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Preface Introduction Compiler Command-line Options Language Extensions Compiler-specific Features Keywords and operators
__align __alignof__ __ALIGNOF__ __asm __forceinline __global_reg __inline __int64 __INTADDR__ __irq __packed __pure __smc __softfp __svc __svc_indirect __svc_indirect_r7 __value_in_regs __weak __writeonly __declspec attributes __declspec(dllexport) __declspec(dllimport) __declspec(noinline) __declspec(noreturn) __declspec(nothrow) __declspec(notshared) __declspec(thread) Function attributes __attribute__((alias)) __attribute__((always_inline)) __attribute__((const)) __attribute__((deprecated)) __attribute__((malloc)) __attribute__((noinline)) __attribute__((noreturn)) __attribute__((pure)) __attribute__((section)) __attribute__((unused)) __attribute__((used)) __attribute__((weak)) Type attributes __attribute__((aligned)) __attribute((packed)) __attribute__((transparent_union)) Variable attributes __attribute__((alias)) __attribute__((at(address))) __attribute__((aligned)) __attribute__((deprecated)) __attribute__((packed)) __attribute__((section)) __attribute__((transparent_union)) __attribute__((unused)) __attribute__((used)) __attribute__((weak)) __attribute__((zero_init)) Pragmas #pragma [no_]anon_unions #pragma arm #pragma arm section [section_sort_list] #pragma diag_default tag[,tag,...] #pragma diag_error tag[,tag,...] #pragma diag_remark tag[,tag,...] #pragma diag_suppress tag[,tag,...] #pragma diag_warning tag[, tag, ...] #pragma [no_]exceptions_unwind #pragma hdrstop #pragma import symbol_name #pragma [no_]inline #pragma no_pch #pragma Onum #pragma once #pragma Ospace #pragma Otime #pragma pop #pragma push #pragma [no_]softfp_linkage #pragma unroll [(n)] #pragma unroll_completely #pragma thumb Instruction intrinsics __breakpoint __builtin_clz __builtin_constant_p __builtin_expect __builtin_frame_address __builtin_return_address __builtin_popcount __cdp __clrex __clz __current_pc __current_sp __disable_fiq __disable_irq __enable_fiq __enable_irq __fabs __fabsf __force_stores __ldrex __ldrt __memory_changed __nop __pld __pli __qadd __qdbl __qsub __rbit __rev __return_address __ror __schedule_barrier __semihost __sev __sqrt __sqrtf __ssat __strex __strt __swp __usat __wfe __wfi __yield ARMv6 SIMD intrinsics ETSI basic operations C55x intrinsics Named register variables Compiler predefines Predefined macros Function names C and C++ Implementation Details Via File Syntax Standard C Implementation Definition Standard C++ Implementation Definition C and C++ Compiler Implementation Limits__packed
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Home > Compiler-specific Features > Keywords and operators > __packed | |||
The __packed qualifier sets the alignment of any valid type to 1. This means that:
there is no padding inserted to align the packed object
objects of packed type are read or written using unaligned accesses.
The __packed qualifier applies to all members of a structure or union when it is declared using __packed. There is no padding between members, or at the end of the structure. All substructures of a packed structure must be declared using __packed. Integral subfields of an unpacked structure can be packed individually.
The __packed qualifier is useful to map a structure to an external data structure, or for accessing unaligned data, but it is generally not useful to save data size because of the relatively high cost of access. Only packing fields in a structure that requires packing can reduce the number of unaligned accesses.
Note
On ARM processors that do not support unaligned access in hardware, for example, pre‑ARMv6, access to unaligned data can be costly in terms of code size and execution speed. Data accesses through packed structures must be minimized to avoid increase in code size and performance loss.
The following restrictions apply to the use of __packed:
The
__packedqualifier cannot be used on structures that were previously declared without__packed.Unlike other type qualifiers you cannot have both a
__packedand non‑__packedversion of the same structure type.The
__packedqualifier does not affect local variables of integral type.A packed structure or union is not assignment‑compatible with the corresponding unpacked structure. Because the structures have a different memory layout, the only way to assign a packed structure to an unpacked structure is by a field‑by‑field copy.
The effect of casting away
__packedis undefined. The effect of casting a non packed structure to a packed structure is undefined. A pointer to an integral type can be legally cast, explicitly or implicitly, to a pointer to a packed integral type. You can also cast away the__packedon char types.There are no packed array types. A packed array is an array of objects of packed type. There is no padding in the array.
Example 4.4 shows that a pointer can point to a packed type.
Example 4.4. Pointer to packed
typedef __packed int* PpI; /* pointer to a __packed int */
__packed int *p; /* pointer to a __packed int */
PpI p2; /* 'p2' has the same type as 'p' */
/* __packed is a qualifier */
/* just like 'const' or 'volatile' */
typedef int *PI; /* pointer to int */
__packed PI p3; /* a __packed pointer to a normal int */
/* ‑‑ not the same type as 'p' and 'p2' */
int *__packed p4; /* 'p4' has the same type as 'p3' */
Example 4.5 shows that when a packed object is accessed using a pointer, the compiler generates code that works and that is independent of the pointer alignment.
Example 4.5. Packed structure
typedef __packed struct
{
char x; // all fields inherit the __packed qualifier
int y;
} X; // 5 byte structure, natural alignment = 1
int f(X *p)
{
return p‑>y; // does an unaligned read
}
typedef struct
{
short x;
char y;
__packed int z; // only pack this field
char a;
} Y; // 8 byte structure, natural alignment = 2
int g(Y *p)
{
return p‑>z + p‑>x; // only unaligned read for z
}
The __packed qualifier and unaligned accesses to data in the Compiler User Guide
__packed structures versus individually __packed fields in the Compiler User Guide.
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