GETENV(3) BSD Library Functions Manual GETENV(3)
getenv, putenv, setenv, unsetenv -- environment variable functions
Standard C Library (libc, -lc)
getenv(const char *name);
setenv(const char *name, const char *value, int overwrite);
unsetenv(const char *name);
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. The
application should not modify the string pointed to by the getenv() function.
The setenv() function inserts or resets the environment variable name in the current envi-
ronment 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'' and puts it directly into
the current environment, so altering the argument shall change the environment. If the
variable name does not exist in the list, it is inserted with the given value. If the vari-
able name does exist, it is reset to the given value.
The unsetenv() function deletes all instances of the variable name pointed to by name from
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.
[EINVAL] The function getenv(), setenv() or unsetenv() failed because the name is
a NULL pointer, points to an empty string, or points to a string contain-
ing an ``='' character.
The function putenv() failed because string is a NULL pointer, string is
without an ``='' character or ``='' is the first character in string.
This does not follow the POSIX specification.
[ENOMEM] The function setenv(), unsetenv() or putenv() failed because they were
unable to allocate memory for the environment.
[EFAULT] The functions setenv(), unsetenv() or putenv() failed to make a valid
copy of the environment due to the environment being corrupt (i.e., a
name without a value). A warning will be output to stderr with informa-
tion about the issue.
csh(1), sh(1), execve(2), 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() func-
tion appeared in 4.3BSD-Reno.
Until FreeBSD 7.0, putenv() would make a copy of string and insert it into the environment
using setenv(). This was changed to use string as the memory location of the ``name=value''
pair to follow the POSIX specification.
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.
BSD June 20, 2007 BSD