcji[i] = funkcja;
    stos->poz++;
}

int main(void)
{
    char *p = "makro";

    // calloc to initialize stos variables to 0
    stack *stos = calloc(sizeof(stack), 1);

    printf("stos->poz before: %d\n", stos->poz);
    put_on_fun_stack(1, p, stos);
    printf("stos->poz after: %d\n", stos->poz);
    printf("stos->nazwa_funkcji[0]: %s\n", stos->nazwa_funkcji[0]);


    free(stos->nazwa_funkcji[0]);
    free(stos);
    return 0;
}

Output:

stos->poz before: 0
stos->poz after: 1
stos->nazwa_funkcji[0]: makro

edited Dec 8, 2016 at 17:55

answered Dec 8, 2016 at 15:55

yLaguardia

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7 Comments

chrislacorunna Over a year ago

Thats pretty the thing i wanted to do, but still ive got error : 3[Error] invalid conversion from 'void*' to 'stack* {aka e*}' [-fpermissive] in Dev-C++ or Initializer element is not constant in VIM

yLaguardia Over a year ago

I've answered in C (because of the C tag in your question). In C++ you have to explicitly cast the return value of malloc & co to the type you want. So try stack *stos = (stack*) calloc(sizeof(stack), 1);

chrislacorunna Over a year ago

I meant ive used C++ compiler from Dev and C compiler from VIM (gcc/cc command)

chrislacorunna Over a year ago

Still, there is an error "initializer element is not constant stack stos = (stack) calloc(sizeof(stack), 1); ^ Btw. Im using pointer to a structure and cant change it, cause thats the part of project.

chqrlie Over a year ago

@chrislacorunna: you did not make the changes suggested by yLaguardia. You must change your typedef to typedef struct e stack;. It is bad style to hide pointers behind typedefs, it is error prone and leads to confusing code for both the programmer and the reader.

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