I know it is a Macro we are passing to a function. How do you explain what is the use of this macro, and in which scenario i have to use this ?.
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4 Answers
_GNU_SOURCE enables GNU extensions to the C and OS standards supported by the GNU C library, such as asprintf. Define it when you're using such non-standard functions and macros.
3 Comments
Whoami
What is disadvantage of using such non-standard ?. It is not portable across platform?
Fred Foo
@Whoami: non-portability, indeed. The manpages for the C library document which standards functionality adheres to.
Whoami
Thanks for the reply. Between, i tried the following code: #include <stdio.h> int main ( int argc, char ** argv) { char *p; asprintf (&p, "%s", "Welcome"); printf ("String is %s ", p); } gcc -o mysample mysample.c : i m able to compile and execute the code. I have included nowhere _GNU_SOURCE in the soure code, nor i used macro while compiling. But still how it is compiling ?
From glibc manual:
Macro: _GNU_SOURCE
If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.
http://www.gnu.org/software/libc/manual/html_node/Feature-Test-Macros.html
Comments
Whereas applications developed conforming to POSIX.1 - (IEEE 1003.1-2008, ISO/IEC 9945:2009) and / or Single Unix Specification 2008 (ISO/IEC 9945:2009 with X/Open Curses) help ensure how portable your application is. The _GNU_SOURCE will most likely limits source code usage to GNU/Linux and GNU/Hurd systems, unless additional work is done to address the non-portability of such functionality on other platforms.
Some companies, and government / military contracts may require certain platform standards to be used.
If you are developing an Open Source / Free Software application that you wish to be available on multiple Unix and Unix-like systems (including Microsoft Windows NT, 2000, and newer which have a POSIX compatibility available) then limiting your development to POSIX.1 library functions makes this an easier task. Other targets include the free/open BSD platforms NetBSD, FreeBSD, OpenBSD, DragonflyBSD, as well as the commercial Unix systems (Solaris, AIX, HP/UX, etc.) that do not include _GNU_SOURCE functionality.
If you use an altered functionality of a portable function, I can't think of an example but I believe they do exist, it may create subtle bugs in non-GNU platforms.
So in general, if your development is already locked into GNU / Linux and GNU / Hurd then feel free to use such extensions, but avoid such usages for any applications that may be deployed on other Unix and Unix-like operating systems.
I do work on a large-ish code base that has been ported from two other Unix platforms to Linux, and we do use _GNU_SOURCE extensions sparingly, though most of the development is limited to modern POSIX or IEEE 1003.1 / Single Unix Spec and C99 (Standard C Library) standards for future compatibility.
3 Comments
Whoami
Thanks for the reply. Between, i tried the following code: #include <stdio.h> int main ( int argc, char ** argv) { char *p; asprintf (&p, "%s", "Welcome"); printf ("String is %s ", p); } gcc -o mysample mysample.c : i m able to compile and execute the code. I have included nowhere _GNU_SOURCE in the soure code, not i used macro while compiling. But still how it is compiling ?
mctylr
The GCC documentation, on C Dialect Options lists that 'gnu89' / 'gnu90' is default (in version 4.6.2 and many other versions as well).
gcc -v to check your version, and cpp -dM </dev/null to look for predefined headers. _GNU_SOURCE is not defined, so as the gcc manual explains extensions that don't conflict with ANSI/ISO standards are allowed by default. _GNU_SOURCE would also allow GNU extensions that modify or conflict with ANSI/ISO standards.
hmijail
Regarding the "altered functionality of a portable function": I was interested in finding an example of functions with different declarations under different Feature Test Macros, and found one: mmap is declared differently depending on the POSIX compliance level, at least on Solaris.
Some functions not mandated by the standard are provided in libraries, but are only available when some feature test macro is defined. _GNU_SOURCE is one of the macros that makes such functions available.
