Asynchronous I/O is supported if the program is linked against the POSIX thread library. If that is not the case, all I/O is performed as synchronous. On systems which do not support pthread condition variables, such as AIX, I/O is also performed as synchronous.
On some systems, such as Darwin or Solaris, the POSIX thread library is always linked in, so asynchronous I/O is always performed. On other sytems, such as Linux, it is necessary to specify -pthread, -lpthread or -fopenmp during the linking step.
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