Interpose `open` library function

`open` is not as side effect free as I had imagined - i.e. if the flag `O_TRUNC` is passed it truncates the file contents alongside opening the file descriptor. In practice this is done by _emacs_ prior to writing the new file content and as such needs to be intercepted so we can start tracking the file before it is changed.

Interposing `open` required some changes to make the library work without including `fcntl.h`. This header not only defines some of the flags we require to check if a library call actually is able to change files but also defines the `open` library function.

While implementing this change I noticed that the function interpositions implemented in C++ actually need to be declared as `external "C"` so their names do not get wrangled during compilation. I suspect that this was previously implicitly done for e.g. `mmap` and `write` by the included C standard library headers. However this did not work for `open` which is why all function interpositions are now explicitly declared external.

End result: _emacs_ file changes are now tracked correctly.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/bootstrap.cc b/src/bootstrap.cc
index 309ff20..1d509d8 100644
--- a/src/bootstrap.cc
+++ b/src/bootstrap.cc
@@ -2,6 +2,8 @@
 
 #include "init/alloc.h"
 
+extern "C" {
+
 void free(void* ptr) {
 	static actual::ptr<void, void*> actual_free{};
 
@@ -41,3 +43,5 @@ void* calloc(size_t block, size_t size) {
 		return actual_calloc(block, size);
 	}
 }
+
+}