How Do Compilers Actually Handle the Unspecified Order of Function Arguments in C ?

Compilers and the Indeterminate Evaluation Order of Function Arguments

Although the C standard allows implementations to evaluate function arguments in an unspecified order, it remains unclear whether compilers actually exploit this flexibility. One might assume that left-to-right evaluation would be prevalent to avoid breaking poorly written code.

However, the evaluation order depends on factors such as the argument type, calling convention, architecture, and compiler settings. For instance, x86 processors with the Pascal calling convention evaluate arguments left to right, while the C calling convention (__cdecl) evaluates them right to left.

As a result, it's crucial for cross-platform programs to consider calling conventions to avoid surprises. According to Raymond Chen's blog and the Stack and Calling section of the GCC manual, compilers may deviate from left-to-right evaluation under certain circumstances.

It's important to note that the language standard leaves the evaluation order unspecified, meaning it's non-deterministic and not guaranteed. This allows for different compilers and platforms to have varying execution sequences for a given program and input.

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