UNIX nonblocking I O O NONBLOCK vs FIONBIO

Question

In every example and discussion I run across in the context of BSD socket programming  it seems that the recommended way to set a file descriptor to nonblocking I O mode is using the O NONBLOCK flag to fcntl    e g   int flags   fcntl fd  F GETFL  0   fcntl fd  F SETFL  flags   O NONBLOCK     I ve been doing network programming in UNIX for over ten years  and have always used the FIONBIO ioctl   call to do this   int opt   1  ioctl fd  FIONBIO   amp opt     Never really gave much thought to why   Just learned it that way     Does anyone have any commentary on the possible respective merits of one or the other   I imagine the portability locus differs somewhat  but do not know to what extent as ioctl list 2  doesn t speak to that aspect of individual ioctl methods

User · Answer

I believe fcntl   is a POSIX function  Where as ioctl   is a standard UNIX thing  Here is a list of POSIX io   ioctl   is a very kernel driver OS specific thing  but i am sure what you use works on most flavors of Unix  some other ioctl   stuff might only work on certain OS or even certain revs of it s kernel

User · Answer

Prior to standardization there was ioctl    FIONBIO     and fcntl    O NDELAY      but these behaved inconsistently between systems  and even within the same system   For example  it was common for FIONBIO to work on sockets and O NDELAY to work on ttys  with a lot of inconsistency for things like pipes  fifos  and devices   And if you didn t know what kind of file descriptor you had  you d have to set both to be sure   But in addition  a non-blocking read with no data available was also indicated inconsistently  depending on the OS and the type of file descriptor the read may return 0  or -1 with errno EAGAIN  or -1 with errno EWOULDBLOCK   Even today  setting FIONBIO or O NDELAY on Solaris causes a read with no data to return 0 on a tty or pipe  or -1 with errno EAGAIN on a socket   However 0 is ambiguous since it is also returned for EOF   POSIX addressed this with the introduction of O NONBLOCK  which has standardized behavior across different systems and file descriptor types   Because existing systems usually want to avoid any changes to behavior which might break backward compatibility  POSIX defined a new flag rather than mandating specific behavior for one of the others   Some systems like Linux treat all 3 the same  and also define EAGAIN and EWOULDBLOCK to the same value  but systems wishing to maintain some other legacy behavior for backward compatibility can do so when the older mechanisms are used   New programs should use fcntl    O NONBLOCK      as standardized by POSIX

User · Answer

As  Sean said  fcntl   is largely standardized  and therefore available across platforms   The ioctl   function predates fcntl   in Unix  but is not standardized at all   That the ioctl   worked for you across all the platforms of relevance to you is fortunate  but not guaranteed   In particular  the names used for the second argument are arcane and not reliable across platforms   Indeed  they are often unique to the particular device driver that the file descriptor references    The ioctl   calls used for a bit-mapped graphics device running on an ICL Perq running PNX  Perq Unix  of twenty years ago never translated to anything else anywhere else  for example

[c] UNIX nonblocking I/O: O_NONBLOCK vs. FIONBIO

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