getenv
Environment variable functions
Interface
#include <stdlib.h>
char* | getenv (const char *name) |
int | setenv (const char *name, const char *value, int overwrite) |
int | putenv (const char *string) |
void | unsetenv (const char *name) |
Description
These functions set, unset and fetch environment variables from the host environment list. For compatibility with differing environment conventions, the given argumentsname
and value
may be appended and prepended, respectively, with an equal sign "=".
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, otherwise 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 unsetenv function deletes all instances of the variable name pointed to by
name
from the list.
The code below displays the current value of the PATH environment variable.Example 1
#include <stdio.h> #include <stdlib.h> int main() { printf("PATH = %s\n", getenv("PATH")); return 0; }
Return Values
The getenv function returns the value of the environment variable as a NULL-terminated string. If the variablename
is not in the current
environment, NULL is returned.Errors
[ENOMEM] | The setenv or putenv function failed because they were unable to allocate memory for the environment. |
Bugs
Successive calls to setenv or putenv assigning a differently sizedvalue
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 versions 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 RenoBSD.Last Modified: 18 Dec 11 @ 14:41 Page Rendered: 2022-03-14 15:49:09