C Segmentation fault: 11 error

Question

I am rather new to C and although this code compiles (using gcc -Wall -o test test.c), running ./test just results in the output dbecher$: Segmentation fault: 11. Does anybody know what might cause this?

Well, I know instead of using a pointer I could just assign a normal struct, but I am using this to test some behavior of pointers. So I want to keep using a pointer variable for that struct.

Here the code snippet:

#include <stdio.h>

typedef struct HumidityMessage {
  int nodeId;
  int sequenceNumber;
  int humidity;
} HumidityMessage;

HumidityMessage* packet;

int main() {
    packet->nodeId = 0;
    packet->sequenceNumber = 1;
    packet->humidity = 3;

    printf("This is the address of packet: %d", packet->nodeId);
}

Global variables are initialized to 0. Hence, packet is a null pointer. And then you are trying to dereference it. — The Paramagnetic Croissant
– The Paramagnetic Croissant, Commented Apr 24, 2015 at 12:32

Bob · Accepted Answer · 2015-04-24 12:33:08Z

4

Segmentation faults are caused by invalid memory accesses. In this case you're getting it because you're dereferencing the packet pointer without first allocating space for it.

answered Apr 24, 2015 at 12:33

Bob

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Comments

Sadique · Accepted Answer · 2015-04-24 12:32:17Z

2

HumidityMessage* packet; is pointing to nothing. Initialize it:

HumidityMessage* packet = malloc(sizeof(HumidityMessage));

answered Apr 24, 2015 at 12:32

Sadique

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4 Comments

Quentin Over a year ago

No, it's global so it's initialized to 0.

The Paramagnetic Croissant Over a year ago

it's not pointing to garbage. it's NULL because it's global.

Sadique Over a year ago

Actually that is what i wrote before - then got confused then corrected myself again lol.

DeBe Over a year ago

Ah! of course, I totally forgot that in the beginning the pointer is pointing pretty much to nothing!

Bathsheba · Accepted Answer · 2015-04-24 12:34:42Z

1

Don't use a pointer. Use HumidityMessage packet; instead.

Currently packet is initialised to nullptr and it's illegal to dereference that.

Alternatively you could retain the pointer and allocate memory for it before you need it, but finding a place for the corresponding free is problematic for globals.

answered Apr 24, 2015 at 12:34

Bathsheba

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Comments

Rndp13 · Accepted Answer · 2015-04-24 12:44:03Z

0

Allocate memory to the packet pointer using the c inbuilt malloc function. Packet = (HumidityMessage * )malloc(size of(HumidityMessage)); Or you can directly take a variable of the structure and assign the value.

answered Apr 24, 2015 at 12:44

Rndp13

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1 Comment

user3629249 Over a year ago

in C, do not cast the returned value from malloc() (and family of function)

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