Gcc packed struct. Below is the example code and output which is self explanatory. Under G...
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Gcc packed struct. Below is the example code and output which is self explanatory. Under GCC it's __attribute__((__packed__)). Six attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated and may_alias Aug 29, 2014 · The gcc has introduced the __attribute__((packed)) precisely to avoid the dangerous effects you are seeking: the definition of the structure should binary compatible between all the user applications and libraries which use the same definition. The -mno-unaligned-access flag used on ARM to prevent GCC from generating unaligned accesses (obviously) will only do so on packed structures. 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). In particular it does not support the "#pragma pack" directive. 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 with the `aligned' attribute. 37 Specifying Attributes of Types The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. Seven attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated, visibility Using the GNU Compiler Collection (GCC) Specifying Attributes of Types The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. So when using the GNU C compiler, you can ensure structure packing in one of two ways Define the structure appropriately so that it is intrinsically packed. Specifying the -fshort-enums flag on the line is equivalent to specifying the packed attribute on all enum definitions. Consider this example on a 64-bit machine: char *p; /* 8 bytes */ char c; /* 1 byte */ long x; /* 8 bytes */ I stumbled upon a very bizarre struct member packing in 32-bit clang. This keyword is followed by an attribute specification inside double parentheses. Some compilers provide to suppress padding or to make it packed to n number of bytes. Jul 23, 2025 · We can pack a struct by using the #pragma directive with the pack argument. Here is the link to the compiler explorer experiment. I started using bit fields and the "packed" attribute and I came across a wierd behavior: struct __attribute__ ( (packed)) { int a:12; int b Jan 13, 2016 · Packed structs guarantee that when treated as an array of bytes it will not contain any padding, which is both wasteful, a possible security risk and possibly incompatible when with interfacing hardware. 1 is a bit stricter than previous gcc versions on this, and using it lead to data abort for unaligned accesses when generating network traffic. When packed used during structure declaration, compiler will not add any padding to the members of the same structure. . Is there no standard for packing structs that works in all C compilers? Jan 4, 2016 · You want the struct to be packed, not the single fields (which are already packed as they are scalar types). Aug 20, 2021 · You have mistaken the position of keyword `struct` or `union` with the position of `__packed` or `__attribute__ ( (packed))`. Using the GNU Compiler Collection (GCC) 6. It can't have a struct representing your data just by packing - you'd have to know how 8-bit bytes are converted to 16-bit bytes when transferred between the two systems. The n value below always is required to be a small power of two and specifies the new alignment in bytes. Specifying this attribute for struct and union types is equivalent to specifying the packed attribute on each of the structure or union members. Feb 11, 2026 · I’ll walk you through how padding happens, how to pack structs with pragmas and attributes across common compilers, how to verify the layout, and the patterns I use in production code (2026-era tooling included) to keep packed layouts correct and safe. Six attributes are currently defined for types: aligned, packed, transparent_union, unused, deprecated and may_alias Using the GNU Compiler Collection (GCC) 5. It seems like gcc 7. This is hard to do and requires an understanding of how the compiler behaves with respect to alignment on the target machine. Even with `typedef`, you should still write like `typedef struct __packed`. This overrides the default packing scheme and can make the packing scheme of the structure as defined. I am working with structs in c on linux. By default structures are padded in C. Using the GNU Compiler Collection (GCC) For compatibility with Microsoft Windows compilers, GCC supports a set of #pragma directives which change the maximum alignment of members of structures (other than zero-width bitfields), unions, and classes subsequently defined. 32 Specifying Attributes of Types The keyword __attribute__ allows you to specify special attributes of struct and union types when you define such types. Structure packing suppresses structure padding, padding used when alignment matters most, packing used when space matters most. In essence, I have the following structs: Suppose your packed struct has 8-bit fields in it, and consider some system with a 16-bit byte. If you want to avoid this behaviour, you have to explicitly request it.
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