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I'm documenting FreeBSD's departures from known standards/APIUE/UNP as I come across them in various man pages and source code. The entirety of the FreeBSD man pages can be browsed at This page serves as a companion to "Linux APIs".

File descriptors

Socket Options


  • SO_ACCEPTFILTER (Introduced in FreeBSD 4.0): Apply an accept_filter(9) kernel module to the listen(2)ing socket. The connection will not be accept(2)able until the filter is satisfied. accf_data(9), satisfied by the receipt of any connection payload data, is roughly equivalent to Linux's TCP_DEFER_ACCEPT.


  • TCP_NOPUSH: Normally, every write operation to a TCP socket will result in an attempt to set the TCP PSH bit and transmit the queued data. With TCP_NOPUSH set, packets will be coalesced until either the MSS is reached, or the socket is closed. Compare to Linux's TCP_CORK.
    • It is my experience that clearing this flag will result in the transmission of queued data, but I cannot find documentation supporting this. Tony Finch's kernel patch seems to have been applied to FreeBSD 4.5, providing this behavior.