Environment

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Overview

The basic idea behind the Environment module is to load configuration values (i.e. values that a program needs at run-time) from a shell script which exports some variables in a given namespace (let's say PFX_).

We let the shell act in the background - using just some env(1) and pipe(2) tricks to redirect and capture the variables - which in turn gives us a great deal of power and flexibility in just a couple of C lines.

The following is a very simple configuration file in which three variables are set and export'ed to the environment:

    SET_PFX_VAR1=""

    [ ! -z "$SET_PFX_VAR1" ] && export PFX_VAR1="tip"
    export PFX_VAR2="tap"
    export PFX_VAR3=$((10 * 100 * 1000))

Things worth noting are:

• the conditional behaviour by which single or sets of variables can be pulled in/out;
• bash does the math to eval PFX_VAR3;
• bash can spawn external utilities;
• it is possible to do complex substitutions;
• every information one can get through the shell (e.g. cat /proc/something) is available;
• aggregation of other configuration modules is trivial via the source (aka ".") shell builtin(1).

The C code needed to access the former configuration file is:

    size_t i;
    const char *v, *vp[] = { "PFX_VAR1", "PFX_VAR2", "PFX_VAR3" };

    // load, parse, eval, etc. variables with PFX_ prefix from my.conf 
    // into the process environment
    dbg_err_if (u_env_init("PFX_", "./my.conf"));

    // access PFX_VAR{1,2,3} variables and print their values
    for (i = 0; i < 3; ++i)
    {
        v = u_env_var(vp[i]);

        con("%s = %s", vp[i], v ? v : "UNSET");
    }

Note:

Configuration files handled by the Environment module are flat. If you need tree-like structured configuration files use the Configuration module instead.

Functions
int	u_env_init (const char *prefix, const char *cfile)
	Load configuration environment.
const char *	u_env_var (const char *name)
	Get a configuration variable value.

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Function Documentation

int u_env_init	(	const char *	prefix,
		const char *	cfile
	)

Parameters:

	prefix	variables namespace
	cfile	configuration file

Load all configuration variables in the given prefix namespace from the configuration file cfile into the environment of the calling process. Complex parameter substitution, conditional evaluations, arithmetics, etc. is done by the shell: the caller has a simple name=value view of the configuration file.

Return values:

	0	on success
	~0	on failure

Definition at line 87 of file env.c.

References u_snprintf().

const char* u_env_var

(

const char *

name

)

Return a configuration variable by its name name

Parameters:

name

the name of the variable to get

Returns:

the value of the requested variable or NULL if the variable is not defined

Definition at line 134 of file env.c.