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Date: Thu, 19 Oct 2006 18:10:00 -0000
From: "vda dot linux at googlemail dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sources.redhat.com
Message-ID: <20061019181000.3400.vda.linux@googlemail.com>
Reply-To: sourceware-bugzilla@sourceware.org
Subject: [Bug libc/3400] New: gcc -Wl,--gc-sections -static build doesn't flush stdout
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t.c:
#include <stdio.h>
int main() {
    printf("Hello world\n");
}

# gcc -O2 t.c -o shared
# gcc -O2 t.c -static -o static
# gcc -O2 t.c -Wl,--gc-sections -static -o gc-sect
# ./shared | cat
Hello world
# ./static | cat
Hello world
# ./gc-sect | cat

The last command outputs nothing.
Adding fflush(stdout) after printf makes it work.
(Pipe is needed to make stdout buffered).

IOW: static link with section garbage collection desn't work correctly,
which is a pity because it is such a handy optimization.
This is why:

Makerules:
                 PROVIDE(__start___libc_atexit = .);\
                 __libc_atexit : { *(__libc_atexit) }\
                 PROVIDE(__stop___libc_atexit = .);\


libio/genops.c:

#ifdef text_set_element
text_set_element(__libc_atexit, _IO_cleanup);
#endif


include/libc-symbols.h:

#ifdef HAVE_ELF

/* Make SYMBOL, which is in the text segment, an element of SET.  */
# define text_set_element(set, symbol)  _elf_set_element(set, symbol)

/* These are all done the same way in ELF.
   There is a new section created for each set.  */
# ifdef SHARED
/* When building a shared library, make the set section writable,
   because it will need to be relocated at run time anyway.  */
#  define _elf_set_element(set, symbol) \
  static const void *__elf_set_##set##_element_##symbol##__ \
    __attribute__ ((used, section (#set))) = &(symbol)
# else


And of course:

void
exit (int status)
{
...
  RUN_HOOK (__libc_atexit, ());

  _exit (status);
}

You have to explain to ld that it should never drop __libc_atexit sections...

-- 
           Summary: gcc -Wl,--gc-sections -static build doesn't flush stdout
           Product: glibc
           Version: 2.4
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: vda dot linux at googlemail dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: i386-pc-linux-gnu
  GCC host triplet: i386-pc-linux-gnu
GCC target triplet: i386-pc-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=3400

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