Optimizing Enum Storage in C with -fshor-enum flag

kmika

1 tydzień ago

When working with C, efficiency in memory usage can be a critical factor, especially in embedded systems or low-level programming. One lesser-known yet powerful GCC compiler flag that can help optimize storage is -fshort-enums. This flag changes the way the compiler allocates memory for enumerations (enum), potentially reducing the size of compiled code. In this article, we’ll look at how -fshort-enums works, when you might want to use it, and the trade-offs involved. Whether you’re working on embedded systems, optimizing for low-memory environments, or just curious about how GCC handles enum types, this deep dive will help you make informed decisions.

Understanding the -fshor-enum flag

In standard C compilation, enum types are usually stored as integers, which means they are assigned the same size as an int (usually 4 bytes on most platforms). This is done to maintain consistency and alignment across platforms. However, many enum values contain only a small range of values that could fit into a smaller data type, such as a char or short. This is where the -fshort-enums flag in GCC comes in. When the -fshort-enums flag is enabled, GCC automatically determines the smallest integer type that can hold all the possible values of an enum, and uses that type instead of defaulting to an int. For example

unsigned char (1 byte) if all enum values fit within 0–255
signed char (1 byte) if values fit within -128 to 127.
unsigned short (2 bytes) if values fit within 0–65,535.
signed short (2 bytes) if values fit within -32,768 to 32,767.

This results in a more compact representation which reduces memory consumption, especially in scenarios such as embedded systems where every byte counts.

Let’s see some examples

To check the effect of -fshort-enums, we can use the sizeof operator:

#include <stdio.h>

enum Color {
    RED,
    GREEN,
    BLUE
};

int main() {
    printf("Size of enum Color: %zu bytes\n", sizeof(enum Color));
    return 0;
}

Compile this code without specifing any additional flags and check the result in aterminal. You can run this code in an online compiler such as https://godbolt.org/ or use gcc in the terminal:

gcc example.c -o example

As a result of this code snippet you should see the following in the terminal:

Size of enum Color: 4 bytes

Compile the same code again, but now with the -fshort-enums flag. You can add your own compiler options in the goodbolt online compiler. You can also compile it this way on your machine:

gcc -fshort-enums example.c -o example

Since the maximum value for the colour enum is 2, the compiler will use the smallest possible integer type to minimise the memory footprint. You should now be able to see the result:

Size of enum Color: 1 bytes

Disadvantages of using this -fshort-enums flag

One of the biggest risks of -fshort-enums is incompatibility between two binary program modules. By default, compilers treat enum types as int (4 bytes). If different parts of a program (or a program and a library it links to) are compiled with and without -fshort-enums, they may have different assumptions about enum sizes, leading to undefined behaviour or crashes. Consider a shared library is compiled without -fshort-enums

// Compiled without -fshort-enums
struct Data {
    enum Color color;
    int value;
};

The structure that normally looks like this in memory:

| 4-byte enum (Color) | 4-byte int (value) |

Now, if an application with -fshort-enums reads the same structure:

| 1-byte enum (Color) | 4-byte int (value) |

This results in misaligned data access, potentially corrupting memory or crashing the program. Similar situations occur with serialisation and network communication. When sending enum values over a network, writing them to files, or working with structured binary data, using -fshort-enums can be risky. If an enum is stored as 1 byte on one system and read as 4 bytes on another, this can lead to incorrect data interpretation.

Summary

Using the -fshort-enums flag can help optimise memory usage on embedded systems & low memory environments. This is because most enums are used for small amounts of data. Typically it can fit 1 byte, but without specifying to use -fshort-enums at compile time, this will be stored as 4 bytes by default. As you can see, this can be very beneficial in terms of memory usage, but don’t forget that the size of an enum can vary between compilers or compilation flags, leading to compatibility problems when exchanging binary data between different systems. There is a potential risk of misunderstanding the data when referencing it with pointers. If an enum is used within a struct, changing its size may affect the layout and alignment of the struct. Using -fshort-enums can make debugging more difficult: gdb or other debuggers may assume a 4-byte enum, leading to incorrect variable display. Compiler warnings/errors can be misleading when dealing with mismatched sizes. As you can see, the benefits of using -fshort-enums are undeniable, but there are many other considerations to make sure you don’t run into problems just because you want to save a few bytes of memory.

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