reduce syscalls on opening files - roll CLOEXEC into open

on linux open supports() O_CLOEXEC. add test case for this in meson
build and ifdef. this rolls 3 syscalls into 1 as we were doing open,
then fnctl to get and fcntl to set flags.

less syscalls is a good thing as syscalls are not cheap on some
architectures or systems. I've seen a syscall on 1 system take 2-3x
as long as another and another syscall in the same 2 system
comparison take 10x as long. depending on the syscall you may only
have a budget of something like 5000 syscalls "per frame" (60fps)
before you spend all of your frame time just in syscalls not
doing any processing, so we should keep these down if possible
and that is what this does.

1 files changed, 14 insertions, 0 deletions

diff --git a/header_checks/meson.build b/header_checks/meson.build
index ff5b21ee0c..4cc855e218 100644
--- a/header_checks/meson.build
+++ b/header_checks/meson.build
@@ -123,6 +123,20 @@ function_checks = [
   ['dladdr', ['dlfcn.h'],                               ['dl'],  '-D_GNU_SOURCE=1']
 ]
 
+open_cloexec = cc.run('''#include <sys/types.h>
+                      #include <sys/stat.h>
+                      #include <fcntl.h>
+                      int main(int argc, char **argv) {
+                        int res = open(argv[0], O_RDONLY | O_CLOEXEC);
+                        if (res < 0) return 1;
+                        return 0;
+                      }
+                      ''',
+                      name : 'open works with O_CLOEXEC')
+if open_cloexec.compiled() and open_cloexec.returncode() == 0
+  config_h.set10('HAVE_OPEN_CLOEXEC', true)
+endif
+
 strerror_r_char_p = cc.compiles('''#define _GNU_SOURCE
                   #include <string.h>
                   int func (void)