Top 5 Pragma Pack The 189 Detailed Answer

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Structure Packing in C

pack pragma | Microsoft Docs

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Syntax

Remarks

Examples

Category

Purpose

Syntax

Defaults

Parameters

Usage

Examples

#pragma pack

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B.2.13 #pragma pack(n) (Sun Studio 12: C++ User’s Guide)

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C static code analysis

Noncompliant Code Example

Compliant Solution

C static code analysis: “#pragma pack” should be used correctly

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Pragma Pack

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pack pragma

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pack pragma

Article

07/08/2022

3 minutes to read

9 contributors

In this article

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:

alignof

align

__unaligned

x64 structure alignment examples Warning In Visual Studio 2015 and later you can use the standard alignas and alignof operators, which unlike __alignof and __declspec( align ) are portable across compilers. The C++ standard doesn’t address packing, so you must still use pack (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 #include 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

“, offsetof(S, k)); printf(“%zu “, offsetof(T, i)); printf(“%zu “, offsetof(T, j)); printf(“%zu

“, 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

See also

#pragma pack effect

#pragma pack instructs the compiler to pack structure members with particular alignment. Most compilers, when you declare a struct, will insert padding between members to ensure that they are aligned to appropriate addresses in memory (usually a multiple of the type’s size). This avoids the performance penalty (or outright error) on some architectures associated with accessing variables that are not aligned properly. For example, given 4-byte integers and the following struct:

struct Test { char AA; int BB; char CC; };

The compiler could choose to lay the struct out in memory like this:

| 1 | 2 | 3 | 4 | | AA(1) | pad……………… | | BB(1) | BB(2) | BB(3) | BB(4) | | CC(1) | pad……………… |

and sizeof(Test) would be 4 × 3 = 12, even though it only contains 6 bytes of data. The most common use case for the #pragma (to my knowledge) is when working with hardware devices where you need to ensure that the compiler does not insert padding into the data and each member follows the previous one. With #pragma pack(1) , the struct above would be laid out like this:

| 1 | | AA(1) | | BB(1) | | BB(2) | | BB(3) | | BB(4) | | CC(1) |

And sizeof(Test) would be 1 × 6 = 6.

With #pragma pack(2) , the struct above would be laid out like this:

| 1 | 2 | | AA(1) | pad.. | | BB(1) | BB(2) | | BB(3) | BB(4) | | CC(1) | pad.. |

And sizeof(Test) would be 2 × 4 = 8.

Order of variables in struct is also important. With variables ordered like following:

struct Test { char AA; char CC; int BB; };

and with #pragma pack(2) , the struct would be laid out like this:

| 1 | 2 | | AA(1) | CC(1) | | BB(1) | BB(2) | | BB(3) | BB(4) |

#pragma pack

If the byte boundary number is smaller than the natural alignment of a member, padding bytes are removed, thereby reducing the overall structure or union size.

Members of aggregates (structures, unions, and classes) are aligned on their natural boundaries and a structure ends on its natural boundary. The alignment of an aggregate is that of its strictest member (the member with the largest alignment requirement).

num:C_Compat aligns structure members so that the class layout will be compatible with the layout produced by the XL C compiler. This applies when:

Usage

The #pragma pack directive applies to the definition of an aggregate type, rather than to the declaration of an instance of that type; it therefore automatically applies to all variables declared of the specified type.

The #pragma pack directive modifies the current alignment rule for only the members of structures whose declarations follow the directive. It does not affect the alignment of the structure directly, but by affecting the alignment of the members of the structure, it may affect the alignment of the overall structure.

The #pragma pack directive cannot increase the alignment of a member, but rather can decrease the alignment. For example, for a member with data type of short , a #pragma pack(1) directive would cause that member to be packed in the structure on a 1-byte boundary, while a #pragma pack(4) directive would have no effect.

#pragma pack directive applies only to complete declarations of structures or unions; this excludes forward declarations, in which member lists are not specified. For example, in the following code fragment, the alignment for struct S is 4, since this is the rule in effect when the member list is declared: #pragma pack(1) struct S; #pragma pack(4) struct S { int i, j, k; }; Thedirective applies only to complete declarations of structures or unions; this excludes forward declarations, in which member lists are not specified. For example, in the following code fragment, the alignment foris 4, since this is the rule in effect when the member list is declared:

#pragma pack (4) // 4-byte alignment struct nested { int x; char y; int z; }; #pragma pack(1) // 1-byte alignment struct packedcxx{ char a; short b; struct nested s1; // 4-byte alignment }; A nested structure has the alignment that precedes its declaration, not the alignment of the structure in which it is contained, as shown in the following example:

If more than one #pragma pack directive appears in a structure defined in an inlined function, the #pragma pack directive in effect at the beginning of the structure takes precedence.

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Top 5 Pragma Pack The 189 Detailed Answer

pack pragma | Microsoft Docs

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#pragma pack

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Pragma Pack

pack pragma

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#pragma pack

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pack pragma | Microsoft Docs

c – #pragma pack effect – Stack Overflow

#pragma pack

Documentation â Arm Developer

B.2.13 #pragma pack(n) (Sun Studio 12: C++ User’s Guide)

Structure-Packing Pragmas – Using the GNU Compiler Collection (GCC)

#pragma pack() – C++ Forum

C static code analysis: “#pragma pack” should be used correctly

Pragma Pack

pack pragma

#pragma pack effect

#pragma pack

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Documentation â Arm Developer