pack pragma
Specifies the packing alignment for structure, union, and class members.
Syntax
#pragma pack( show )
#pragma pack( push[,identifier] [,n])
#pragma pack( pop[,{identifier|n} ])
#pragma pack([n])
Parameters
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 isn't specified, the current packing alignment value is pushed.
pop
(Optional) Removes the record from the top of the internal compiler stack. If n isn't 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 using an 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 gets popped, and the packing value associated with the record found on the top of the stack becomes the new packing alignment value. If you pop using an identifier that isn't found in any record on the stack, then the pop is ignored.
The statement #pragma pack (pop, r1, 2) is equivalent to #pragma pack (pop, r1) followed by #pragma pack(2).
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 isn't 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 isn't 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 is on a boundary that's either a multiple of n, or a multiple of the size of the member, whichever is smaller.
Remarks
To pack a class is to place its members directly after each other in memory. It can mean that some or all members can be aligned on a boundary smaller than the default alignment of the target architecture. pack gives control at the data-declaration level. It 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 isn't set, the default value is 8 for x86, ARM, and ARM64. The default is 16 for x64 native and ARM64EC.
If you change the alignment of a structure, it may not use as much space in memory. However, you may see a loss of performance or even get a hardware-generated exception for unaligned access. You can modify this exception behavior by using SetErrorMode.
For more information about how to modify alignment, see these articles:
x64 structure alignment examples
Warning
In Visual Studio 2015 and later you can use the standard
alignasandalignofoperators, which unlike__alignofand__declspec( align )are portable across compilers. The C++ standard doesn't 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.
Examples
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
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
// pop to the identifier and then set
// the value of the current packing alignment:
#pragma pack(pop, r1, 2) // n = 2 , stack popped
#pragma pack(show) // C4810