Struct pointer errors

Question

int d() {return 0;} int i() {return 7;}

struct a { int(*b)(); }c={d};

typedef struct e{ struct a f; }g;

main() { struct e *h; h->f.b = i; }

I am getting segmentation fault when I try to run this program. Can anyone justify the reason?

And I also tried like

int d() {return 0;} int i() {return 7;}

struct a { int(*b)(); }c={d};

typedef struct e{ struct a f; }g;

main() { struct e *h; h = (g)malloc(sizeof(g)); h->f.b = i; }

Now I am getting errors like

funptrinstrct.c: In function `main': funptrinstrct.c:17: error: conversion to non-scalar type requested

Answer for this also would be appreciable.

paxdiablo · Accepted Answer · 2010-08-20 07:15:12Z

5

For the first question, you create a pointer h without initialising it, then you immediately try to dereference it with h->f.b.

For the second one, you should be casting to g*, not g:

#include <stdio.h>

int d (void) { return 0; }
int i (void) { return 7; }

struct a { int(*b)(void); } c = {d};
typedef struct e { struct a f; } g;

int main (void) {
    struct e *h = (g*)malloc (sizeof (g));
    h->f.b = i;
    printf ("%d\n", h->f.b());
}

That's because g is a structure, not a pointer to a structure. The code above outputs 7 as expected.

edited Aug 20, 2010 at 7:15

answered Aug 20, 2010 at 7:09

paxdiablo

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Comments

Bwmat · Accepted Answer · 2010-08-20 07:07:59Z

0

h is an uninitialized pointer.

Trying to dereference it is a big no-no

answered Aug 20, 2010 at 7:07

Bwmat

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Comments

Mattias Holm · Accepted Answer · 2010-08-20 07:08:42Z

0

struct e *h is an undefined pointer, either declare it on the stack by dropping the '*' or allocate it with malloc.

answered Aug 20, 2010 at 7:08

Mattias Holm

Comments

Alam · Accepted Answer · 2010-08-20 07:14:40Z

0

change your main body to

struct e *h = (struct e *)malloc(sizeof(struct e)); 
h->f.b = i;

It will work. moral:

Never ignore warnings

.

answered Aug 20, 2010 at 7:14

Alam

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3 Comments

paxdiablo Over a year ago

You can still use g just fine, the problem was simply that it wasn't a pointer being cast to. And your moral (however condescending) is irrelevant since it was an error, not a warning. Hard to ignore something that won't give you executable code :-)

Alam Over a year ago

For first case it gives warning

paxdiablo Over a year ago

Right, that's me sorted then. Apologies for that :-)

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