I have noticed g++ failing to compile a program when a directory is included
with '-I' but succeeding when '-isystem' is used. I am trying to understand
the exact nature of special treatment by gcc/g++ when searching directories
that are to be treated as 'system' directories . All the notes I came across
talk of suppressing some warnings but nothing that would explain why the
compilation fails or succeeds.
The version of g++ is
g++ (GCC) 3.3.3 (SuSE Linux)
I am enclosing a specific example below with two command lines, identical
except for the fact that one uses
'-isystem' and the latter uses '-I' . The former can compile the test
program , but the latter fails.
To test the issue, local copies of the following system directories were
used in compiling the above file. The names of these system directories (in
the same order shown below) have been reported by g++ when the '-v' flag was
used.
/usr/include/g++
/usr/include/g++/x86_64-suse-linux
/usr/include/g++/backward
/usr/local/include
/usr/lib64/gcc-lib/x86_64-suse-linux/3.3.3/include
/usr/x86_64-suse-linux/include
/usr/include
For the test case, the -'nostdinc' and '-nostdinc++' flags were used to
prevent g++ from searching the actual system directories under /usr.
Instead, -I and/or -isystem were used to force it to search only the local
copies of the same directories (note the '.' at the beginning of the paths).
Details follow : The test program is
/////////////////// test.c
////////////////////////////////////////////////
#include <stdio.h>
#include <time.h>
extern "C" struct tm *localtime_r(const time_t *, struct tm *) ;
int main (int argc, char ** argv)
{
time_t now;
struct tm l_time;
now = time((time_t *)NULL);
localtime_r(&now, &l_time);
printf("%s", asctime(&l_time));
return 0;
}
///////////////////////////////////////////////////////////////////////////////////////
The following two command lines illustrate the example. They are identical
except for the use of '-isystem' for ./usr/include , as opposed to '-I'.
The following command line compiles successfully.
g++ -nostdinc -nostdinc++ test.c -I./usr/include/g++
-I./usr/include/g++/x86_64-suse-linux -I./usr/include/g++/backward
-I./usr/local/include
-I./usr/lib64/gcc-lib/x86_64-suse-linux/3.3.3/include
-I./usr/x86_64-suse-linux/include -isystem./usr/include
The following command line fails to compile.
g++ -nostdinc -nostdinc++ test.c -I./usr/include/g++
-I./usr/include/g++/x86_64-suse-linux -I./usr/include/g++/backward
-I./usr/local/include -I./usr/lib64/gcc-lib/x86_64-suse-linux/3.3.3/include
-I./usr/x86_64-suse-linux/include -I./usr/include
The compile error is
error: declaration of `tm* localtime_r(const time_t*, tm*)' throws
different exceptions
usr/include/time.h:244: error: than previous declaration `tm*
localtime_r(const
time_t*, tm*) throw ()'
can somebody please explain what is going on ?
TIA
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