CODEWITHSUNDEEP
gcc
mort
mort

Reputation: 13618

What are those gcc options doing?

In a makefile I work on, gcc is used with the -D XOPEN_SOURCE=500 and -D_BSD_SOURCE options. gcc --help does not tell me what these are; a quick google search didn't help either. I'm quite a newbie with gcc, could someone give me a hint?

Upvotes: 0

Views: 720

Answers (5)

Antoine
Antoine

Reputation: 5198

-D sets a define. It's like adding a header file that contains:

#define XOPEN_SOURCE 500
#define _BSD_SOURCE 1

You can then use #ifdef _BSD_SOURCE to enable conditional compilation of certain part of the code.

Upvotes: 1

X-Istence
X-Istence

Reputation: 16675

-D is used to set defines. The source code you are compiling most likely is using those defines to include specific header files.

Think of -D as doing the same thing as:

#define XOPEN_SOURCE 500
#define _BSD_SOURCE 1

at the top of the file it is currently compiling.

Upvotes: 2

In silico
In silico

Reputation: 52217

According to the GCC documentation ("3.11 Options Controlling the Preprocessor"), the -D switch defines the macros XOPEN_SOURCE and _BSD_SOURCE with the values 500 and 1 respectively. It's as though you have this code at the beginning of all the source files you pass to GCC:

#define XOPEN_SOURCE 500
#define _BSD_SOURCE 1

Build scripts usually take advantage of the compiler's ability to "insert" macros like these to "communicate" to the source code details about the platform being targeted (e.g. operating system version).

The "opposite" command-line switch for -D is -U, which #undefs a macro.

Most (if not all) modern C/C++ compilers include similar switches. For example, Visual C++ compilers accept the /D compiler switch, which essentially serves the same purpose as GCC's -D switch.

For future reference, the GCC option index is great if you need to look up compiler switches for the GCC compiler.

Upvotes: 4

Ed Heal
Ed Heal

Reputation: 60037

-D is equlivant of a #define

i.e.

#define XOPEN_SOURCE 500

Upvotes: 1

Cougar
Cougar

Reputation: 651

These do not nothing for gcc. These are definitions like similar you have in your .c, .cpp or .h files:

#define XOPEN_SOURCE 500
#define _BSD_SOURCE

Upvotes: 1

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