Issue 0325: strerror()

This issue has been automatically converted from the original issue lists and some formatting may not have been preserved.

Authors: Stoughton (US)
Date: 2006-03-20
Reference document: XSH Aardvark, ERN 137
Submitted against: C99
Status: Closed
Converted from: summary-c99.htm, dr_325.htm

Summary: Is an implementation permitted to return an empty string for strerror()?

This is a potential defect forwarded from the Austin Group. Is an implementation of strerror permitted to return an empty string if there is no associated error message for the given errnum?

The C Standard, although not perfectly clear, strongly implies that the string returned by strerror() cannot be empty. The C Standard says

strerror shall map any value of type int to a message.

It doesn't state that the message cannot be empty, but the fact that it uses the word "message" means that any interpretation that allows this message to be empty would also have to allow the diagnostic messages produced by the compiler to be empty. Clearly such an interpretation is very much not intended. Note that the relationship between the term "diagnostic message" and the "message" produced by strerror() is clear from section 3.10:

diagnostic message
message belonging to an implementation-defined subset of the implementation's message output

On the other hand, some have argued that "An implementation-defined subset" does not preclude the empty string from being included in the set of messages, provided the implementation has defined the error that equates to the message.

Suggested Technical Corrigendum

Clarification Required.

Change 7.21.6.2, p2 from:

The strerror function maps the number in errnum to a message string. Typically, the values for errnum come from errno, but strerror() shall map any value of type int to a message.

To:

The strerror function maps the number in errnum to a message string. If the value of errnum is a valid error number, the message string shall indicate what error occurred; otherwise, if this functions completes successfully, the message string shall indicate that an unknown error occurred. Typically, the values for errnum come from errno, but strerror shall map any value of type int to a message.

Comment from WG14 on 2006-10-25:

Proposed Committee Response

The intention is to allow implementations to decide what form of message is appropriate. There is no consensus to make the suggested change or any change along this line.