Why gcc need -D_GNU_SOURCE

Question

When I make a gcc compile command with the option "-L",I should add the gblic library path or add -D_GNU_SOURCE or the gcc will run error.

$gcc sscanf_test.c -I/si/usr/aa/include/arch -o sscanf_test_2  -m64  -L$ORACLE_HOME/precomp/lib/ -L$ORACLE_HOME/lib/ -L$ORACLE_HOME/lib/stubs/ -lc
$/tmp/ccq1Z6T1.o：function ‘main’：
  sscanf_test.c:(.text+0x32)：undefined reference to ‘__isoc99_sscanf’
  sscanf_test.c:(.text+0xf8)：undefined reference to ‘__isoc99_sscanf’
  sscanf_test.c:(.text+0x1e4)：undefined reference to ‘__isoc99_sscanf’
  collect2: ld return 1
$gcc sscanf_test.c -I/si/usr/aa/include/arch -o sscanf_test_2  -m64  -L$ORACLE_HOME/precomp/lib/ -L$ORACLE_HOME/lib/ -L$ORACLE_HOME/lib/stubs/ -D_GNU_SOURCE
$gcc sscanf_test.c -I/si/usr/aa/include/arch -o sscanf_test_2  -m64  -L$ORACLE_HOME/precomp/lib/ -L$ORACLE_HOME/lib/ -L$ORACLE_HOME/lib/stubs/ /lib64/libc.so.6

Now, I have puzzle that why gcc need -D_GNU_SOURCE.Another machine have a similar environment can run normally without -D_GNU_SOURCE.

@SouravGhosh I mean the superset of all other standards under GNU C libraries. — hrl, Nov 29 '17 at 08:29
@hrl, gcc is a compiler C++ is a programming language. How can one be another? gcc is a compiler for many languages, C and C++ among them. — Jens Gustedt, Nov 29 '17 at 08:35
@SouravGhosh Thanks for your recommendation.It may related to my question ,but I don't understand its conclusion and can't get a solution from it. — hrl, Nov 29 '17 at 08:37
@JensGustedt I am sorry for that. I will add gcc tag and remove the c++ tag,thanks.But as you know ,gcc is a compiler for many languages , I just think someone who is good at c++ may also know more about gcc complier and help me to find the answer .So that I add the c++ tag. — hrl, Nov 29 '17 at 08:40
"So that I add the c++ tag." This is called "tag spamming" and it's considered as bad behavior. — Scheff's Cat, Nov 29 '17 at 11:16

score -1 · Answer 1 · edited Sep 15 '20 at 18:46

You need -D_GNU_SOURCE in-order to use GNU extension features/functions. You can use low level functions such as socket, ifconfig, etc.

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.

Compiling without -D_GNU_SOURCE may run on one Linux machine without any problem but then fail on other Linux variants if they use a different C standard library.

Very straight forward answer:

#define _GNU_SOURCE 1 enables GNU extensions supported by the GNU C library. Define it when you're using non-standard functions and macros. In your case, you're trying to use a non-standard function that is part of POSIX, that's why you need to #define _GNU_SOURCE 1.
You want to get the full effect of _GNU_SOURCE but make the BSD definitions take precedence over the POSIX definitions, the use the _GNU_SOURCE.

Check this answer for more info about defining _GNU_SOURCE.

This is so wrong. First of all, it's `-D_GNU_SOURCE`, not `-DGNU_SOURCE` or `-GNU_SOURCE`. Second of all, header files are located in `/usr/include`, not `/usr/lib`. — S.S. Anne, Jun 14 '19 at 04:17

Why gcc need -D_GNU_SOURCE

1 Answers1