Inline Specifier

inline is a specifier for a function in C/C++ that tells the compiler to replace the function call in assembly with the contents of the function. It is most often used when tying to speed up a program as it takes out the overhead of a function call. it is particularly useful with small functions that are called often. However, the specifier will also increase the size of your program as it takes away the benefit of re-usability at the assembly level.

Basic Example of the inline Specifier

In the following example there are two functions that do the same thing. Except one uses the inline specifier and the other does not. When compiled using compiler optimizations, the function with the inline specifier will not have a function call. Rather, the functions contents will be put directly in the main function of the assembly output. Note: If compiled without any optimizations -O0, the inline specifier will be ignored.

#include "stdio.h"

void print_NON_inline()
{
	printf("%d", 2);
}

inline void print_WITH_inline()
{
	printf("%d", 3);
}

int main(void)
{
	print_NON_inline();
	print_WITH_inline(); 
	return 1;
}

The following output was compiled with an optimization of -O1

00000000 <print_NON_inline()>:
_Z16print_NON_inlinev():
   0:	83 ec 10             	sub    $0x10,%esp
   3:	6a 02                	push   $0x2
   5:	68 00 00 00 00       	push   $0x0	6: R_386_32	.rodata.str1.1
   a:	6a 01                	push   $0x1
   c:	e8 fc ff ff ff       	call   d <print_NON_inline()+0xd>	d: R_386_PC32	__printf_chk
  11:	83 c4 1c             	add    $0x1c,%esp
  14:	c3                   	ret    

00000015 <main>:
main():
  15:	8d 4c 24 04          	lea    0x4(%esp),%ecx
  19:	83 e4 f0             	and    $0xfffffff0,%esp
  1c:	ff 71 fc             	pushl  -0x4(%ecx)
  1f:	55                   	push   %ebp
  20:	89 e5                	mov    %esp,%ebp
  22:	51                   	push   %ecx
  23:	83 ec 04             	sub    $0x4,%esp
  26:	e8 fc ff ff ff       	call   27 <main+0x12>	27: R_386_PC32	print_NON_inline()
  2b:	83 ec 04             	sub    $0x4,%esp
  2e:	6a 03                	push   $0x3
  30:	68 00 00 00 00       	push   $0x0	31: R_386_32	.rodata.str1.1
  35:	6a 01                	push   $0x1
  37:	e8 fc ff ff ff       	call   38 <main+0x23>	38: R_386_PC32	__printf_chk
  3c:	83 c4 10             	add    $0x10,%esp
  3f:	b8 01 00 00 00       	mov    $0x1,%eax
  44:	8b 4d fc             	mov    -0x4(%ebp),%ecx
  47:	c9                   	leave  
  48:	8d 61 fc             	lea    -0x4(%ecx),%esp
  4b:	c3                   	ret