HP 3000 Manuals HP 3000 Manuals
Alignment within Structures
In a structure, each member is allocated sequentially at the next
alignment boundary corresponding to its type. Therefore, the structure
might be padded internally if its members' types have different alignment
requirements. In a union, all members are allocated starting at the same
memory location. Both structures and unions can have padding at the end,
in order to make the size a multiple of the alignment.
The code fragment in Figure 2-2 illustrates the alignment on various
systems.
______________________________________________________________
| |
| struct x { |
| char y[3]; |
| short z; |
| char w[5]; |
| }; |
| |
| struct q { |
| char n; |
| struct x v[2]; |
| double u; |
| char t; |
| int s:6; |
| char m; |
| } a = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,|
| 20.0,21,22,23}; |
______________________________________________________________
Figure 2-2. Code Fragment for Comparing Storage and Alignment
HP C/HP-UX Series 700/800 and HP C/iX
Figure 2-3 shows how the data in Figure 2-2 is stored in memory when
using HP C on the HP 9000 Series 700/800 and HP 3000 Series 900. The
values are shown above the variable names. Shaded cells indicate padding
bytes.
Click here to view figure.
Figure 2-3. Storage with HP C on the HP 9000 Series 700/800 and HP 3000 Series 900
The struct q is aligned on an 8-byte boundary because the most
restrictive data type within the structure is the double u.
Table 2-2 shows the padding for the example code fragment:
Table 2-2. Padding on HP 9000 Series 700/800 and HP 3000 Series 900
---------------------------------------------------------------------------------------------
| | |
| Padding Location | Reason for Padding |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+1 | The most restrictive type of the structure x is short; therefore, the |
| | structure is 2-byte aligned. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+5 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+13 | Fills out the struct x to a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+17 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+25 | Fills out the structure to a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+26 through a+31 | Aligns the double u on an 8-byte boundary. The bit-field s begins |
| | immediately after the previous item at a+41. Two bits of padding is |
| | necessary to align the next byte properly. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+43 through a+47 | Fills out the struct q to an 8-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
HP C on the Series 300/400
The differences between HP C on the HP 9000 Series 300/400 and HP C on
the HP 9000 Series 700/800 and HP 3000 Series 900 are:
* On the Series 300/400, a structure is aligned on a 2-byte
boundary. On the HP 9000 Series 700/800 and HP 3000 Series 900,
it is aligned according to the most restrictive data type within
the structure.
* On the Series 300/400, the double data type is 2-byte aligned
within structures. It is 8-byte aligned on the HP 9000 Series
700/800 and HP 3000 Series 900.
* On the Series 300/400, the long double, available in ANSI mode
only, is 2-byte aligned within structures. The long double is
8-byte aligned on the HP 9000 Series 700/800 and HP 3000 Series
900.
* On the Series 300/400, the enumerated data type is 2-byte aligned
in a structure, array, or union. The enumerated type is always
4-byte aligned on the HP 9000 Series 700/800 and HP 3000 Series
900, unless a sized enumeration is used.
When the sample code fragment is compiled and run, the data is stored as
shown in Figure 2-4 :
Click here to view figure.
Figure 2-4. Storage with HP C on the HP 9000 Series 300/400
Table 2-3 shows the padding for the example code fragment:
Table 2-3. Padding on the HP 9000 Series 300/400
---------------------------------------------------------------------------------------------
| | |
| Padding Location | Reason For Padding |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+1 | Within structures align short on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+5 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+14 | Structures within structures are aligned on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+17 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+25 | Doubles are 2-byte aligned within structures. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+37 | Pads a to a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
CCS/C on the HP 1000 and HP 3000
Figure 2-5 shows how the members of the structure defined in Figure 2-2
are aligned in memory when using CCS/C on the HP 1000 or HP 3000:
Click here to view figure.
Figure 2-5. Storage with CCS/C
NOTE All data types and structures are 2-byte aligned when using CCS/C
on the HP 1000 or HP 3000.
Table 2-4 shows the padding for the example code fragment:
Table 2-4. Padding with CCS/C
---------------------------------------------------------------------------------------------
| | |
| Padding Location | Reason for Padding |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+1 | Aligns the structure on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+5 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+13 | Fills out the struct x to a 2-byte boundary. (Aligns the character |
| | on a 2-byte boundary.) |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+17 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+25 | Fills out the structure to a 2-byte boundary and aligns the double u |
| | on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+37 | Pads a to a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
VAX/VMS C
The differences between HP C and VAX/VMS C are:
* In HP C Series 700/800, the double type is 8-byte aligned; in
VAX/VMS C, the double type is 4-byte aligned.
* In HP C, bit-fields are packed from left to right. In VAX/VMS C,
the fields are packed from right to left.
* HP C uses big-endian data storage with the most significant byte
on the left. VAX/VMS C uses little-endian data storage with the
most significant byte on the right. (See the swab function in the
HP-UX Reference manual for information about converting from
little-endian to big-endian.)
In VAX/VMS C, the data from the program in Figure 2-2 is stored as shown
in Figure 2-6:
Click here to view figure.
Figure 2-6. Storage on VAX/VMS C
Table 2-5 shows the padding for the example code fragment:
Table 2-5. Padding on VAX/VMS C
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| | |
| Padding Location | Reason for Padding |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+1 | The most restrictive type of any struct x member is short; therefore, |
| | struct x is 2-byte aligned. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+5 | Aligns the short z on a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+13 | Fills out the struct x to a 2-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+17 | Needed for alignment of the short z. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+25 through a+27 | Fills out the structure to a 2-byte boundary and aligns the double u |
| | on a 4-byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+37 | Aligns the char m on a byte boundary. |
| | |
---------------------------------------------------------------------------------------------
| | |
| a+39 | Fills out the structure to a 4-byte boundary. |
| | |
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