Unix Domain Socket Error Codes?

Question

I've been occasionally reviewing an unsolved mystery on a Solaris 10 setup we have been running, and I noticed something in the error message that might yield a clue to help me solve the mystery.

The error message is when connecting to MySQL on a UNIX Domain Socket.

I have a specific question here, which is about the error code at the end.

See these three error messages:

• mysql -S /tmp/missing.sock outputs
  Can't connect to local MySQL server through socket '/tmp/missing.sock' (2)

• mysql -S /dev/null outputs
  Can't connect to local MySQL server through socket '/dev/null' (95)

• The rare and intermittent error I am trying to resolve is
  Can't connect to local MySQL server through socket '/tmp/mysql.sock' (146)

That number at the end: Is this a UNIX Domain Socket error code? If so, is there a place I can look up the meaning of that code?

As I said, this is a specific question. Other helpful input should be posted to the other question.

Answer 1

The numbers in parentheses are almost certainly system error numbers, normally reported via errno, the definitions for which are found via #include <errno.h> though on Solaris the numbers are usually in /usr/include/sys/errno.h (but can be in other places, especially on Linux and Mac OS X). You could write a simple program to see the 3 errors.

#include <stdio.h>
#include <string.h>

int main(void)
{
    puts(strerror(2));
    puts(strerror(95));
    puts(strerror(146));
    return 0;
}

Conjecture: 2 is probably ENOENT, no such file or directory; 95 may be ENOTSOCK (not a socket); 146 might be ENOTSUPP (operation not supported).

George Bailey confirms:

On my system, the answer was in /usr/include/sys/errno.h:

• 2=ENOENT
• 95=ENOTSOCK
• 146=ECONNREFUSED

Note that error numbers up to the mid-twenties tend to be consistent across systems as the error codes existed in 7th Edition Unix. Higher numbers diverge. For example, on Mac OS X 10.9:

• 2 (ENOENT): No such file or directory
• 95 (EMULTIHOP): Reserved
• errno: no message for errno = 146
• ENOTSOCK (38): Socket operation on non-socket
• ECONNREFUSED (61): Connection refused

On SuSE (SLES 10 SP2 — antique, but these numbers don't change much):

• 2 (ENOENT): No such file or directory
• 95 (EOPNOTSUPP): Operation not supported on transport endpoint
• errno: no message for errno = 146
• ENOTSOCK (88): Socket operation on non-socket
• ECONNREFUSED (111): Connection refused

These answers were obtained via a program errno that reports on error numbers and names. It has to be compiled for each different system.

---

Note that there is a consistent MySQL-provided component to the messages:

Can't connect to local MySQL server through socket '/dev/null' (95)

roughly as if the format string for the printf() statement was:

"Can't connect to local MySQL server through socket '%s' (%s)\n"

The name of the 'socket' file is being provided — very helpful — and (educated guess) the system error number, collected at some point from errno. However, errno is volatile — almost any library function may set it to a non-zero value — so you need to preserve a specific value (copy it) before doing much in the way of error reporting work, such as reading message files to get the correct translation of the format string.