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6.5.11 Structure-Layout Pragmas ¶

For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma pack directives that change the maximum alignment of members of structures (other than zero-width bit-fields), unions, and classes subsequently defined. The n value below specifies the new alignment in bytes and may have the value 1, 2, 4, 8, and 16. A value of 0 is also permitted and indicates the default alignment (as if no #pragma pack were in effect) should be used.

#pragma pack(n)

Sets the new alignment according to n.

#pragma pack()

Sets the alignment to the one that was in effect when compilation started (see also command-line option -fpack-struct[=n]. See Options for Code Generation Conventions).

#pragma pack(push[,n])

Pushes the current alignment setting on an internal stack and then optionally sets the new alignment.

#pragma pack(pop)

Restores the alignment setting to the one saved at the top of the internal stack (and removes that stack entry). Note that #pragma pack([n]) does not influence this internal stack; thus it is possible to have #pragma pack(push) followed by multiple #pragma pack(n) instances, with the original state restored by a single #pragma pack(pop).

You can also use the packed type attribute (see Common Type Attributes) to pack a structure. However, the packed attribute interferes with #pragma pack, and attempting to use them together may cause spurious warnings or unexpected behavior.

Some targets, e.g. x86 and PowerPC, support the #pragma ms_struct directive, which causes subsequent structure and union declarations to be laid out in the same way as __attribute__ ((ms_struct)); see x86 Variable Attributes.

#pragma ms_struct on

Turns on the Microsoft layout.

#pragma ms_struct off

Turns off the Microsoft layout.

#pragma ms_struct reset

Goes back to the default layout.

Most targets also support the #pragma scalar_storage_order directive which lays out subsequent structure and union declarations in in the same way as the documented __attribute__ ((scalar_storage_order)); see Common Type Attributes.

#pragma scalar_storage_order big-endian

#pragma scalar_storage_order little-endian

Set the storage order of scalar fields to big- or little-endian, respectively.

#pragma scalar_storage_order default

Goes back to the endianness that was in effect when compilation started (see also command-line option -fsso-struct=endianness see Options Controlling C Dialect).