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# 3. Message Content

This code:

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

produces this messsage:

> 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

This breaks down into three pieces:

> system-call failed, system-error because explanation

## 3.1 Before Because

slide

> 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

slide

> \&...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

slide

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

## 3.4 Postmortem

slide

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

## 3.5 Philosophy

slide

- 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