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NAME

getenv, putenv, setenv, unsetenv — environment variable functions

SYNOPSIS

#include <stdlib.h>

char *
getenv(const char *name);

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

int
putenv(char *string);

int
unsetenv(const char *name);

DESCRIPTION

These functions set, unset, and fetch environment variables from the host environment list.

The getenv() function obtains the current value of the environment variable name. If the variable name is not in the current environment, a null pointer is returned.

The setenv() function inserts or resets the environment variable name in the current environment list. If the variable name does not exist in the list, it is inserted with the given value. If the variable does exist, the argument overwrite is tested; if overwrite is zero, the variable is not reset, otherwise it is reset to the given value.

The putenv() function takes an argument of the form name=value. The memory pointed to by string becomes part of the environment and must not be deallocated by the caller. If the variable already exists, it will be overwritten. A common source of bugs is to pass a string argument that is a locally scoped string buffer. This will result in corruption of the environment after leaving the scope in which the variable is defined. For this reason, the setenv() function is preferred over putenv().

The unsetenv() function deletes all instances of the variable name pointed to by name from the list.

RETURN VALUES

The putenv(), setenv(), and unsetenv() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The getenv() function returns a pointer to the requested value, or NULL if it could not be found. If getenv() is successful, the string returned should be considered read-only.

ERRORS

[EINVAL]

The setenv() or unsetenv() function was passed an empty name or a NULL pointer, or was passed a name containing an ‘=’ character.

The putenv() function was passed a string that did not contain an ‘=’ character.

[ENOMEM]

The setenv() or putenv() function failed because it was unable to allocate memory for the environment.

SEE ALSO

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

STANDARDS

The getenv() function conforms to ANSI X3.159-1989 (“ANSI C89”). The putenv(), setenv(), and unsetenv() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY

The function getenv() appeared in Version 7 AT&T UNIX and 3BSD. The functions setenv() and unsetenv() appeared in 4.3BSD-Tahoe. The putenv() function appeared in 4.3BSD-Reno.

CAVEATS

Library code must be careful about using getenv() to read untrusted environment variables in setuid programs. The issetugid() function is provided for this purpose.