Bad manpage

DESCRIPTION
     Select() examines the I/O descriptor sets whose addresses are passed in readfds, writefds, and errorfds to see if some of their descriptors are ready
     for reading, are ready for writing, or have an exceptional condition pending, respectively.  The first nfds descriptors are checked in each set; i.e.,
     the descriptors from 0 through nfds-1 in the descriptor sets are examined.  (Example: If you have set two file descriptors "4" and "17", nfds should
     not be "2", but rather "17 + 1" or "18".)  On return, select() replaces the given descriptor sets with subsets consisting of those descriptors that are
     ready for the requested operation.  Select() returns the total number of ready descriptors in all the sets.

     The descriptor sets are stored as bit fields in arrays of integers.  The following macros are provided for manipulating such descriptor sets:
     FD_ZERO(&fdset) initializes a descriptor set fdset to the null set.  FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.  FD_CLR(fd,
     &fdset) removes fd from fdset.  FD_ISSET(fd, &fdset) is non-zero if fd is a member of fdset, zero otherwise.  FD_COPY(&fdset_orig, &fdset_copy) replaces
     an already allocated &fdset_copy file descriptor set with a copy of &fdset_orig.  The behavior of these macros is undefined if a descriptor value is
     less than zero or greater than or equal to FD_SETSIZE, which is normally at least equal to the maximum number of descriptors supported by the system.

     If timeout is a non-nil pointer, it specifies a maximum interval to wait for the selection to complete.  If timeout is a nil pointer, the select blocks
     indefinitely.  To effect a poll, the timeout argument should be non-nil, pointing to a zero-valued timeval structure.  Timeout is not changed by
     select(), and may be reused on subsequent calls, however it is good style to re-initialize it before each invocation of select().

Good manpage

DESCRIPTION
       select() and  pselect()  allow  a  program  to  monitor  multiple  file
       descriptors,  waiting  until one or more of the file descriptors become
       "ready" for some class of I/O operation (e.g., input possible).  A file
       descriptor  is considered ready if it is possible to perform the corre‐
       sponding I/O operation (e.g., read(2)) without blocking.

       The operation of select() and pselect() is identical, with  three  dif‐
       ferences:

       (i)    select()  uses  a timeout that is a struct timeval (with seconds
              and microseconds), while pselect() uses a struct timespec  (with
              seconds and nanoseconds).

       (ii)   select()  may  update  the timeout argument to indicate how much
              time was left.  pselect() does not change this argument.

       (iii)  select() has no  sigmask  argument,  and  behaves  as  pselect()
              called with NULL sigmask.

       Three  independent  sets of file descriptors are watched.  Those listed
       in readfds will be watched to see if characters  become  available  for
       reading  (more  precisely, to see if a read will not block; in particu‐
       lar, a file descriptor is also ready on end-of-file), those in writefds
       will  be  watched  to  see  if  a  write  will  not block, and those in
       exceptfds will be watched for exceptions.  On exit, the sets are  modi‐
       fied  in place to indicate which file descriptors actually changed sta‐
       tus.  Each of the three file descriptor sets may be specified  as  NULL
       if no file descriptors are to be watched for the corresponding class of
       events.