Hi
On 1/18/23 21:37, Flávio Heleno wrote:
This may be a silly question, but in that case, wouldn't #ifdef guards keep
the compiler from including/parsing a.h twice?
It's complicated. Modern compilers may include an optimization for
include guards
(https://gcc.gnu.org/onlinedocs/cppinternals/Guard-Macros.html), but
those rules may only be applied fairly narrowly.
I just recompiled gammasection.c in ext/random [1] which is a fairly
simple file containing just a handful of files and traced the compiler
with 'strace' for the 'openat' syscalls:
In total 2488 openat syscalls (many of them resulting in ENOENT, because
of multiple include paths) were performed by the compiler. Multiple of
my system headers were opened several times, because the include guard
optimization could not be applied to them. I'm also seeing duplicated
PHP headers:
20 "/usr/lib/gcc/x86_64-linux-gnu/9/include/stddef.h",
13 "/usr/include/x86_64-linux-gnu/bits/mathcalls-narrow.h",
11 "/usr/include/x86_64-linux-gnu/bits/wordsize.h",
8 "/usr/include/x86_64-linux-gnu/bits/mathcalls.h",
6 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h",
5 "/usr/lib/gcc/x86_64-linux-gnu/9/include/limits.h",
4 "/usr/include/x86_64-linux-gnu/bits/mathcalls-helper-functions.h",
3 "/usr/include/assert.h",
3 "./php-src/Zend/zend_hash.h",
3 "./php-src/Zend/zend.h",
2 "/usr/lib/gcc/x86_64-linux-gnu/9/include/stdarg.h",
2 "/usr/lib/gcc/x86_64-linux-gnu/9/include/mm_malloc.h",
2 "/usr/lib/gcc/x86_64-linux-gnu/9/include/emmintrin.h",
2 "/usr/include/x86_64-linux-gnu/bits/long-double.h",
2 "./php-src/Zend/zend_string.h",
2 "./php-src/Zend/zend_stream.h",
2 "./php-src/Zend/zend_stack.h",
[1] https://github.com/php/php-src/blob/master/ext/random/gammasection.c
When a.h is *not* required by any of b.h, c.h nor foo.c, I agree that it
should *not* be included at all, but when any of them,
Ideally the scope of each individual header would also be narrowed to
reduce the number of required dependencies.
Staying at the gammasection.c example from above: Ideally it should not
be necessary to include the 20 kB zend_string.h twice, because the
gammasection.c does not use strings at all. Likewise the 50 kB
zend_hash.h is included three times and not used it all. With the amount
of C files this adds up.
Best regards
Tim Düsterhus
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