unsetenv

Environment variable functions

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Contents

Interface
Description
Return Values
Errors
Bugs
History
Page Comments

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 arguments name 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 variable name 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 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 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.

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)