What is the Static Initialization Order Fiasco (SIOF) in C ?

Static Initialization Order Fiasco

The "static initialization order fiasco" (SIOF) is a potential issue in C that can arise when multiple translation units (e.g., .cpp files) contain statically initialized variables that depend on each other.

Consider the following example:

// file1.cpp
extern int y;
int x = y + 1;

// file2.cpp
extern int x;
int y = x + 1;

When compiling this code, the following steps will occur:

• File1.cpp:

  • The compiler encounters y and declares it as an external variable (without allocating storage).
  • It allocates storage for x but does not initialize it.

• File2.cpp:

  • The compiler encounters x and declares it as an external variable (without allocating storage).
  • It allocates storage for y but does not initialize it.

During linking, the order in which the object files are initialized is significant. If file2.o is initialized before file1.o, the following will happen:

• x will be initialized to 0 by default initialization.
• y will be initialized to x 1, which evaluates to 1.

On the other hand, if file1.o is initialized before file2.o, the same values will be set for x and y. The order in which the object files are initialized is therefore crucial for the correct execution of the program.

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