pack
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The latest version of this topic can be found at pack. Specifies packing alignment for structure, union, and class members.
Syntax
#pragma pack
( [ show ] | [ push | pop ] [, identifier ] , n )
Remarks
To pack a class is to place its members directly after each other in memory, which can mean that some or all members can be aligned on a boundary smaller than the default alignment the target architecture. pack gives control at the data-declaration level. This differs from compiler option /Zp, which only provides module-level control. pack takes effect at the first struct, union, or class declaration after the pragma is seen. pack has no effect on definitions. Calling pack with no arguments sets n to the value set in the compiler option /Zp. If the compiler option is not set, the default value is 8.
If you change the alignment of a structure, it may not use as much space in memory, but you may see a decrease in performance or even get a hardware-generated exception for unaligned access. You can modify this exception behavior by using SetErrorMode.
show (optional)
Displays the current byte value for packing alignment. The value is displayed by a warning message.
push (optional)
Pushes the current packing alignment value on the internal compiler stack, and sets the current packing alignment value to n. If n is not specified, the current packing alignment value is pushed.
pop (optional)
Removes the record from the top of the internal compiler stack. If n is not specified with pop, then the packing value associated with the resulting record on the top of the stack is the new packing alignment value. If n is specified, for example, #pragma pack(pop, 16), n becomes the new packing alignment value. If you pop with identifier, for example, #pragma pack(pop, r1), then all records on the stack are popped until the record that has identifier is found. That record is popped and the packing value associated with the resulting record on the top of is the stack the new packing alignment value. If you pop with an identifier that is not found in any record on the stack, then the pop is ignored.
identifier (optional)
When used with push, assigns a name to the record on the internal compiler stack. When used with pop, pops records off the internal stack until identifier is removed; if identifier is not found on the internal stack, nothing is popped.
n (optional)
Specifies the value, in bytes, to be used for packing. If the compiler option /Zp is not set for the module, the default value for n is 8. Valid values are 1, 2, 4, 8, and 16. The alignment of a member will be on a boundary that is either a multiple of n or a multiple of the size of the member, whichever is smaller.
#pragma pack(pop, identifier , n ) is undefined.
For more information about how to modify alignment, see these topics:
Examples of Structure Alignment (x64 specific)
Warning
Note that in Visual Studio 2015 and later you can use the standard alignas and alignof operators which, unlike
__alignofanddeclspec( align )are portable across compilers. The C++ standard does not address packing, so you must still usepack(or the corresponding extension on other compilers) to specify alignments smaller than the target architecture’s word size.
Example
The following sample shows how to use the pack pragma to change the alignment of a structure.
// pragma_directives_pack.cpp
#include <stddef.h>
#include <stdio.h>
struct S {
int i; // size 4
short j; // size 2
double k; // size 8
};
#pragma pack(2)
struct T {
int i;
short j;
double k;
};
int main() {
printf("%zu ", offsetof(S, i));
printf("%zu ", offsetof(S, j));
printf("%zu\n", offsetof(S, k));
printf("%zu ", offsetof(T, i));
printf("%zu ", offsetof(T, j));
printf("%zu\n", offsetof(T, k));
}
0 4 8
0 4 6
Example
The following sample shows how to use the push, pop, and show syntax.
// pragma_directives_pack_2.cpp
// compile with: /W1 /c
#pragma pack
() // n defaults to 8; equivalent to /Zp8
#pragma pack
(show) // C4810
#pragma pack
(4) // n = 4
#pragma pack
(show) // C4810
#pragma pack
(push, r1, 16) // n = 16, pushed to stack
#pragma pack
(show) // C4810
#pragma pack
(pop, r1, 2) // n = 2 , stack popped
#pragma pack
(show) // C4810