CODEWITHSUNDEEP
cparentheses
maximilliano
maximilliano

Reputation: 163

parentheses in defining a pointer C programming

I am a newbie in C progrmming and I have looked at the following code:

struct iphdr *iph = (struct iphdr *)Buffer;

What does the expression mean?

here is the link to the code http://www.binarytides.com/packet-sniffer-code-c-linux/

Upvotes: 0

Views: 722

Answers (2)

Havenard
Havenard

Reputation: 27854

buffer is a pointer to a unsigned char array that is being used to storage the raw packet data.

This is recasting the type of buffer to be unsderstood as a pointer to struct iphdr, so you can assign it to iph and read it as a structure. Technically this is just supressing the compiler error that would say that the value of buffer cannot be assigned to iph because they are of different types, despite the fact they are both pointers.

Alternatively, you can make buffer of type void* instead.

A pointer to void can be assigned to any other pointer type without need to recast it's type:

void* buffer = malloc(65536);
...
void ProcessPacket(void* buffer, int size) {
    struct iphdr *iph = buffer;
    ...
}

Upvotes: 0

Arkku
Arkku

Reputation: 42129

It casts buffer to a pointer to struct iphdr and then initializes iph to that pointer. This is used because buffer is a pointer to a buffer of raw bytes, but in this function it is known that the bytes stored in the buffer follow the format of a struct iphdr. Hence the struct iphdr can be used to access the contents of the buffer, rather than having to interpret and manipulate the raw bytes.

edit: To clarify (as per comments): The cast does not copy or convert anything. It is basically just telling the compiler “I know that buffer is supposed to contain unsigned chars but at this particular time those bytes are actually a struct iphdr so let me access them through the pointer iph in a more convenient way”.

Upvotes: 2

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