The putenv() function makes the value of the environment variable name equal to value by altering an existing variable or creating a new one. In either case, the string pointed to by string becomes part of the environment, so altering the string will change the environment.

The string argument points to a string of the form name=value. The space used by string is no longer used once a new string-defining name is passed to putenv().

The putenv() function uses malloc(3C) to enlarge the environment.

After putenv() is called, environment variables are not in alphabetical order.

The putenv() functions returns a non-zero value if it was unable to obtain enough space using malloc(3C) for an expanded environment. Otherwise, 0 is returned.

The putenv() function may fail if:

ENOMEM

Insufficient memory was available.

The putenv() function can be safely called from multithreaded programs. Caution must be exercised when using this function and getenv(3C) in multithreaded programs. These functions examine and modify the environment list, which is shared by all threads in a program. The system prevents the list from being accessed simultaneously by two different threads. It does not, however, prevent two threads from successively accessing the environment list using putenv() or getenv().

See attributes(5) for descriptions of the following attributes:

exec(2), getenv(3C), malloc(3C), attributes(5), environ(5)

The string argument should not be an automatic variable. It should be declared static if it is declared within a function because it cannot be automatically declared. A potential error is to call putenv() with a pointer to an automatic variable as the argument and to then exit the calling function while string is still part of the environment.