Re: reading config files from C

[MJ <mafsys1234%gmail.com@localhost>]

Hi George,

On 30/01/2020 10:07 am, George Georgalis wrote:
> Hi Mark,
>
> On Wed, Jan 29, 2020 at 2:07 PM MJ <mafsys1234%gmail.com@localhost> wrote:
> > On 30/01/2020 5:19 am, George Georgalis wrote:
> > > Hi, I would like to introduce configuration files to a software
> > > project I am supporting with build infrastructure. The C programs
> > > depend on quite a few libraries from different sources. As a result of
> > > development evolution, we have hardcoded paths in the source which
> > > would be better served as parameters read from configuration files, so
> > > the location of dependencies can be adjusted without modifying the
> > > code. We need this functionality for runtime as well as compile time.
> > Can you not just change all the hardcoded paths to defines and then pass those defines to the programs via make?
> ...
> > Am I over-simplifying the issue?
> That sounds like a great solution, compile time defines solves the
> immediate problem of getting it to work on another platform, accept
> that I'm not proficient enough in C to just code it out. How do you
> pass make env to programs at compile time? Would you provide an
> example?

How are you at C? Can you use pointers, manipulate "strings"? I'm not averse to helping you, but I can't teach you C :-)

To your issue and using MACROS/defines , for example, suppose this code snippet:

some_function(void) {

config_path="/usr/local/etc/prog.txt"

...

...

}


Change it to:

some_function(void) {

#if defined (MY_CONFIG_PATH)

strcpy(config_path,MY_CONFIG_PATH);

#else

#error "compile with -DMY_CONFIG_PATH="/usr/local/etc/prog.txt"

#endif
...

...

}


Then compile (using make), with cc -DMY_CONFIG_PATH="/usr/local/etc/prog.txt"


Of course, if you've got a large suite of programs each with a different CONFIG_PATH, then the command line might get overrun with data! That's a decision you will have to confront based on your knowledge of the configuration path lengths hardcoded into the programs.

> What about runtime paths and parameters? I guess a config file still
> needs parsing from C, for that? I know a lot of compiled programs do
If they are runtime, then this is different altogether.

Suppose your configuration file is made up of some passed arguments like so:

int main(int argc, char **argv)

{
/* config_path depends on argument 1 for some of the path */
sprintf(config_path,"/usr/local/etc/%s/prog.txt", argv[1]);

}


Then this cannot be circumvented with -D, obviously. Then again, how are you doing it now, if they're runtime? Are they runtime but still static?

That is, even though the information is added at runtime, it never varies each time the program runs?


> this, but in the source I've looked at, I'm not seeing exactly
> how/where. :-\ ...this would be the first thing I would make templates
> for, but I'm not seeing any templates either....

If I was building a simple configuration file and parsing it, I would use something like this:

key=value

In the tradition of unix, I would skip all lines that are blank and begin with #

I'm "old skool" with this format. It's simple, it's effective.

You can then use something like strtok() to parse it, or roll your own. In the latter case, I would just use basic fopen(), fgets(), parse the line by pulling it apart using pointers, then placing the result into a KEY and DATA. You would then compare the KEY(s) to what you're looking for and when a match is obtained, use that DATA value.

Do you want some more specific code rather than this vague notion?

cheers

Mark