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   pselect.3p    ( 3 )

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Возвращаемое значение (Return value)

Upon successful completion, the pselect() and select() functions
       shall return the total number of bits set in the bit masks.
       Otherwise, -1 shall be returned, and errno shall be set to
       indicate the error.

       FD_CLR(), FD_SET(), and FD_ZERO() do not return a value.
       FD_ISSET() shall return a non-zero value if the bit for the file
       descriptor fd is set in the file descriptor set pointed to by
       fdset, and 0 otherwise.