Inline function in C

Inline Function are those function whose definitions are small and be substituted at the place where its function call is happened. Function substitution is totally compiler choice.

Let’s take below example:

#include <stdio.h> 
  
// Inline function in C 
inline int foo() 
{ 
    return 2; 
} 
  int main() 
{ 
      int ret; 
      // inline function call 
    ret = foo(); 
      printf("Output is: %d\n", ret); 
    return 0; 
} 

Compiler Error:

In function `main':
undefined reference to `foo'
Why this error happened?

This is one of the side effect of GCC the way it handle inline function. When compiled, GCC performs inline substitution as part of optimisation. So there is no function call present (foo) inside main. 



Normally GCC’s file scope is “not extern linkage”. That means the inline function is never ever provided to the linker which is causing linker error, mentioned above.

How to remove this error?

To resolve this problem use “static” before inline. Using static keyword forces the compiler to consider this inline function in the linker, and hence the program compiles and run successfully.

Example:

#include <stdio.h> 
  
// Inline function in C 
static inline int foo() 
{ 
    return 2; 
} 
  
int main() 
{ 
      int ret; 
    // inline function call 
    ret = foo(); 
      printf("Output is: %d\n", ret); 
    return 0; 
}

Output:
Output is: 2