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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)) function attribute __attribute__((always_inline)) function attribute __attribute__((const)) function attribute __attribute__((constructor[(priority)])) function __attribute__((deprecated)) function attribute __attribute__((destructor[(priority)])) function a __attribute__((format_arg(string-index))) function __attribute__((malloc)) function attribute __attribute__((noinline)) function attribute __attribute__((nomerge)) function attribute __attribute__((nonnull)) function attribute __attribute__((noreturn)) function attribute __attribute__((notailcall)) function attribute __attribute__((pcs("calling_convention") __attribute__((pure)) function attribute __attribute__((section("name"))) functio __attribute__((unused)) function attribute __attribute__((used)) function attribute __attribute__((visibility("visibility_type&qu __attribute__((weak)) function attribute __attribute__((weakref("target"))) funct Type attributes __attribute__((bitband)) type attribute __attribute__((aligned)) type attribute __attribute((packed)) type attribute Variable attributes __attribute__((alias)) variable attribute __attribute__((at(address))) variable attribute __attribute__((aligned)) variable attribute __attribute__((deprecated)) variable attribute __attribute__((noinline)) constant variable attrib __attribute__((packed)) variable attribute __attribute__((section("name"))) variabl __attribute__((unused)) variable attribute __attribute__((used)) variable attribute __attribute__((visibility("visibility_type&qu __attribute__((weak)) variable attribute __attribute__((weakref("target"))) varia __attribute__((zero_init)) variable attribute Pragmas #pragma anon_unions, #pragma no_anon_unions #pragma arm #pragma arm section [section_type_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 exceptions_unwind, #pragma no_exceptions_u #pragma hdrstop #pragma import symbol_name #pragma import(__use_full_stdio) #pragma import(__use_smaller_memcpy) #pragma inline, #pragma no_inline #pragma no_pch #pragma Onum #pragma once #pragma Ospace #pragma Otime #pragma pack(n) #pragma pop #pragma push #pragma softfp_linkage, #pragma no_softfp_linkage #pragma unroll [(n)] #pragma unroll_completely #pragma thumb #pragma weak symbol, #pragma weak symbol1 = symbol Instruction intrinsics __breakpoint intrinsic __cdp intrinsic __clrex intrinsic __clz intrinsic __current_pc intrinsic __current_sp intrinsic __disable_fiq intrinsic __disable_irq intrinsic __enable_fiq intrinsic __enable_irq intrinsic __fabs intrinsic __fabsf intrinsic __force_stores intrinsic __ldrex intrinsic __ldrexd intrinsic __ldrt intrinsic __memory_changed intrinsic __nop __pld intrinsic __pldw intrinsic __pli intrinsic __promise intrinsic __qadd intrinsic __qdbl intrinsic __qsub intrinsic __rbit intrinsic __rev intrinsic __return_address intrinsic __ror intrinsic __schedule_barrier intrinsic __semihost intrinsic __sev intrinsic __sqrt intrinsic __sqrtf intrinsic __ssat intrinsic __strex intrinsic __strexd intrinsic __strt intrinsic __swp intrinsic __usat intrinsic __wfe intrinsic __wfi intrinsic __yield intrinsic ARMv6 SIMD intrinsics See also ETSI basic operations Example See also C55x intrinsics Example See also VFP status intrinsic __vfp_status intrinsic Fused Multiply Add (FMA) intrinsics Named register variables Syntax Usage Examples See also Compiler predefines Predefined macros Function names C and C++ Implementation Details ARMv6 SIMD Instruction Intrinsics Via File Syntax Summary Table of GNU Language Extensions Standard C Implementation Definition Standard C++ Implementation Definition C and C++ Compiler Implementation Limits#pragma once
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This pragma enables the compiler to skips subsequent includes of that header file.
#pragma once is accepted for compatibility with other compilers, and enables you to use other forms of header guard coding. However, it is preferable to use #ifndef and #define coding because this is more portable.
ExampleThe following example shows the placement of a #ifndef guard around the body of the file, with a #define of the guard variable after the #ifndef.
#ifndef FILE_H
#define FILE_H
#pragma once // optional ... body of the header file ...#endif
The #pragma once is marked as optional in this example. This is because the compiler recognizes the #ifndef header guard coding and skips subsequent includes even if #pragma once is absent.
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