What Is a Packed Data Structure?


Scott Campbell

What Is a Packed Data Structure?

Data structures are a fundamental concept in computer science and programming. They allow us to store and organize data efficiently, making it easier for us to access and manipulate it. One such data structure is a packed data structure.

A packed data structure is a way of organizing data in memory so that it occupies the least amount of space possible. It achieves this by eliminating any unnecessary padding or alignment requirements, which are usually implemented for performance reasons.

Why Use Packed Data Structures?

There are several reasons why you might choose to use packed data structures in your programs:

  • Memory Efficiency: Packed data structures can save memory by eliminating padding. This can be particularly useful when dealing with large amounts of data or when memory is limited.
  • Data Transmission: Packed data structures can also be beneficial when transmitting data over a network or storing it in files. By reducing the size of the data, you can save bandwidth and storage space.
  • Performance: In some cases, packed data structures can improve performance by reducing cache misses and improving memory access patterns.

The Trade-Offs

While using packed data structures has its advantages, there are also some trade-offs to consider:

  • Memory Alignment Issues: Without padding, accessing individual members of a packed structure may require additional calculations or slower access times due to alignment constraints.
  • Inefficient Memory Accesses: When accessing members of a packed structure, the CPU may need to read extra bytes before getting to the desired member. This can lead to slower performance compared to aligned structures.
  • Data Corruption: Packed data structures can be more prone to data corruption if not used carefully. For example, if you mix packed and unpacked data structures, it can lead to unpredictable behavior and bugs.

Examples of Packed Data Structures

Here are a few examples of packed data structures in different programming languages:

C Example:

#pragma pack(push, 1)
struct PackedStruct {
    char a;
    int b;
#pragma pack(pop)

Python Example:

import ctypes

class PackedStruct(ctypes.Structure):
    _pack_ = 1
    _fields_ = [
        ('a', ctypes.c_char),
        ('b', ctypes.c_int),


Packed data structures are a useful tool for optimizing memory usage and improving performance in certain scenarios. However, they also come with trade-offs that need to be carefully considered. It’s important to weigh the benefits against potential drawbacks before deciding to use packed data structures in your programs.

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