#include <stdlib.h> int setenv(const char *envname, const char *envval, int overwrite);
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. The environment variable is set to the value to which envval points. The function fails if envname points to a string which contains an '=' character. If the environment variable named by envname already exists and the value of overwrite is non-zero, the function returns successfully and the environment is updated. If the environment variable named by envname already exists and the value of overwrite is zero, the function returns successfully and the environment remains unchanged.
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.
Upon successful completion, 0 is returned. Otherwise, -1 is returned, errno set to indicate the error, and the environment is left unchanged.
The setenv() function will fail if:
The envname argument is a null pointer, points to an empty string, or points to a string containing an '=' character.
Insufficient memory was available to add a variable or its value to the environment.
See attributes(5) for descriptions of the following attributes: