CODEWITHSUNDEEP
c
Damiano Barbati
Damiano Barbati

Reputation: 3496

Environment variabile in Macro path

I need to define some path to files with macros. How can I use the $HOME environment variable? I can't find it on the GNU manual, and this doesn't work:

#define LOGMMBOXMAN    "$HOME/mmbox/LOGmmboxman"

Upvotes: 3

Views: 2622

Answers (3)

IanNorton
IanNorton

Reputation: 7282

Sounds like you are really asking "how can I set some cpp macro from my environment?"

If sothen you should just be able to add it to CPPFLAGS.

export CPPFLAGS="$CPPFLAGS -D LOGMMBOXMAN=$HOME/mmbox/LOGmmboxman"

Then in your code

#ifndef LOGBOXMAN
#error LOGBOXMAN not defined
#endif

Then make sure your source is built using the CPPFLAGS in the command line to gcc:

$ gcc -c file.c $CPPFLAGS

Upvotes: 2

Chris Lutz
Chris Lutz

Reputation: 75399

You can't. You need to use your build system to define a macro with the $HOME value (or equivalent on a non-unix system), i.e. something like this:

gcc -DHOME="/home/username" file.c

Or "/Users/username" for Mac OS X, or "C:\Users\username" (or something) for Windows. Basically, GCC provides the -D flag to define a macro on the command line. You can set up a script (or your build system) to take care of this macro definition for you, or perhaps make a system-dependent include file to define the HOME macro properly.

Then, in your C header, you can do:

#define LOGMMBOXMAN HOME "/mmbox/LOGmmboxman"

Note that, in C, consecutive string literals are concatenated. So this macro expands to:

"/home/username" "/mmbox/LOGmmboxman"

Which C interprets as

"/home/username/mmbox/LOGmmboxman"

EDIT: All that thinking, and I didn't think! D'oh!

As others have pointed out, you probably don't want to do this. This will hard-code your program to work for one specific user's home directory. This will likely cause problems if you want each user to use your program, but for each to keep his (or her) own separate files.

Ninefingers' answer is what you're most likely looking for. In the event that you ever find yourself in need of the above technique (i.e. storing application files in a system-specific place) I will leave my answer unchanged, but I expect it won't help you here.

Upvotes: 1

user257111
user257111

Reputation:

No it shouldn't and you probably don't want constant-defined settings like that in any case. If you did that and it worked as you're intending to use it, your home directory would be built in as whatever $HOME is for whoever's doing the building. The executable then depends on that specific home directory existing. If that's OK, just #define your own home. I suspect it isn't though, so you need to deduce it at runtime.

For run-time deduction what you want is this, such that:

const char* home_dir = getenv("HOME");

If there is no $HOME defined, you get NULL returned so be sure to test for this.

You can then build your string based on that. You'll need #include <stdlib.h>.

Upvotes: 5

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