GCC precompiled headers not working

[Shachar Shemesh]

Hi all,

I'm having trouble getting gcc to do pre-compiled headers. The 
instructions for doing it are at 
http://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html.

Here's what I got:
precomp.h:
#ifndef PRECOMP_H
#define PRECOMP_H

//#error Include twice

#include <stdio.h>

#endif

hello.c:
#include "precomp.h"

int main()
{
        printf("Hello, world.\n");

        return 0;
}
Here's what I do:
$ gcc -x c -o precomp.h.gch -c precomp.h -g -Wall -O2
Uncomment the "error" line
$ gcc -x c -o hello.o -c hello.c -g -Wall -O2
In file included from hello.c:1:
precomp.h:4:2: error: #error Include twice

Now, here's the strange part, when I run the compile command through 
strace, strace shows:
20522 open("hello.gcda", O_RDWR)        = -1 ENOENT (No such file or 
directory)
20522 stat64("precomp.h.gch", {st_mode=S_IFREG|0644, st_size=3056, 
...}) = 0
20522 open("precomp.h.gch", O_RDONLY|O_NOCTTY) = 4
20522 fstat64(4, {st_mode=S_IFREG|0644, st_size=3056, ...}) = 0
20522 read(4, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\1\0\3\0\1\0\0\0"..., 
24) = 24
20522 close(4)                          = 0
20522 open("precomp.h", O_RDONLY|O_NOCTTY) = 4
20522 fstat64(4, {st_mode=S_IFREG|0644, st_size=86, ...}) = 0
20522 read(4, "#ifndef PRECOMP_H\n#define PRECOMP"..., 86) = 86
20522 close(4)                          = 0
So, to recap:

   * My version of gcc DOES support precompiled headers.
   * I am placing the precompiled headers file at the right location
   * gcc DOES manage to find my precompiled header file when it
     searches for it
   * But it ignores it!

Now the gcc page does give a list of reasons why a precompiled header 
might be ignored, by my test case does not appear to cover any of those 
reasons. The header file is a simple one, no defines before the 
precompiled header, exact same command line to both header and body 
code, only one per file.

Anyone either understanding the man page or having an idea what I'm 
doing wrong is welcome to let me know.

Shachar

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