GCC/G++'s treatment of missing return statement

gcc will not complain the missing of return statement.

foo.c

int add(int l, int r) {
}
int main(int argc, const char *argv[]) {
  int result = add(1, 2);
  result++;
  return 0;
}

$ gcc foo.c

By add -Wall, gcc gives a warning

$ gcc -Wall foo.c
foo.c: In function ‘add’:
foo.c:2: warning: control reaches end of non-void function

In 6.8.6.4 of C99 standaard, there is the following text:

A return statement with an expression shall not appear in a function whose
return type is void. A return statement without an expression shall only appear
in a function whose return type is void.

So gcc treats this constraint as a warning.

g++ is similar to gcc regarding return statement missing. In 7.3 of The C++
Programming Languge (3rd edition), there is the following text:

A value must be returned from a function that is not declared void (however,
main() is special; see void main §3.2). Conversely, a value cannot be returned
from a void function.

In 6.6.3 of ISO/IEC 14882:2003, there is the following text:

Flowing off the end of a function is equivalent to a return with no value; this
results in undefined behavior in a value-returning function.

http://old.nabble.com/missing-return-statement-td22487507.html is a discussion
about why g++ treats missing return statement in the current way.