setenv - add or change environment variable

Syntax	#include <stdlib.h> int setenv (const char *envname, const char *envval, int overwrite);
Description	The setenv() function updates or adds a variable in the environment of the calling process. The envname argument points to a string containing the name of an environment variable to be added or altered. If the environment variable already exists, two cases must be distinguished: If the value of overwrite is not zero, the environment is changed; if the value is zero, the environment remains unchanged. In both cases the function is terminated successfully. If the application modifies environ or the pointers to which it points, the behavior of setenv is undefined. The setenv function updates the list of pointers to which environ points. The strings described by envname and envval are copied by this function. setenv() is not thread-safe.
Return val.	0 -1	if successful. otherwise. errno is set to indicate the error. The environment remains unchanged.
Errors	setenv() will fail if:
	EINVAL	The envname argument is a null pointer, points to an empty string, or points to a string containing an '=' character.
	ENOMEM	Insufficient memory was available to add a variable or its value to the environment.
See also	environ, exec, getenv(), malloc(), putenv(), unsetenv(), stdlib.h, section“Environment variables”.