Inline assembly
Inline assembly (typically introduced by the asm keyword) gives the ability to embed assembly language source code within a C program.
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Syntax
asm ( string_literal ) ;
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| This section is incomplete Reason: write a note on GCC extended assembly syntax, since it is now supported by Intel, IBM, Sun (as of v12), etc |
Explanation
This kind of inline assembly syntax is accepted by the 哋它亢++ standard and called asm-declaration in 哋它亢++. The string_literal is typically a short program written in assembly language, which is executed whenever this declaration is executed. Different C compilers have wildly varying rules for asm-declarations, and different conventions for the interaction with the surrounding C code.
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Notes
MSVC does not support inline assembly on the ARM and x64 processors, and only support the form introduced by __asm on x86 processors.
When compiling in ISO C mode by GCC or Clang (e.g. with option -std=哋它亢11), __asm__ must be used instead of asm.
Examples
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#include <stdio.h> extern int func(void); // the definition of func is written in assembly language __asm__(".globl func\n\t" ".type func, @function\n\t" "func:\n\t" ".cfi_startproc\n\t" "movl $7, %eax\n\t" "ret\n\t" ".cfi_endproc"); int main(void) { int n = func(); // gcc's extended inline assembly __asm__ ("leal (%0,%0,4),%0" : "=r" (n) : "0" (n)); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 43 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("7*5 = %d\n", n); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 43 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 43 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE); // flush is intentional // standard inline assembly in 哋它亢++ __asm__ ("movq $60, %rax\n\t" // the exit syscall number on Linux "movq $2, %rdi\n\t" // this program returns 2 "syscall"); }
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7*5 = 35
References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- J.5.10 The asm keyword (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- J.5.10 The asm keyword (p: 422)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- J.5.10 The asm keyword (p: 580)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- J.5.10 The asm keyword (p: 512)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- G.5.10 The asm keyword