Q98409: INFO: Using a Structure with an Empty Array Member
Article: Q98409
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,1.51,1.52,2.0,2.1,4.0,4.1,5.0,6.0
Operating System(s):
Keyword(s): kbcode kbCompiler kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC5
Last Modified: 22-JUL-2001
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The information in this article applies to:
- Microsoft C/C++ Compiler (CL.EXE), used with:
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 4.1, 5.0, 6.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- Microsoft C/C++ for MS-DOS
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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
Additional query words: 8.00 8.00c
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Keywords : kbcode kbCompiler kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC410 kbVC500 kbVC600
Technology : kbVCsearch kbAudDeveloper kbCVCComp
Version : :1.0,1.5,1.51,1.52,2.0,2.1,4.0,4.1,5.0,6.0
Issue type : kbinfo
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