---
title: Macros
TARGET DECK: Obsidian::STEM
FILE TAGS: c17::macro
tags:
  - c17
---

## Overview

Macros refer to `#define` directives that specify terms that should be textually replaced by the preprocessor during compilation:

```c
#define NAME ...
```

For types that don't have literals that describe their constants, we can use **compound literals** on the replacement side of the macro:

```c
#define NAME (T){ INIT }
```

%%ANKI
Basic
What preprocessor directive is used to define macros?
Back: `#define`
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419429-->
END%%

%%ANKI
Basic
How are compound literals specified in a macro definition, say `MACRO`?
Back:
```c
#define MACRO (T){ INIT }
```
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419447-->
END%%

%%ANKI
Basic
What term is used to refer to the replacement side of the following macro?
```c
#define MACRO (T){ INIT }
```
Back: A compound literal.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419481-->
END%%

%%ANKI
Basic
What is the difference between the following two lines?
```c
#define MACRO (T){ INIT }
# define MACRO (T){ INIT }
```
Back: N/A. They are equivalent.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419485-->
END%%

%%ANKI
Basic
What is the difference between the following two lines?
```c
#define MACRO (T){ INIT }
#define MACRO(T){ INIT }
```
Back: The first defines a compound literal. The latter defines a function-like macro.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419489-->
END%%

%%ANKI
Basic
What is `T` a reference to in the following compound literal?
```c
#define MACRO (T){ INIT }
```
Back: A type-specifier.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419492-->
END%%

%%ANKI
Basic
What is `INIT` a reference to in the following compound literal?
```c
#define MACRO (T){ INIT }
```
Back: An initializer.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419495-->
END%%

%%ANKI
Basic
Why aren't compound literals suitable for ICE?
Back: They are objects, not constants.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419498-->
END%%

%%ANKI
Basic
How can the following be rewritten so that `MACRO` is an object?
```c
#define MACRO 5
```
Back:
```c
#define MACRO (int){5}
```
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419500-->
END%%

%%ANKI
Basic
What is the difference between the following two lines?
```c
#define MACRO 5
#define MACRO (int){5}
```
Back: The former is a literal whereas the latter is a compound literal.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419503-->
END%%

%%ANKI
Basic
Why should compound literals be, generally speaking, `const`-qualified?
Back: Doing so gives the optimizer more room to generate good binary code.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419506-->
END%%

%%ANKI
Basic
How do we write macro definitions that span more than one line?
Back: Ending all but the last line with a `\` character.
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419508-->
END%%

%%ANKI
Basic
Generally speaking, what character should *not* be specified at the end of a macro definition?
Back: `;`
Reference: Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).
<!--ID: 1727432419511-->
END%%

## Bibliography

* Jens Gustedt, _Modern C_ (Shelter Island, NY: Manning Publications Co, 2020).