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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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.
Greg Hewgill
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Thanks! A quick check in /usr/include/bits/errno.h confirmed that errno is indeed defined to be per-thread when using threads, on my Ubundu machine. – Erik Öjebo Jan 16 '09 at 08:04
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man errno says:
errno is defined by the ISO C standard to be a modifiable lvalue of type int, and must not be explicitly declared; errno may be a macro. errno is thread-local; setting it in one thread does not affect its value in any other thread.
qrdl
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