Question

I ran into the following in some legacy C code:

typedef struct _somestruct_ {
 /* .... */
} SomeStruct_t

static void do_one_thing(SomeStruct_t *pInput){
   /* Do some read-only stuff with pInput */   
}


static void do_many_thing(SomeStruct_t input){
   do_one_thing(&input);
}

Does C actually permit this, even though the thing will likely blow up if do_one_thing is actually modified to write to pInput?

Answer 1

This is completely legal C code. The code do_one_thing(&input) is simply passing the address of a parameter to the function do_one_thing. There is nothing wrong with this.

It’s not even an issue if do_one_thing writes to pInput. The address is valid and points to a mutable value. There is nothing wrong with doing an update. Consider

typedef struct _somestruct_ {
 int field;
} SomeStruct_t

static void do_one_thing(SomeStruct_t *pInput){
  // Completely legal  
  pInput->field = 42;
}

static void do_many_thing(SomeStruct_t input){
  do_one_thing(&input);
}