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If you are writing a multi-threaded application that uses system/library calls that make use of errno to indicate the error type, is there a safe way to use errno? If not, is there some other way to indicate the type of error that occurred rather than just that an error has occurred?
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Yes, it is thread safe. On Linux, the global errno variable is thread-specific. POSIX requires that errno be threadsafe.
write() is certainly thread-safe. The problem is that a partial write() could require multiple calls in order to completely write the data, and while that is "thread-safe" it could result in interleaved data.
If your standard library is multithread aware, then it probably has a #define that changes errno into a function call that returns a thread-local error return value. However, to use this you generally must include <errno.h>, rather than relying on an extern declaration.
I found an article Thread-safety and POSIX.1 which addresses this very question.
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