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GETENV(3) Library Functions Manual GETENV(3)

getenv, putenv, setenv, unsetenv — environment variable functions

Standard C Library (libc, -lc)

#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);

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

The () function obtains the current value of the environment variable, name. The application should not modify the string pointed to by the getenv() function.

The () 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 () function takes an argument of the form ``name=value'' and is equivalent to:

setenv(name, value, 1);

The string pointed to by string becomes part of the environment. A program should not alter or free the string, and should not use stack or other transient string variables as arguments to (). The setenv() function is strongly preferred to putenv().

The () function deletes all instances of the variable name pointed to by name from the list. Note that only the variable name (e.g., "NAME") should be given; "NAME=value" will not work.

The getenv() function returns the value of the environment variable as a NUL-terminated string. If the variable name is not in the current environment, NULL is returned.


The setenv(), putenv(), 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 function getenv(), setenv() or unsetenv() failed because the name is a NULL pointer, points to an empty string, or points to a string containing an “=” character.

The function putenv() failed because string is a NULL pointer or string is without an “=” character.

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

#include <stdlib.h>

void
(const char *name);;

() doesn't return a value.

putenv() no longer copies its input buffer. This often appears in crash logs as a crash in getenv(). Avoid passing local buffers or freeing the memory that is passed to putenv(). Use setenv(), which still makes an internal copy of its buffers.

unsetenv() no longer parses the variable name; e.g., unsetenv ("FOO=BAR") no longer works. Use unsetenv("FOO"). unsetenv() also now returns a status value and will set errno to EINVAL if name is not a defined environment variable.

csh(1), sh(1), execve(2), compat(5), environ(7)

The getenv() function conforms to ISO/IEC 9899:1990 (“ISO C90”). The setenv(), putenv() and unsetenv() functions conforms to IEEE Std 1003.1-2001 (“POSIX.1”).

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

Successive calls to setenv() that assign a larger-sized value than any previous value to the same name will result in a memory leak. The FreeBSD semantics for this function (namely, that the contents of value are copied and that old values remain accessible indefinitely) make this bug unavoidable. Future versions may eliminate one or both of these semantic guarantees in order to fix the bug.

June 20, 2007 macOS