C issue 0073: Another definition of an object question

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

Authors: Clive Feather, WG14
Date: 1993-12-03
Submitted against: C90
Status: Closed
Cross-references: 0072, 0178
Converted from: dr.htm, dr_073.html

#include <stdlib.h>
 struct complex
        {
        double real [2];
        double imag;
        }
 #define D_PER_C (sizeof (struct complex) / sizeof (double))
 struct complex *f (double x)
        {
        struct complex *array = malloc(sizeof (struct complex) +
                sizeof (double));
        struct complex *pc;
        double *pd;

        if (array == NULL)
                return NULL;
        array [1].real [0] = x;                         /* Line A /*
        array [1].real [1] = x;                         /* Line B /*
        array [1].imag = x;                                     /* Line C /*
        pc = array + 1;                                 /* Line D /*
        pc = array + 2;                                 /* Line E /*
        pd = &(array [1].real [0]);                 /* Line F /*
        pd = &(array [1].real [1]);                 /* Line G /*
        pd = &(array [1].imag);                             /* Line H /*
        pd = &(array [0].real [0]) + D_PER_C;               /* Line I /*
        pd = &(array [0].real [1]) + D_PER_C;               /* Line J /*
        pd = &(array [0].imag) + D_PER_C;           /* Line K /*
        pd = &(array [0].real [0]) + D_PER_C * 2;           /* Line L /*
        pd = &(array [0].real [0]) + D_PER_C + 1;           /* Line M /*
        pd = &(array [0].real [0]) + D_PER_C + 2;           /* Line N /*
        return array;
        }

Subscripting is strictly conforming if the array is “large enough” (subclause 6.3.6). For each of the marked lines, is the assignment strictly conforming?

Response

Lines A, B, C. The identifier array points to an object that is not large enough to hold two struct complex objects. The dot selection operator is at liberty to require the complete structure denoted by its left hand side to be accessed. Such an access would result in undefined behavior.

Line D. If array is regarded as pointing to a single structure then creating a pointer to one past the end of that object is permitted.

Line E. If array is regarded as pointing to a single structure then creating a pointer two past the end of that object is not permitted. Since there is insufficient storage allocated to create a second struct complex object, it is not permitted to point one past this partial struct complex object.

Lines I, J, K, L, M, N. All of these calculations will result in pointers that point outside the original object (arrays or structures) and result in undefined behavior.

Issue 0073: Another definition of an object question

Response