Sometimes I have trouble finding the information I’m looking for in the Linux man pages.
Recently, I wanted to do the equivalent of the following mount(8) command in Go using the mount(2) syscall:
mount --make-rprivate /
The signature of mount(2) looks like this:
int mount(const char *source, const char *target,
const char *filesystemtype, unsigned long mountflags,
const void *data);
--make-rprivate will translate to mountflags and / is the target.
But I wasn’t sure if the mount --make-rprivate / command would make some assumptions or gather information about the current state of the mount to fill in the other fields.
man 8 mount didn’t seem to say.
So, I just used strace to see for myself:
$ strace mount --make-rprivate /
...
mount("none", "/", NULL, MS_REC|MS_PRIVATE, NULL) = 0
...
And there it was, dummy/null values for source, filesystemtype, and data.
Later, with a fresh pair of eyes and a cup of coffee, I read the mount(2) man pages:
Changing the propagation type of an existing mount
...
The source, filesystemtype, and data arguments are ignored.
This makes sense; the command changes the propagation type, so what purpose would the other arguments have?