Hi Malcom,
Recently, I had to work on a function to get system error message on
various systems. Here is the piece of code I came up with. Hope it helps.
static void get_system_error_message(char * buf, int buf_len, int code)
{
#if defined(_WIN32)
LPVOID lpMsgBuf;
DWORD status = FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, code,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
/* Default language
*/
(LPTSTR) &lpMsgBuf, 0, NULL);
if (status > 0)
{
strncpy(buf, (char *)lpMsgBuf, buf_len-1);
buf[buf_len-1] = '\0';
/* Free the buffer returned by system */
LocalFree(lpMsgBuf);
}
else
{
_snprintf(buf, buf_len-1 , "%s %d",
"Can't get system error message for code", code);
buf[buf_len-1] = '\0';
}
#else
#if defined(_HPUX_SOURCE)
{
char * msg;
errno = 0;
msg = strerror(code);
if (errno == 0)
{
strncpy(buf, msg, buf_len-1);
buf[buf_len-1] = '\0';
}
else
{
snprintf(buf, buf_len, "%s %d",
"Can't get system error message for code", code);
}
}
#else
if (strerror_r(code, buf, buf_len) != 0)
{
snprintf(buf, buf_len, "%s %d",
"Can't get system error message for code", code);
}
#endif
#endif
}
Note that HPUX does not have strerror_r() since strerror() itself is
thread-safe. Also Windows does not have snprintf(). The equivalent function
_snprintf() has a subtle difference in its interface.
-- Asokan
________________________________________
From: malcolm [malcolm.kaval...@oracle.com]
Sent: Thursday, December 11, 2014 11:02 AM
To: common-dev@hadoop.apache.org
Subject: Re: Solaris Port
Fine with me, I volunteer to do this, if accepted.
On 12/11/2014 05:48 PM, Allen Wittenauer wrote:
sys_errlist was removed for a reason. Creating a fake sys_errlist on Solaris
will mean the libhadoop.so will need to be tied a specific build
(kernel/include pairing) and therefore limits upward mobility/compatibility.
That doesn’t seem like a very good idea.
IMO, switching to strerror_r is much preferred, since other than the brain-dead
GNU libc version, is highly portable and should work regardless of the kernel
or OS in place.
On Dec 11, 2014, at 5:20 AM, malcolm <malcolm.kaval...@oracle.com> wrote:
FYI, there are a couple more files that reference sys_errlist directly (not
just terror within exception.c) , but also hdfs_http_client.c and NativeiO.c
On 12/11/2014 07:38 AM, malcolm wrote:
Hi Colin,
Exactly, as you noticed, the problem is the thread-local buffer needed to
return from terror.
Currently, terror just returns a static string from an array, this is fast,
simple and error-proof.
In order to use strerror_r inside terror, would require allocating a buffer
inside terror and depend on the caller to free the buffer after using it, or
to pass a buffer to terrror (which is basically the same as strerror_r,
rendering terror redundant).
Both cases require modification outside terror itself, as far as I can tell, no
simple fix. Unless you have an alternative which I haven't thought of ?
As far as I can tell, we have two choices:
1. Remove terror and replace calls with strerror_r, passing a buffer from the
callee.
Advantage: a more modern portable interface.
Disadvantage: All calls to terror need to be modified, though all seem to
be in a few files as far as I can tell.
2. Adding a sys_errlist array (ifdeffed for Solaris)
Advantage: no change to any calls to terror
Disadvantage: 2 additional files added to source tree (.c and .h) and
some minor ifdefs only used for Solaris.
I think it is more a question of style than anything else, so I leave you to
make the call.
Thanks for your patience,
Malcolm
On 12/10/2014 09:54 PM, Colin McCabe wrote:
On Wed, Dec 10, 2014 at 2:31 AM, malcolm <malcolm.kaval...@oracle.com> wrote:
Hi Colin,
Thanks for the hints around JIRAs.
You are correct errno still exists, however sys_errlist does not.
Hadoop uses a function terror (defined in exception.c) which indexes
sys_errlist by errno to return the error message from the array. This
function is called 26 times in various places (in 2.2)
Originally, I thought to replace all calls to terror with strerror, but
there can be issues with multi-threading (it returns a buffer which can be
overwritten), so it seemed simpler just to recreate the sys_errlist message
array.
There is also a multi-threaded version strerror_r where you pass the buffer
as a parameter, but this would necessitate changing every call to terror
with mutiple lines of code.
Why don't you just use strerror_r inside terror()?
I wrote that code originally. The reason I didn't want to use
strerror_r there is because GNU libc provides a non-POSIX definition
of strerror_r, and forcing it to use the POSIX one is a pain. But you
can do it. You also will require a thread-local buffer to hold the
return from strerror_r, since it is not guaranteed to be static
(although in practice it is 99% of the time-- another annoyance with
the API).
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