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Re: How to check for error with write(2)

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Philip Guenther <guenther@gmail.com> writes:

> Uh, actually look at the return value and see if it matches the
> number of bytes you asked to write: on partial success, write() will
> return less than the requested count.  Note that a short write won't
> change errno; if you need to know why it was unable to write more
> than you need to keep calling write() until it completely fails
> (usually the next call) and check errno then.
> 
> To quote the (next issue) of the POSIX spec's description of write():
>          If a write( ) requests that more bytes be written than
>          there is room for (for example, the file size limit of the
>          process or the physical end of a medium), only as many
>          bytes as there is room for shall be written. For example,
>          suppose there is space for 20 bytes more in a file before
>          reaching a limit. A write of 512 bytes will return 20. The
>          next write of a non-zero number of bytes would give a
>          failure return (except as noted below).
> 
>          If the request would cause the file size to exceed the soft
>          file size limit for the process and there is no room for
>          any bytes to be written, the request shall fail and the
>          implementation shall generate the SIGXFSZ signal for the
>          thread.
> 
> And then in the ERRORS section:
> ERRORS
>             These functions shall fail if:
> ...
> [EFBIG]      An attempt was made to write a file that exceeds the
>              implementation-defined maximum file size and there was
>              no room for any bytes to be written.
> 
> [EFBIG]      An attempt was made to write a file that exceeds the
>              file size limit of the process, and there was no room
>              for any bytes to be written. A SIGXFSZ signal shall
>              also be sent to the thread.

Ah, thanks.  How to handle a short write, i.e. with no error return
code, is exactly what I was looking for.  This information is not in
the Linux write(2) man page.

BTW, is it possible that after a short write the next write() syscall
for the remaining data will write some more data, possibly even the
whole remaining data successfully?  I think of something like a pipe
or a socket where not the whole data can be sent immediately.

Or will the write() syscall always block until all data could be
written?  I.e. will the next write syscall after a short write always
fail and return -1?

Steve

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Thread

How to check for error with write(2) Steve Keller <keller.steve@gmx.de> - 2021-12-16 18:58 +0100
  Re: How to check for error with write(2) scott@slp53.sl.home (Scott Lurndal) - 2021-12-16 18:26 +0000
    Re: How to check for error with write(2) Geoff Clare <geoff@clare.See-My-Signature.invalid> - 2021-12-20 13:52 +0000
  Re: How to check for error with write(2) Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2021-12-16 18:49 +0000
    Re: How to check for error with write(2) Rainer Weikusat <rweikusat@talktalk.net> - 2021-12-16 19:00 +0000
  Re: How to check for error with write(2) Rainer Weikusat <rweikusat@talktalk.net> - 2021-12-16 19:06 +0000
  Re: How to check for error with write(2) Jorgen Grahn <grahn+nntp@snipabacken.se> - 2021-12-16 22:34 +0000
    Re: How to check for error with write(2) Steve Keller <keller.steve@gmx.de> - 2021-12-17 04:38 +0100
      Re: How to check for error with write(2) Rainer Weikusat <rweikusat@talktalk.net> - 2021-12-17 15:14 +0000
      Re: How to check for error with write(2) Jorgen Grahn <grahn+nntp@snipabacken.se> - 2021-12-18 10:25 +0000
  Re: How to check for error with write(2) Philip Guenther <guenther@gmail.com> - 2021-12-16 15:04 -0800
    Re: How to check for error with write(2) Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2021-12-16 20:10 -0700
    Re: How to check for error with write(2) Steve Keller <keller.steve@gmx.de> - 2021-12-17 04:35 +0100
      Re: How to check for error with write(2) Philip Guenther <guenther@gmail.com> - 2021-12-18 14:58 -0800
        Re: How to check for error with write(2) scott@slp53.sl.home (Scott Lurndal) - 2021-12-19 16:07 +0000

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