Tag:clearenv

This article will outline how to modify environment variables from a running C process. It should be noted that setting variables from your C process into the environment will only persist during process lifetime. Environment variables are propagated forward to a child program, but not backward to the parent.

Earlier I wrote about creating, removing, changing environment variables directing in Bash, now I will show you how to do these modifications using C.

Creating An Environment Variable In C process

Creating the environment variable can easily be achieved using the C function int setenv(const char *name, const char *value, int overwrite);. It takes two parameters, the first parameter being the name of the environment variable, and the second being a flag outlining whether to overwrite a pre-existing variable of the same name.

#include<stdio.h>
#include <stdlib.h>
 
int main()
{
	char *name = "DATA";
	char *value = "erik";
 
	if(setenv(name, value, 1) < 0){
		fprintf(stderr, "Could not create environment variable.\n");
		return -1;
	}
 
	fprintf(stdout, "-%s-\n", getenv("DATA"));
	return 0;
}

The variable will persist for any children you may fork, or if you execute a system call to grab the variables.

Removing An Environment Variable Using unsetenv

To remove an environment variable using your C process, you may call int unsetenv(const char *name);. Where char *name is the name from the environment. If name does not exist in the environment, then the function succeeds, and the environment is unchanged.

Clear The Environment Using clearenv

To clear the entire environment you can:

Or

Reading An Environment Variable Using getenv

To just read an environment variable there is the C call, char *getenv(const char *name);. This will return the value of the environment variable.

Those are the basic functions you can execute from your C process to set, remove, create those environment variables.