inline serves two purposes: it suggests that the compiler substitute the function body at call sites rather than performing a function call, and it controls the linkage of functions defined in headers. An inline function definition allows multiple translation units to share the same function definition without causing "multiple definition" linker errors. The compiler may ignore the inlining hint.

Function calls have overhead: saving registers, pushing arguments, jumping, and returning. For small, frequently called functions, this overhead can dominate. Inlining eliminates this overhead and enables further optimizations by exposing the function body to the calling context. Without inline, placing a function definition in a header file causes linker errors when multiple translation units include that header. inline allows functions to be defined in headers and shared across translation units while maintaining proper linkage.

#include <stdio.h>

static inline int max(int a, int b) {
    return a > b ? a : b;
}

static inline int square(int x) {
    return x * x;
}

int main(void) {
    int a = 10;
    int b = 20;

    // Compiler may inline these calls
    printf("max(%d, %d) = %d\n", a, b, max(a, b));
    printf("square(%d) = %d\n", a, square(a));

    // Expression using inline functions
    printf("max of squares: %d\n", max(square(a), square(b)));
}