Understanding Flexible Array Members in C: Syntax, Benefits, and Considerations

Introduction

This article explores the concept of flexible array members in C, providing insights into their syntax, advantages, drawbacks, and broader considerations related to memory manipulation in the C language.

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Syntax

In C, a flexible array member is declared by defining the last data member of a structure as an array without a specified length or with a length of 0:

struct Bar {
    int a;
    char b[]; // Also written as char b[0];
};

Memory Allocation

The size of a structure with a flexible array does not include the array size itself. For example:

struct Bar bar;
printf("%d %d\n", sizeof(bar), sizeof(bar.a));

The result indicates that the size of the bar variable is equal to the size of the bar.a data member.

To allocate memory for a flexible array, use:

struct Bar *bar = malloc(sizeof(struct Bar) + 128);

This allocates a contiguous block of 132 bytes, with the first 4 bytes assigned to bar->a and the subsequent 128 bytes accessible via bar->b.

Advantages and Trade-offs