getenv, setenv, unsetenv - manipulate environmental variables

char *getenv(name)
const char *name;

int setenv(name, value, overwrite)
const char *name, value;
int overwrite;

void unsetenv(name)
const char *name;

Getenv searches the environment list (see environ(7)) for a string
of the form name=value and returns a pointer to the string value if
such a string is present, and 0 (NULL) if it is not.

Setenv searches the environment list as getenv does; if the string
name is not found, a string of the form name=value is added to the
environment. If it is found, and overwrite is non-zero, its value
is changed to value. Setenv returns 0 on success and -1 on
failure, where failure is caused by an inability to allocate space
for the environment.

Unsetenv removes all occurrences of the string name from the
environment. There is no library provision for completely removing
the current environment. It is suggested that the following code
be used to do so.

static char *envinit[1];
extern char **environ;
environ = envinit;

All of these routines permit, but do not require, a trailing equals
("=") sign on name or a leading equals sign on value.

csh(1), sh(1), execve(2), environ(7)