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* Re: [rfa/doc/threads] thread breakpoints and system calls
@ 2003-10-27 14:01 Michael Elizabeth Chastain
  0 siblings, 0 replies; 9+ messages in thread
From: Michael Elizabeth Chastain @ 2003-10-27 14:01 UTC (permalink / raw)
  To: drow, gdb-patches

Committed!

Michael C

===

2003-10-26  Michael Chastain  <mec@shout.net>

	* gdb.texinfo (Thread Stops): Document the issue with
	premature return from system calls in multi-threaded programs.


^ permalink raw reply	[flat|nested] 9+ messages in thread
* Re: [rfa/doc/threads] thread breakpoints and system calls
@ 2003-10-26 21:35 Michael Elizabeth Chastain
  2003-10-27  6:01 ` Eli Zaretskii
  0 siblings, 1 reply; 9+ messages in thread
From: Michael Elizabeth Chastain @ 2003-10-26 21:35 UTC (permalink / raw)
  To: drow, gdb-patches; +Cc: eliz

Okay, how about this?

This version eschews the words 'legal' and 'illegal'.  It also adds a
sentence about the root cause, which is the interaction between
multiple threads and the signals used to implement breakpoints and
other events that stop execution.

Tested with makeinfo for syntax.

Michael C

2003-10-26  Michael Chastain  <mec@shout.net>

	* gdb.texinfo (Thread Stops): Document the issue with
	premature return from system calls in multi-threaded programs.

Index: gdb.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
retrieving revision 1.183
diff -c -3 -p -r1.183 gdb.texinfo
*** gdb.texinfo	23 Oct 2003 00:11:59 -0000	1.183
--- gdb.texinfo	26 Oct 2003 21:28:00 -0000
*************** allows you to examine the overall state 
*** 3761,3766 ****
--- 3761,3807 ----
  switching between threads, without worrying that things may change
  underfoot.
  
+ @cindex thread breakpoints and system calls
+ @cindex system calls and thread breakpoints
+ @cindex premature return from system calls
+ There is an unfortunate side effect.  If one thread stops for a
+ breakpoint, or for some other reason, and another thread is blocked in a
+ system call, then the system call may return prematurely.  This is a
+ consequence of the interaction between multiple threads and the signals
+ that @value{GDBN} uses to implement breakpoints and other events that
+ stop execution.
+ 
+ To handle this problem, your program should check the return value of
+ each system call and react appropriately.  This is good programming
+ style anyways.
+ 
+ For example, do not write code like this:
+ 
+ @smallexample
+   sleep (10);
+ @end smallexample
+ 
+ The call to @code{sleep} will return early if a different thread stops
+ at a breakpoint or for some other reason.
+ 
+ Instead, write this:
+ 
+ @smallexample
+   int unslept = 10;
+   while (unslept > 0)
+     unslept = sleep (unslept);
+ @end smallexample
+ 
+ A system call is allowed to return early, so the system is still
+ conforming to its specification.  But @value{GDBN} does cause your
+ multi-threaded program to behave differently than it would without
+ @value{GDBN}.
+ 
+ Also, @value{GDBN} uses internal breakpoints in the thread library to
+ monitor certain events such as thread creation and thread destruction.
+ When such an event happens, a system call in another thread may return
+ prematurely, even though your program does not appear to stop.
+ 
  @cindex continuing threads
  @cindex threads, continuing
  Conversely, whenever you restart the program, @emph{all} threads start


^ permalink raw reply	[flat|nested] 9+ messages in thread
* [rfa/doc/threads] thread breakpoints and system calls
@ 2003-10-26 15:00 Michael Elizabeth Chastain
  2003-10-26 17:35 ` Eli Zaretskii
  0 siblings, 1 reply; 9+ messages in thread
From: Michael Elizabeth Chastain @ 2003-10-26 15:00 UTC (permalink / raw)
  To: gdb-patches; +Cc: drow, eliz

How embarrassing, my first doco on this was all wrong.

How about this?

Tested with makeinfo for syntax.

Looking for approval from a thread guy such as Daniel J for content,
and Eli Z for syntax.

Michael C

2003-10-26  Michael Chastain  <mec@shout.net>

	* gdb.texinfo (Thread Stops): Document the issue with
	premature return from system calls in multi-threaded programs.

Index: gdb.texinfo
===================================================================
RCS file: /cvs/src/src/gdb/doc/gdb.texinfo,v
retrieving revision 1.183
diff -c -3 -p -r1.183 gdb.texinfo
*** gdb.texinfo	23 Oct 2003 00:11:59 -0000	1.183
--- gdb.texinfo	26 Oct 2003 14:53:09 -0000
*************** allows you to examine the overall state 
*** 3761,3766 ****
--- 3761,3804 ----
  switching between threads, without worrying that things may change
  underfoot.
  
+ @cindex thread breakpoints and system calls
+ @cindex system calls and thread breakpoints
+ @cindex premature return from system calls
+ There is an unfortunate side effect.  If one thread stops for a
+ breakpoint, or for some other reason, and another thread is blocked in a
+ system call, then the system call may return prematurely.
+ 
+ To handle this problem, your program should check the return value of
+ each system call and react appropriately.  This is good programming
+ style anyways.
+ 
+ For example, do not write code like this:
+ 
+ @smallexample
+   sleep (10);
+ @end smallexample
+ 
+ The call to @code{sleep} will return early if a different thread stops
+ at a breakpoint or for some other reason.
+ 
+ Instead, write this:
+ 
+ @smallexample
+   int unslept = 10;
+   while (unslept > 0)
+     unslept = sleep (unslept);
+ @end smallexample
+ 
+ It is legal behavior for a system call to return early, so @value{GDBN}
+ does not cause your program to behave illegally.  But @value{GDBN} does
+ cause your multi-threaded program to behave differently than it would
+ without @value{GDBN}.
+ 
+ Also, @value{GDBN} uses internal breakpoints in the thread library to
+ monitor certain events such as thread creation and thread destruction.
+ When such an event happens, a system call in another thread may return
+ prematurely, even though your program does not appear to stop.
+ 
  @cindex continuing threads
  @cindex threads, continuing
  Conversely, whenever you restart the program, @emph{all} threads start


^ permalink raw reply	[flat|nested] 9+ messages in thread
end of thread, other threads:[~2003-10-27 20:04 UTC | newest]

Thread overview: 9+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2003-10-27 14:01 [rfa/doc/threads] thread breakpoints and system calls Michael Elizabeth Chastain
  -- strict thread matches above, loose matches on Subject: below --
2003-10-26 21:35 Michael Elizabeth Chastain
2003-10-27  6:01 ` Eli Zaretskii
2003-10-27  6:04   ` Daniel Jacobowitz
2003-10-26 15:00 Michael Elizabeth Chastain
2003-10-26 17:35 ` Eli Zaretskii
2003-10-26 17:39   ` Daniel Jacobowitz
2003-10-27 16:12   ` Andrew Cagney
2003-10-27 20:04     ` Eli Zaretskii

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