INFO: Using a Structure with an Empty Array Member

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Summary

The information below is derived from the Microsoft C/C++ version 7.0 online help file. To view the topic that corresponds to this article, search for the word "struct" and choose the "Unsized Arrays in Structures" topic.

In the C language, an application can define structure that contains an unsized array as one of its fields. However, an application cannot define an array of structures when the structure contains an unsized array because the structure declaration does not allocate any memory for the unsized array.

More information

Background

The following structure declaration includes an unsized array field:
struct test
{
   int x, y, z;
   char empty[];
}
				
The code example below demonstrates how the compiler processes the structure declaration. The sizeof macro returns the size of the structure, but does not include the unsized array. For the structure sample above, the sizeof macro returns the value of 6 bytes for the 16-bit compiler and 12 bytes for the 32-bit compiler.

Declaring Structures with Unsized Array Fields

An application can declare an instance of a structure type that includes an unsized array field, but the behavior of the compiler is unpredictable.
   struct test x;
				
In the declaration above, the compiler allocates six bytes for x and the address of the empty field is set to six bytes beyond the address of x. This may present a problem because empty is a character array. If the application interprets empty as a string, the string has no length and ends only when a null terminating character (zero value) appears in memory.

An attempt to create an array of structures (for example, with a struct text x[10] declaration) fails and the compiler generates the following messages:
warning C4001: nonstandard extension 'zero sized array in struct/union' was used

error C2087: 'x' : missing subscript
The compiler that is included with Visual C++ versions 5.0 and 6.0 generates the following error:
error C2233: '<Unknown>' : arrays of objects containing zero-size arrays are illegal
An application can initialize an instance of a type that contains an unsized array field using the following syntax:
   struct test x={1, 2, 3, "Test String");
				
In this example the empty field in x points to the 'T' in "Test String" and the string is correctly terminated with a null terminating character after the 'g'.

An application can declare a pointer to a structure that contains an unsized array field. The application must allocate memory for to store the contents of the array. Given the structure declaration above, the following code declares a pointer to structure and allocates memory to store an instance of the structure:
   struct test *right;
   right = malloc(sizeof(struct test) + stringlength + 1);
				
The stringlength variable contains the length of the string stored in the structure and the constant 1 provides space to store the terminating null character. Even though the application allocates memory to store the string, the sizeof macro returns the size of the elements in the structure except for the unsized array element.

Using this technique, an application can use malloc() to declare an array of structures that contain unsized array fields; however, the application must correctly calculate the proper amount of memory to allocate to store the structures and the strings and it must correctly implement pointers to access elements of the array. Because the size of the structure does not include the storage required by the unsized array element, the value (ptr+1) points to the sixth (or twelfth) byte of the allocated array which is not necessarily the first field of the second element.

An attempt to allocate an array of structures produces the C4001 warning described above. The C2087 error does not occur because the size of the array is not specified.

Sample Code

/*
 * Compile options needed: /W4
 */ 

#include <stdio.h>

struct test
{
   int x, y, z;
   char str[];
};

struct test trythis;

void main(void)
{
   printf("the size of test is %d bytes\n",sizeof(struct test));
}

/* Output */ 

/* 16-bit compiler output */ 

the size of test is 6 bytes

/* 32-bit compiler output */ 

the size of test is 12 bytes
				

Properties

Article ID: 98409 - Last Review: October 26, 2013 - Revision: 3.0
Applies to
  • Microsoft Visual C++ 2.1
  • Microsoft Visual C++ 4.0 Standard Edition
  • Microsoft Visual C++ 1.0 Professional Edition
  • Microsoft Visual C++ 1.52 Professional Edition
Keywords: 
kbnosurvey kbarchive kbcode kbcompiler kbinfo KB98409

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