How can we know the caller function’s name?

There’s nothing you can do only in a.

However, with a simple standard macro trick, you can achieve what you want, IIUC showing the name of the caller.

void a()
{
    /* Your code */
}

void a_special( char const * caller_name )
{
    printf( "a was called from %s", caller_name );
    a();
}

#define a() a_special(__func__)

void b()
{
    a();
}