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In this noncompliant code example, the function f() is called from within a multithreaded application but encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number.

The C Standard, 7.2426.6.2 3 paragraph 3 [ISO/IEC 9899:20112024], specifically states that strerror() is not required to avoid data races.

The strerror function is not required to avoid data races with other calls to the strerror function.

An implementation could write the error string into a static array and return a pointer to it, and that array might be accessible and modifiable by other threads.

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