the allocation of structure members in Compaq C v6.5 for openVMS system

very important about the size

the allocation of structure members in Compaq C v6.5 for openVMS system

Size of structure members (very important!)

OS is OpenVMS (TM) Alpha Operating System(Version V7.3-2),I had made a program for testing:

#include

 void main()
{
    struct test1{
                char        a;  // 1 byte(+3 bytes as filler follow a)
                int         b;   // 4 bytes
                short       c;  // 2 bytes(+2 bytes as filler follow c)
    }abc;                       //12 bytes
    struct test2{
                char     d;     // 1 byte
    }ddd;

    struct test{
                struct test3{
                char        a;  // 1 byte(+3 bytes as filler follow a)
                int         b;   // 4 bytes
                short       c;  // 2 bytes(+2 bytes as filler follow c)
        }_abc;                  //12 bytes

        struct test4{
                char     d;     // 1 bytes(+3 bytes as filler follow d)
        }_ddd;
    }abcd;                      // 16 bytes  4*4=16

    float  e;                                   // 4 bytes
    double f;                                   // 8 bytes

        printf("abc   = %d n",sizeof (abc));
        printf("ddd   = %d n",sizeof (ddd));
        printf("abcd  = %d n",sizeof (abcd));
        printf("e     = %d n",sizeof (e));
        printf("f     = %d n",sizeof (f));

}

ca *.c    //ca="CC/FLOAT=IEEE_FLOAT/INCLUDE_DIRECTORY=(???ignored)/LIST"

link *  

link.com:

$ SET VER
$ LINK/MAP='P1' /EXE='P1' 'P1', ???ignored/OPT, -
                       ora_proc:oraclient/opt,-
                       .../LIB,-
                       ???ignored/LIB,-
                       ???ignored/LIB,-
                       ???ignored/LIB,-
                       ???ignored/LIB,-
                       ???ignored/LIB,-
                       ???ignored/LIB,-
                      ???ignored/OPT, ???ignored:???ignored/LIB
$ SET NOVER

the result is :

abc    = 12
ddd    = 1
abcd  = 16
e       = 4
f        = 8

If under turbo c 2.0 & window xp :

 pattern            size

 char                    1 byte

 int           2 bytes

unsigned int  2 bytes

signed int    2 bytes

float        4bytes

double      8 bytes

#pragma [no]member_alignment Directive

By default, Compaq C for OpenVMS VAX systems does not align structure members on natural boundaries; they are stored on byte boundaries (with the exception of bit-field members).

By default, Compaq C for OpenVMS Alpha systems does align structure members on natural boundaries.

The #pragma member_alignment preprocessor directive can be used to force natural-boundary alignment of structure members. The #pragma nomember_alignment preprocessor directive restores byte-alignment of structure members.

This pragma has the following formats:

When #pragma member_alignment is used, the compiler aligns structure members on the next boundary appropriate to the type of the member, rather than on the next byte. For example, a long variable is aligned on the next longword boundary; a short variable is aligned on the next word boundary.

Consider the following example:

When this example is executed, it shows the difference between #pragma member_alignment and #pragma nomember_alignment .

Once used, the member_alignment pragma remains in effect until the nomember_alignment pragma is encountered; the reverse is also true.

The optional base_alignment parameter can be used to specify the base-alignment of the structure. Use one of the following keywords for the base_alignment:

• byte (1 byte)

• word (2 bytes)

• longword (4 bytes)

• quadword (8 bytes)

• octaword (16 bytes)

The #pragma member_alignment save and #pragma member_alignment restore directives can be used to save the current state of the member_alignment and to restore the previous state, respectively. This feature is necessary for writing header files that require member_alignment or nomember_alignment , or that require inclusion in a member_alignment that is already set.