#pragma pack
heißt es bei MSVC.
#pragma pack( [ n] )
Specifies packing alignment for structure and union members. Whereas the
packing alignment of structures and unions is set for an entire
translation unit by the /Zp option, the packing alignment is set at the
data-declaration level by the pack pragma. The pragma takes effect at
the first structure or union declaration after the pragma is seen; the
pragma has no effect on definitions.
When you use #pragma pack(n), where n is 1, 2, 4, 8, or 16, each
structure member after the first is stored on the smaller member type or
n-byte boundaries. If you use #pragma pack without an argument,
structure members are packed to the value specified by /Zp. The default
/Zp packing size is /Zp8.
The compiler also supports the following enhanced syntax:
#pragma pack( [ [ { push | pop}, ] [ identifier, ] ] [ n ] )
This syntax allows you to combine program components into a single
translation unit if the different components use pack pragmas to specify
different packing alignments.
Each occurrence of a pack pragma with a push argument stores the current
packing alignment on an internal compiler stack. The pragma’s argument
list is read from left to right. If you use push, the current packing
value is stored. If you provide a value for n, that value becomes the
new packing value. If you specify an identifier, a name of your
choosing, the identifier is associated with the new packing value.
Each occurrence of a pack pragma with a pop argument retrieves the value
at the top of an internal compiler stack and makes that value the new
packing alignment. If you use pop and the internal compiler stack is
empty, the alignment value is that set from the command-line and a
warning is issued. If you use pop and specify a value for n, that value
becomes the new packing value. If you use pop and specify an identifier,
all values stored on the stack are removed from the stack until a
matching identifier is found. The packing value associated with the
identifier is also removed from the stack and the packing value that
existed just before the identifier was pushed becomes the new packing
value. If no matching identifier is found, the packing value set from
the command line is used and a level-one warning is issued. The default
packing alignment is 8.
The new, enhanced functionality of the pack pragma allows you to write
header files that ensure that packing values are the same before and
after the header file is encountered:
1 | /* File name: include1.h */
|
2 | #pragma pack( push, enter_include1 )
|
3 | /* Your include-file code ... */
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4 | #pragma pack( pop, enter_include1 )
|
5 | /* End of include1.h */
|
In the previous example, the current pack value is associated with the
identifier enter_include1 and pushed, remembered, on entry to the header
file. The pack pragma at the end of the header file removes all
intervening pack values that may have occurred in the header file and
removes the pack value associated with enter_include1. The header file
thus ensures that the pack value is the same before and after the header
file.
The new functionality also allows you to use code, such as header files,
that uses pack pragmas to set packing alignments that differ from the
packing value set in your code:
1 | #pragma pack( push, before_include1 )
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2 | #include "include1.h"
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3 | #pragma pack( pop, before_include1 )
|
In the previous example, your code is protected from any changes to the
packing value that might occur in include.h.
