**ptr and ptr or *ptr and &ptr?

**assiduus** · 11-06-2018

Ehlo.

I came back to programming in C after more than a few years and there's a piece of code I'm not sure I understand correctly. It's a couple of lines of code regarding libpcap. What it does it runs a pcap function to get the list of available devices:

Code:

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

int main(void) {

        char ebuf[PCAP_ERRBUF_SIZE];
        pcap_if_t *devs;

        pcap_findalldevs(&devs, ebuf);

        exit(EXIT_SUCCESS);
}

This is the way it's supposed to be done. However, when I first saw this function's prototype:

int pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf);

I assumed that this code would work as well:

Code:

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

int main(void) {

        char ebuf[PCAP_ERRBUF_SIZE];
        pcap_if_t **devs;

        pcap_findalldevs(devs, ebuf);

        exit(EXIT_SUCCESS);
}

It does compile but segfaults on execution. I think I know why but I don't want to suggest any particular point of view so, if you know the answer then....well, answer. :)

Cheers.

**Salem** · 11-06-2018

Try
pcap_if_t *devs;
pcap_findalldevs(&devs, ebuf);

**assiduus** · 11-06-2018

Yup. That's the way it's supposed to be and that's the first piece of code I quoted.

I just wonder why the second piece of code seqfaults.

**Salem** · 11-06-2018

Because the function expects to WRITE a pointer to the address you supply.

**assiduus** · 11-06-2018

So what happens memory-wise when you supply it with a dereferenced pointer to a pointer? I mean, doesn't you supply it with a memory address anyway? When it comes to a program's command line arguments you can use either **argv or *argv[]. Why can't you use **ptr (and then dereference it) instead of *ptr (and then $ptr)?

Code:

/usr/local/include/pcap/pcap.h:495:14: note: expected ‘pcap_if_t ** {aka struct pcap_if **}’

so I give it a **ptr, right?

I know that there is a difference between

Code:

char *ptr = "hello!";

and

Code:

ptr[7] = { h, e, l, l, o, !, \0 };

in that you can't access individual characters of "hello!" using *ptr = "string".

What's the case here?

Cheers.

**Salem** · 11-06-2018

Do you understand this?

Code:

void getUniversalAnswer ( int *a ) {
    *a = 42;
}
int main ( ) {
   int foo;
   getUniversalAnswer(&foo);
}

Do you further understand that this is the same thing.

Code:

void getUniversalAnswer ( int *a ) {
    *a = 42;
}
int main ( ) {
   int foo;
   int *ptr = &foo;
   getUniversalAnswer(ptr);
}

Do you then understand why this is bad?

Code:

void getUniversalAnswer ( int *a ) {
    *a = 42;
}
int main ( ) {
   // null makes it fail, leaving it uninitialised for randomness
   int *ptr = NULL;
   getUniversalAnswer(ptr);
}

**assiduus** · 11-06-2018

Yup. That's what I suspected happened. I just wanted a confirmation. Thanks.

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