using environment variable: gcc -D vs. #define

**MK27** · 03-13-2011

Hi y'all! I have some defines like this:

Code:

#define DIR "path"
#define FILE "file"

That I use like this (eg):

Code:

fopen(DIR"/"FILE, "r");

No problem. However, I want to include another define that is passed to the compiler on the command line or in a makefile, taken from an environment variable, eg:

Code:

export WROOT=/some/path
gcc -DROOT=$WROOT

But different rules seem to apply because of the shell and special characters such as '/'. I could do this:

Code:

gcc -D'ROOT=/some/path'

Which prevents the shell screwing up on "/", but I cannot include an environment variable in single quotes for the same reason -- the shell will then just pass '$WROOT' thru literally.

Anyone know of a way around this?

**quzah** · 03-13-2011

You could just use the char *envp[] arguments to main?

Quzah.

**MK27** · 03-13-2011

Originally Posted by quzah

You could just use the char *envp[] arguments to main?
Quzah.

Sounds like a good suggestion but this is a library/module, so there is no main

**stahta01** · 03-13-2011

Originally Posted by MK27

Code:

gcc -D'ROOT=/some/path'

Have you tried the below; just a guess from a Windows user.

Code:

gcc -DROOT="/some/path"

**Bayint Naung** · 03-13-2011

getenv function?

**grumpy** · 03-13-2011

The "way around this" depends on your command shell, not gcc.

The problem is, every shell has slightly distinct handling of quote (and double quote) characters. Since you are looking to have a #define specified on the gcc command line that expands to something with double quotes, you need to carefully read the documentation for your shell.

**phantomotap** · 03-13-2011

O_o

Code:

gcc "-DMACRO=VALUE"

That virtually always works for me, but I only use a couple of different shells.

I'd still recommend you alter your use of the macro to expand the string inline with the quotes.

Soma

Code:

#if !defined(DIR)
#define DIR /some/path
#endif

#define WRAP2(X) #X
#define WRAP1(X) WRAP2(X)
#define WRAP(X) WRAP1(X)

#define ACTIVEDIR WRAP(DIR)

#include <stdio.h>

int main()
{
	printf(ACTIVEDIR);
	return(0);
}

**cas** · 03-13-2011

Because you're making a string literal, the quotes are part of the macro expansion, so you need them to be passed to gcc. Shells will eat your quotes because they're special characters. Solution: escape the quotes. Any shell worth its salt will allow you to escape with \, so you'd use:

Code:

gcc -DROOT=\"$WROOT\"

This passes actual quotation marks on the command-line, allowing gcc to correctly create a string literal ($WROOT will still be expanded by the shell).

**MK27** · 03-13-2011

Originally Posted by Bayint Naung

getenv function?

Then I can't use it like a define.

Originally Posted by cas

Shells will eat your quotes because they're special characters. Solution: escape the quotes. Any shell worth its salt will allow you to escape with \, so you'd use:

Code:

gcc -DROOT=\"$WROOT\"

This passes actual quotation marks on the command-line, allowing gcc to correctly create a string literal ($WROOT will still be expanded by the shell).

Whew! That's the ticket. It's actually for use with apxs, which compiles (via gcc) and installs apache modules. I thought it was doing it's own strange husking, but what it was because the command gets passed thru the shell twice, ie what works is:

Code:

apxs -D MY_DEF=\\\"$MY_ENV_VAR\\\"

Heh. Thanks all!

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