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Manual Pages  — GETENV


getenv, putenv, setenv, unsetenv – environment variable functions



Standard C Library (libc, -lc)


#include <stdlib.h>

char *
getenv(const char *name);

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

putenv(char *string);

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 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 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 variable 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 list.

If corruption (e.g., a name without a value) is detected while making a copy of environ for internal usage, then setenv(), unsetenv() and putenv() will output a warning to stderr about the issue, drop the corrupt entry and complete the task without error.


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, putenvand 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 "amp;=" character.

The function putenv() failed because string is a NULL pointer, string is without an "amp;=" character or "amp;=" is the first character in string. This does not follow the POSIX specification.

  The function setenv(), unsetenv() or putenv() failed because they were unable to allocate memory for the environment.


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 AT&T v7 . The putenv() function appeared in BSD 4.3 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.

GETENV (3) June 20, 2007

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