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).

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