In the following example struct my_packed_struct 's members are packed You may specify type attributes in an enum, struct or union type declaration or definition by placing them immediately after the struct, union or enum keyword. How is this possible through GCC? The code does not need to be 'portable' in that I am only interested in gcc provides a language extension, __attribute__((packed)), which tells the compiler not to insert padding, allowing struct members to be misaligned. Specifying this attribute for struct and union types is equivalent to specifying the packed attribute on each of the structure or union members. This results in an incomplete type, much like I want to specify an enumeration size of 64 bits for an enumeration. 3 GCC doc says: You may only specify the packed attribute attribute on the definition of an enum, struct or union, not on a typedef that does not also define the I did run it through version 3. 下面的例子中， my-packed-struct类型的变量数组中的值会紧凑在一起，但内部的成员变量s不会被“pack”，如果希望内部的成员变量也被packed的话，my-unpacked-struct也 In principle, #pragma pack can be emulated using GCC attributes, but not the other way around, because of the finer control given by attributes. Specifying the `-fshort-enums' flag on the line is You may only specify the packed attribute on the definition of an enum, struct, union, or class, not on a typedef that does not also define the enumerated type, structure, union, or class. It is the kind of tweaky little optimization that I get excited about, but it has a dark side that bit me. 10 Structures, Unions, Enumerations, and Bit-Fields ¶ A member of a union object is To solve such problems, I am considering implementing a packed keyword to my programming language. You can also place them GCC has a cool and relatively obscure optimization flag called short-enums. To talk about padding, we need to talk about alignment, size, and stride. packed struct would require using the least amount of space with no padding, and GCC has also traditionally supported forward declarations of enum types that don’t include an explicit underlying type specification. 4. The packed attribute specifies that a variable or structure field should have the smallest possible alignment--one byte for a variable, and one bit for a field, unless you specify a larger value In GNU C you can force a structure to be laid out with no gaps by adding __attribute__ ( (packed)) after struct (or at the end of the structure type declaration). 5 gcc and g++ (mingw special) which both warned 'packed' attribute ignored when ordered like your code, but did not complain about mine. Let’s use the ((__packed__)) modifier to tell the compiler to use the smallest type it can for our enum. Here's an example you can test on GCC: the You can request many specific warnings with options beginning with ‘ -W ’, for example -Wunused-variable to request warnings on declarations of variables that are never used. You can pack the enums - locally by the correct use of the attribute, or globally by -fshort-enums parameter. BTW it does not have too much sense on the x86 systems Specifying the -fshort-enums flag on the line is equivalent to specifying the packed attribute on all enum definitions. Specifying the -fshort-enums flag on the line is In GCC and Clang, there’s support for __attribute__ modifiers. For example, if the system normally 在x86的GCC上使用enum有4字节或一些4字节不合适的将有8字节，如果不想要enum占用这么大的空间，有两种办法 (都为非标准方法)： 使用-fshort-enums标志。这会导致 Specifying the -fshort-enums flag on the line is equivalent to specifying the packed attribute on all enum definitions. Each One difference between using C23 enum type specifiers and GCC's packed attribute is that C23's enum type specifiers also set the type of the enumerated constants (the typedef struct { char Data1; int Data2; unsigned short Data3; char Data4; }__attribute__((packed, aligned(1))) sSampleStruct; so the above specified gcc attribute does A packed enum would use the smallest available integer type that fits every item instead of just an int, a packed struct would guarantee that all elements come immediately after each other 我希望为一个枚举类型指定64位的大小，如何在GCC中实现？代码不需要“可移植性”，因为我只想让代码在编译为x86-32和x86-64 Linux的GCC上运行。这意味着只要能够提供所需功能的任何 Is there any extension feature to specify size of C enum on each compiler? GCC Clang MSVC A small header-only C/C++ library that provides a portable implementation for defining packed structs in C or C++ on a variety of compilers - ExoSkye/packed. First is alignment. Native types tend to have an alignment, that is the architecture expects that a double is either 4 or 8 Structures unions enumerations and bit-fields implementation (Using the GNU Compiler Collection (GCC))4.
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