Thursday, February 20, 2014

Fixing "MySQL server has gone away" Errors in C

I ran across an old question on Stack Overflow the other day in which a user was having issues maintaining his connection to MySQL from C. I left a brief answer there for anyone else who might stumble across the same problem in the future, but I felt it was worth expanding on a bit more.

The error "MySQL server has gone away" means the client's connection to the MySQL server was lost. This could be because of many reasons; perhaps MySQL isn't running, perhaps there's network problems, or perhaps there was no activity after a certain amount of time and the server closed the connection. Detailed information on the error is available in the MySQL documentation.

It's possible for the client to attempt to re-connect to the server when it's "gone away" although it won't try to by default. To enable the reconnecting behavior, you need to set the MYSQL_OPT_RECONNECT option to 1 using the mysql_options() function. It should be set after mysql_init() is called and before calling mysql_real_connect(). This should solve the problem if the connection was closed by the server because of a time-out.

The MySQL documentation that discusses the reconnect behavior points out that only one re-connect attempt will be made, which means the query can still fail if the server is stopped or inaccessible. I ran across this problem myself while writing a daemon in C that would periodically pull data from MySQL. The daemon was polling at set intervals far less than the time-out period, so any such errors were the result of an unreachable or stopped server. I simply jumped execution to just prior to my work loop's sleep() call and the daemon would periodically try to re-connect until the server came back up.

#define DBHOSTNAME localhost
#define DBHOSTNAME dbuser
...

MYSQL *db = mysql_init(NULL);
if (db == NULL) {
    fprintf(stderr, "Insufficient memory to allocate MYSQL object.");
    exit(EXIT_FAILURE);
}

/* enable re-connect behavior */
my_bool reconnect = 1;
int success = mysql_options(db, MYSQL_OPT_RECONNECT, &reconnect);
assert(success == 0);

if (mysql_real_connect(db, DBHOSTNAME, DBUSERNAME, DBPASSWORD, DBDATABASE,
    0, NULL, 0) == NULL) {
    fprintf(stderr, "Connection attempt failed: %s\n", mysql_error(db));
    exit(EXIT_FAILURE);
}

for (;;) {
    success = mysql_query(db, "<MYSQL QUERY HERE>");
    if (success != 0) {
        /* The error is most likely "gone away" since the query is
         * hard-coded, doesn't return much data, and the result is
         * managed properly. */
        fprintf(stderr, "Unable to query: %s\n", mysql_error(db));
        goto SLEEP;
    }

    /* call mysql_use_result() and do something with data */
    ...

    SLEEP:
    sleep(SLEEP_SECONDS);
}

Thursday, February 13, 2014

Generating C Code and Compiling from STDIN

Lately I've been exploring some syslog configurations and needed to generate some log messages to verify they were routed correctly. Of course doing so programmatically would provide an easy and repeatable method to generate a batch of fresh log messages whenever I needed, but because of the number of facilities and priorities defined by the syslog protocol, it made sense to write a code generator to iterate the different permutations.

The following Lua script generates boilerplate C code for each of the 64 messages needed to test LOG_LOCAL 0-7 with all priorities. I chose generating the code in this manner over writing a nested facilities/priorities loop directly in C so I could easily include a textual representation of the facility and priority constants in the log message (this seemed like a cleaner solution to me than having to maintain a mapping of constants to char* strings as well). And why Lua? Well, it seemed a better idea than M4. :)

#! /usr/bin/env lua

local facilities = {
    "LOG_LOCAL0",
    "LOG_LOCAL1",
    "LOG_LOCAL2",
    "LOG_LOCAL3",
    "LOG_LOCAL4",
    "LOG_LOCAL5",
    "LOG_LOCAL6",
    "LOG_LOCAL7"
}

local priorities = {
    "LOG_DEBUG",
    "LOG_INFO",
    "LOG_NOTICE",
    "LOG_WARNING",
    "LOG_ERR",
    "LOG_CRIT",
    "LOG_ALERT",
    "LOG_EMERG"
}

print([[
#include <stdlib.h>
#include <syslog.h>
#include <libgen.h>

int main(int argc, char *argv[])
{
    char *appName = basename(argv[0]);
]])

for _, facility in pairs(facilities) do 
    for _, priority in pairs(priorities) do
        print(string.format(
[[
    openlog(appName, LOG_CONS|LOG_NDELAY|LOG_PID, %s);
    syslog(%s, "Test %s.%s message.\n");
    closelog();
]],
            facility, priority, facility, priority
        ))
    end
end

print([[
    return EXIT_SUCCESS;
}]])

Running the script will output the desired C code, which looks like this:

#include <stdlib.h>
#include <syslog.h>
#include <libgen.h>

int main(int argc, char *argv[])
{
    char *appName = basename(argv[0]);

    openlog(appName, LOG_CONS|LOG_NDELAY|LOG_PID, LOG_LOCAL0);
    syslog(LOG_DEBUG, "Test LOG_DEBUG message.\n");
    closelog();

    openlog(appName, LOG_CONS|LOG_NDELAY|LOG_PID, LOG_LOCAL0);
    syslog(LOG_INFO, "Test LOG_INFO message.\n");
    closelog();

    openlog(appName, LOG_CONS|LOG_NDELAY|LOG_PID, LOG_LOCAL0);
    syslog(LOG_NOTICE, "Test LOG_NOTICE message.\n");
    closelog();
...

If I wanted to inspect or tweak the generated code, I could pipe the script's output to a file before compiling it:

./gen-syslog-tests.lua > syslog-tests.c
gcc -o syslog-tests syslog-tests.c

But if I just wanted the compiled binary and had no need to modify the code, it seems inelegant to write things out to a file. Here's where I learned it's possible for gcc to compile code piped in on STDIN.

./gen-syslog-tests.lua | gcc -o syslog-tests -xc -

The two things of note are: gcc can't deduce the programming language from the file extension (since there is no file) so the -x flag is necessary to identify the language, and - is used as the file name (a convention commonly used to indicate reading from STDIN as a file).