help getting IP address in c code

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For user level programs there should virtually *never* be a need to include from theusr-include-linux directory. But there are some tricks to learn about how to avoid it too. Here is the correct list of includes:

#define GNUSOURCE

The define is needed *only* if you enable ISO-ANSI C Standard compliance of the compiler on the command line. Hence, with this command line it is not required,

gcc -O2 -Wall -W foo.c

Which is probably the most minimal command line you should ever use. With this command line, which is probably more appropriate, the define above would be required,

gcc -O2 -Wall -W -std=c99 foo.c

Seeusr-include-features.h for more information about the possible defines that might be useful.

Incidentally, it is handy to have a generic Makefile to allow testing just about anything with the simple "make" command. Mine invokes the compiler like this,

gcc -ggdb -O2 -std=c99 -pedantic -Wall -W -Wcast-align -Wcast-qual -Wmissing-prototypes -Wshadow -Wnested-externs -Wstrict-prototypes -Waggregate-return -Wpointer-arith -c foo.c

There is little point in not letting the compiler catch your mistakes, before they are a real problem.

Here are a couple variations on that code, which may or may not be interesting to you,

int main(void) { int fd; struct ifreq ifr;

* Define POINTER as non-zero to use a pointer here. *

#define POINTER 0 #if POINTER struct sockaddrin *saddr = (struct sockaddrin *) &ifr.ifraddr; #else struct sockaddrin saddr = *((struct sockaddrin *)(&(ifr.ifraddr))); #endif

strcpy(ifr.ifrname,"eth0");

if (-1 == (fd = socket(PFINET,SOCKSTREAM,0))) { perror("socket()"); return EXITFAILURE; }

if (-1 == (ioctl(fd, SIOCGIFADDR, &ifr))) { perror("ioctl SIOGIFADDR"); return EXITFAILURE; }

#if POINTER #else printf("-n%s : %s-n", ifr.ifrname, inetntoa(saddr.sinaddr)); #endif

return EXITSUCCESS; }

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