How Can C Structs Achieve Cross-Platform File I/O Compatibility?

Cross-Platform Compatibility for C Struct File I/O

The issue of cross-platform compatibility arises when attempting to read or write a C struct to a file that may be accessed by different platforms or compilers.

In C , struct padding differs among compilers and platforms due to a lack of standardization at the binary level. This means that the memory layout of a struct can vary depending on the target environment.

Unfortunately, this discrepancy makes it impossible to safely read/write structs across platforms and compilers. Don Box highlights this fundamental weakness in C , emphasizing the absence of a standardized binary runtime model.

Compilers apply different padding rules, even within the same compiler, based on the pragma pack used. Moreover, simply reordering member declarations within structs can alter their sizes.

For instance, consider the following example:

struct A
{
   char c;
   char d;
   int i;
};

struct B
{
   char c;
   int i;
   char d;
};

When compiled with gcc-4.3.4, the output demonstrates different sizes for A and B:

8
12

This variation in struct sizes underscores the lack of consistency in padding across compilers.

As a result, it is not possible to guarantee that a struct written from one platform and compiler will be correctly interpreted by another platform and compiler due to varying padding rules.

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