Adding value to pointer in struct, when using it in a function call

Question

Former I had a function f(double *a), which gets an array to work on. In order to get the correct position it should work on the array, it gets called as f(a+offset), with offset an integer. Now I reworked the code, and I defined a custom struct:

typedef struct{
double *a;
} struName;

and the function changed to f(struName *a). Now I was wondering how I still could get the offset to the function, without having to create a new temporary struct or having to change the function from f(struName *a) to f(struName *a, int offset). Is that even possible?

Why you changed the function prototype to f(struName *a)? It should have still worked with f(variabelname.a + offset) — Sourav Ghosh
– Sourav Ghosh, Commented Apr 28, 2016 at 7:46
Changing the function seems like the cleanest solution by far. If that can't be done, is adding an offset member to the struct an option? — Lundin
– Lundin, Commented Apr 28, 2016 at 8:06

Leandros · Accepted Answer · 2016-04-28 07:54:00Z

2

It's not possible, without changing the contents of the struct, or invoking undefined behaviour.

If you have access to ISO C99, you can change the signature to accept the struct by value, and pass in a compound literal:

f((struct struName){ .a = myStruct.a + offset });

Otherwise your proposed solution with to distinct parameters should work just fine:

f(myStruct, offset);

answered Apr 28, 2016 at 7:54

Leandros

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6 Comments

Leandros Over a year ago

If you do not fear undefined behavior, and want to break the strict aliasing rule, you can do: f(((double*)myStruct)+offset);. DO NOT DO THIS UNLESS YOU LIKE NASAL DAEMONS

Lundin Over a year ago

That UB is easily dodged by changing the struct to a union, in which case the behavior would be well-defined. There's only one member anyhow.

Leandros Over a year ago

@Lundin Isn't it still aliasing a different type?

Lundin Over a year ago

It is well-defined as per 6.7.2.1/16:

"A pointer to a union object, suitably converted, points to each of its members (or if a member is a bitfield, then to the unit in which it resides), and vice versa."

Also note this exception for unions in the "strict aliasing rule": "an aggregate or union type that includes one of the aforementioned types among its members"

Leandros Over a year ago

@Lundin Cool. Thank you!

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Lundin · Accepted Answer · 2016-04-28 08:41:18Z

2

The best solution is to change the function to take an offset parameter.

If that isn't an option, the second best solution is to change the struct definition to contain an offset member (if that makes sense design-wise).

If that isn't an option either, the third best solution is like in the answer by Leandros: create a temporary struct and change the pointer member before passing a pointer to the temporary struct. That is, treat the struct as immutable - if possible make the pointer member const. This keeps the program design clean, but you have to make an extra copy of the struct.

answered Apr 28, 2016 at 8:41

Lundin

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