Question about implementation of Unix FD passing

Thiago Macieira thiago at
Sun Feb 10 09:41:46 PST 2013

On domingo, 10 de fevereiro de 2013 14.34.38, Georg Reinke wrote:
> However, because of the special semantics of out-of-band data on Unix
> sockets, one has to read all data in one operation as remaining data is
> discarded. That means that the buffer for the out-of-band data must big
> enough to hold the greatest number of file descriptors that can be
> possibly transferred, which is 2^32 (according to the spec), in order
> to guarantee that no data is discarded.
> While the actual size probably is much lower because of some kernel
> limit, this behaviour still seems odd to me. So my question is: Is this
> intended or have I misunderstood anything or made wrong assumptions
> anywhere?

It's not possible to query the system to find out how many file descriptors were 
passed. Since the number is most often very small, the reference 
implementation passes a buffer of 1024 entries.

Thiago Macieira - thiago (AT) - thiago (AT)
   Software Architect - Intel Open Source Technology Center
      PGP/GPG: 0x6EF45358; fingerprint:
      E067 918B B660 DBD1 105C  966C 33F5 F005 6EF4 5358
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