How Can I Programmatically Determine Endianness in C ?

Programmatically Detecting Endianness in C

Endianness, a property that indicates the order of bytes in a data structure, is crucial for cross-architecture code compatibility. In C , there is a method to detect endianness programmatically without conditional compilation.

To understand this method, we delve into the concept of unions. A union in C combines multiple data types into a single structure, where they share the same memory space. By utilizing a union, we can access the same data in different data formats, providing a clever solution to detect endianness.

Consider the following code snippet:

bool is_big_endian(void) {
    union {
        uint32_t i;
        char c[4];
    } bint = { 0x01020304 };

    return bint.c[0] == 1;
}

In this code, we define a union with a 32-bit integer (uint32_t) named i and an array of four characters named c. We initialize the union with the value 0x01020304, where the most significant byte is 0x01.

If the system is big-endian, the most significant byte will be stored in the first byte of the array c, making bint.c[0] equal to 1. In contrast, if the system is little-endian, the least significant byte will be stored in the first byte of c, resulting in bint.c[0] being equal to 4.

By comparing bint.c[0] with 1, we can determine the endianness of the system. This method is effective and avoids the use of type punning, which can generate compiler warnings. Additionally, it adheres to C99 standards and is compliant with OSes supporting multiple architectures.

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