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I need to use IP address of the eth0 interface of a PC (Linux OS - IPv4). How can I define it as a constant variable without knowing the IP address in advance?

johan
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  • What OS? You'll need to make a system call to look it up if you don't know it *a priori*. – Flexo May 13 '12 at 13:08
  • Language itself has nothing to do with it. You either need a library that would be more or less portable, or simply a system-call. – ScarletAmaranth May 13 '12 at 13:10
  • This question is very ambiguous. You have not provided sufficient info. What it will be used for? You can store it as `unsigned char[4]` (or 6 in case of IPv6), as a string, in the format XXX.XXX.XXX.XXX, as one of the BSD sockets' host struct types (struct inaddr etc.)... !? –  May 13 '12 at 13:10
  • http://stackoverflow.com/questions/2021549/get-ip-address-in-c-language -> Similar. – ScarletAmaranth May 13 '12 at 13:11
  • I think what's more important question is, why do you need it to be const ? – Roee Gavirel May 13 '12 at 13:22

2 Answers2

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You cannot.

There are two ways to define a constant quantity in C: the preprocessor (#define) and constants (const something). Both of these require that the value of the constant be known at compile time¹. So if your aim was to have a way to refer to this IP without having the possibility of overwriting it with another value during program execution, it's simply not doable.

Jon
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2

If you have them as pointer to an address buffer you can. Export a pointer to const qualified type

extern your_address_type const*const addr;

You'd have to have your library code see something like 

static your_address_type my_address;
your_address_type const*const addr = &my_address;

Inside your library you can then initialize my_address properly, client code only would see addr and wouldn't be able to change it.

Jens Gustedt
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