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strlen is a standard function (in a hosted environment). So it must do exactly what the standard says it does, and the standard doesn't say it has side-effects

strlen is a standard function (in a hosted environment). So it must do exactly what the standard says it does, and the standard doesn't say it has side-effects. The compiler could very well use a built-in implementation of strlen, or even omit the call entirely if it had another way to deduce its would-be return value. Object files are an implementation detail not known by the C standard.

sokoloff 5y ago
> the standard doesn't say it has side-effects The relevant question is "does the standard say that it does not have side-effects?" (is a pure function). @skissane's sibling comment to yours provides the explanation of how the compiler can deduce that it's a pure function.
- foxfluff 5y ago
  The compiler can deduce that it is a pure function using the same logic by which it can deduce it is free to replace the call with a builtin: the function is defined in the standard. Everything @skissane said is implementation details and not particularly relevant (the compiler can do the optimization without the attribute they mentioned). I think the standard would fall apart if you read it under the assumption that anything not explicitly forbidden can happen. Instead, you should read and find out what the side effects are (and same for undefined behavior, unspecified behavior, implementation defined behavior, etcetra). "Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment." (There are more details if you care to dig in) Of course nothing stops you or me from making extensions to the standard, but analysing things from the perspective that some implementation might extend strlen to have visible side effects goes too far into whataboutism for my taste, unless there are real world examples to make it a relevant point.

sokoloff 5y ago

> the standard doesn't say it has side-effects The relevant question is "does the standard say that it does not have side-effects?" (is a pure function). @skissane's sibling comment to yours provides the explanation of how the compiler can deduce that it's a pure function.

foxfluff 5y ago
The compiler can deduce that it is a pure function using the same logic by which it can deduce it is free to replace the call with a builtin: the function is defined in the standard. Everything @skissane said is implementation details and not particularly relevant (the compiler can do the optimization without the attribute they mentioned). I think the standard would fall apart if you read it under the assumption that anything not explicitly forbidden can happen. Instead, you should read and find out what the side effects are (and same for undefined behavior, unspecified behavior, implementation defined behavior, etcetra). "Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects, which are changes in the state of the execution environment." (There are more details if you care to dig in) Of course nothing stops you or me from making extensions to the standard, but analysing things from the perspective that some implementation might extend strlen to have visible side effects goes too far into whataboutism for my taste, unless there are real world examples to make it a relevant point.