Why is the size of a function in C always 1 byte?

Question

When we check the size of a function using sizeof(), we always get 1 byte. What does this 1 byte signify?

Answer 1

This is not undefined behavior - the C language standard requires a diagnostic when using the sizeof operator with a function designator (a function name) since it's a constraint violation for the sizeof operator.

However, as an extension to the C language, GCC allows arithmetic on void pointers and function pointers, which is done by treating the size of a void or a function as 1. As a consequence, the sizeof operator will evaluate to 1 for void or a function with GCC. See http://gcc.gnu.org/onlinedocs/gcc/Pointer-Arith.html#Pointer-Arith

You can get GCC to issue a warning when using sizeof with these operands by using the -pedantic or -Wpointer-arith options to GCC. Or make it an error with -Werror=pointer-arith.