Issue 0042.01: Does memcpy define a (sub)object?

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

Authors: Tom MacDonald, WG14
Date: 1992-12-10
Reference document: X3J11/92-001
Submitted against: C90
Status: Closed
Cross-references: 0054
Converted from: dr.htm, dr_042.html

The description of memcpy in subclause 7.11.2.1 says:

void *memcpy(void *s1, const void *s2, size_t n);

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

The definition of the term object in subclause 3.14 is:

object - A region of data storage in the execution environment, the contents of which can represent values. Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined. When referenced, an object may be interpreted as having a particular type...

Are the objects in the description of memcpy the largest objects into which the arguments can be construed as pointing?

In particular, is the behavior of the call of memcpy in Example 1 defined:

void f1(void) {
        extern char a[2][N];
        memcpy(a[1], a[0], N);
        }

because the arguments point into the disjoint array objects, a[1] and a[0]? Or is the behavior undefined because the arguments both point into the same array object, a?

Comment from WG14 on 1997-09-23:

Response

From subclause 3.14, an object is “a region of data storage ... Except for bit-fields, objects are composed of contiguous sequences of one or more bytes, the number, order, and encoding of which are either explicitly specified or implementation-defined ...” From subclause 7.11.1, “the header <string.h> declares one type and several functions, and defines one macro useful for manipulating arrays of character type and other objects treated as arrays of character type.” “Various methods are used for determining the lengths of the arrays...” From subclause 7.11.2.1, description of memcpy, “if copying takes place between objects that overlap, the behavior is undefined.” Therefore, 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).

a. So, no, the objects are not “the largest objects into which the arguments can be construed as pointing.”

b. In Example 1, the call to memcpy has defined behavior.

c. The behavior is defined because the pointers point into different (non-overlapping) objects.