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(libc.info)Interrupted Primitives

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Primitives Interrupted by Signals
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   A signal can arrive and be handled while an I/O primitive such as
`open' or `read' is waiting for an I/O device.  If the signal handler
returns, the system faces the question: what should happen next?

   POSIX specifies one approach: make the primitive fail right away.
The error code for this kind of failure is `EINTR'.  This is flexible,
but usually inconvenient.  Typically, POSIX applications that use signal
handlers must check for `EINTR' after each library function that can
return it, in order to try the call again.  Often programmers forget to
check, which is a common source of error.

   The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro `TEMP_FAILURE_RETRY':

 - Macro: TEMP_FAILURE_RETRY (EXPRESSION)
     This macro evaluates EXPRESSION once.  If it fails and reports
     error code `EINTR', `TEMP_FAILURE_RETRY' evaluates it again, and
     over and over until the result is not a temporary failure.

     The value returned by `TEMP_FAILURE_RETRY' is whatever value
     EXPRESSION produced.

   BSD avoids `EINTR' entirely and provides a more convenient approach:
to restart the interrupted primitive, instead of making it fail.  If
you choose this approach, you need not be concerned with `EINTR'.

   You can choose either approach with the GNU library.  If you use
`sigaction' to establish a signal handler, you can specify how that
handler should behave.  If you specify the `SA_RESTART' flag, return
from that handler will resume a primitive; otherwise, return from that
handler will cause `EINTR'.  Note: Flags for Sigaction.

   Another way to specify the choice is with the `siginterrupt'
function.  Note: BSD Handler.

   When you don't specify with `sigaction' or `siginterrupt' what a
particular handler should do, it uses a default choice.  The default
choice in the GNU library depends on the feature test macros you have
defined.  If you define `_BSD_SOURCE' or `_GNU_SOURCE' before calling
`signal', the default is to resume primitives; otherwise, the default
is to make them fail with `EINTR'.  (The library contains alternate
versions of the `signal' function, and the feature test macros
determine which one you really call.)  Note: Feature Test Macros.

   The description of each primitive affected by this issue lists
`EINTR' among the error codes it can return.

   There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as `read' or
`write' is interrupted by a signal after transferring part of the data.
In this case, the function returns the number of bytes already
transferred, indicating partial success.

   This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; Note: Datagrams),
where splitting one `read' or `write' into two would read or write two
records.  Actually, there is no problem, because interruption after a
partial transfer cannot happen on such devices; they always transfer an
entire record in one burst, with no waiting once data transfer has
started.