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GETENV(3)                BSD Library Functions Manual                GETENV(3)

NAME
     getenv, putenv, setenv, unsetenv -- environment variable functions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     char *
     getenv(const char *name);

     int
     putenv(char *string);

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

     int
     unsetenv(const char *name);

DESCRIPTION
     These functions set, unset and fetch environment variables from the host environment list.  For compat-ibility compatibility
     ibility with differing environment conventions, the given arguments name and value may be appended and
     prepended, respectively, with an equal sign ``=''.  The behavior is undefined when an equal sign
     appears at any other location in name.

     The getenv() function obtains the current value of the environment variable, name.

     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, other-wise otherwise
     wise it is reset to the given value.

     The putenv() 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 putenv().  The
     setenv() function is strongly preferred to putenv().

     The unsetenv() 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.

RETURN VALUES
     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.

ERRORS
     [EINVAL]           The function unsetenv() failed because name was not found in the environment list.

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

LEGACY SYNOPSIS
     #include <stdlib.h>

     void
     unsetenv(const char *name);

     unsetenv() doesn't return a value.

COMPATIBILITY
     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.

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

STANDARDS
     The getenv() function conforms to ISO/IEC 9899:1990 (``ISO C90'').

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

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

BSD                            December 11, 1993                           BSD

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