3. Message Content

int fd = explain_open_or_die("some/thing", O_RDONLY, 0);

open(pathname = "some/file", flags = O_RDONLY) failed, No such file or directory (2, ENOENT) because there is no "some" directory in the current directory

system-call failed, system-error because explanation

3.1 Before Because

open(pathname = "some/file", flags = O_RDONLY) failed, No such file or directory (2, ENOENT) because ...

• This is the proximal cause.
• Developers find it useful.
• If not in error message, user can't copy-and-paste into bug report.
• Maintainers find that useful (once apon a time, on a file system far, far away...)
• Arguments names used in explanation for context.
• Note: I have been using error messages with this level of detail for many years, and they go over rather well, contrary to expectations.
• Users like it when you don't talk down to them. Give them some credit.

3.2 After Because

...because there is no "some" directory in the current directory

• This portion attempts to explain the proximal cause of the error in plain language.
• It attempts to include as much information as possible,
• possibly delving deeply into the semantics of the system call,
• including looking at owners and permission bits.

3.3 Internationalization

• Most of them — left over proof-of-concept
• No localizations

3.4 Postmortem

• explain
• need to know system call
• need to know errno
• how to turn errno value into string
  explain -e num strerror

3.5 Philosophy

• Tell me everything.
• Including what I didn't know to look for (or how to look for)
• Avoid modifying process (thread) state
• Absolute paths if relative paths would be ambiguous