Issue 0054: What is the behavior of various string functions with a specified length of zero?

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

Authors: Larry Jones, Project Editor (P.J. Plauger)
Date: 1993-04-01
Submitted against: C90
Status: Fixed
Fixed in: C90 TC1
Cross-references: 0042.01
Converted from: dr.htm, dr_054.html

Are the string handling functions defined in subclause 7.11 that have an explicit length specification (memcpy, memmove, strncpy, strncat, memcmp, strncmp, strxfrm, memchr, and memset) well-defined when the length is specified as zero?

Taking memcpy as an example, the description in subclause 7.11.2.1 states:

The memcpy function copies n characters from the object pointed to by s2 into the object pointed to by s1. If copying takes place between objects that overlap, the behavior is undefined.

The response to Defect Report #042 Question 1 indicates that:

... the “objects” referred to by subclause 7.11.2.1 are exactly the regions of data storage pointed to by the pointers and dynamically determined to be of N bytes in length (i.e. treated as an array of N elements of character type).

Since, by definition, objects consist of at least one byte, this would imply that s1 and s2 are not pointing to objects when N is zero and thus are outside the domain of the function leading to undefined behavior.

I do not recall whether this was the Committee's intent or not, but it would seem that some clarification is in order.

Comment from WG14 on 1997-09-23:

Correction

Add to subclause 7.11.1, page 162:

Where an argument declared as size_t n specifies the length of the array for a function, n can have the value zero on a call to that function. Unless explicitly stated otherwise in the description of a particular function in this subclause, pointer arguments on such a call must still have valid values, as described in subclause 7.1.7. On such a call, a function that locates a character finds no occurrence, a function that compares two character sequences returns zero, and a function that copies characters copies zero characters.