putenv(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

PUTENV(3)               Linux Programmer's Manual              PUTENV(3)

NAME         top

       putenv - change or add an environment variable

SYNOPSIS         top

       #include <stdlib.h>

       int putenv(char *string);

   Feature Test Macro Requirements for glibc (see
   feature_test_macros(7)):

       putenv():
           _XOPEN_SOURCE
               || /* Glibc since 2.19: */ _DEFAULT_SOURCE
               || /* Glibc <= 2.19: */ _SVID_SOURCE

DESCRIPTION         top

       The putenv() function adds or changes the value of environment
       variables.  The argument string is of the form name=value.  If
       name does not already exist in the environment, then string is
       added to the environment.  If name does exist, then the value of
       name in the environment is changed to value.  The string pointed
       to by string becomes part of the environment, so altering the
       string changes the environment.

RETURN VALUE         top

       The putenv() function returns zero on success.  On failure, it
       returns a nonzero value, and errno is set to indicate the error.

ERRORS         top

       ENOMEM Insufficient space to allocate new environment.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────┬───────────────┬─────────────────────┐
       │Interface                 Attribute     Value               │
       ├──────────────────────────┼───────────────┼─────────────────────┤
       │putenv()                  │ Thread safety │ MT-Unsafe const:env │
       └──────────────────────────┴───────────────┴─────────────────────┘

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

NOTES         top

       The putenv() function is not required to be reentrant, and the
       one in glibc 2.0 is not, but the glibc 2.1 version is.

       Since version 2.1.2, the glibc implementation conforms to SUSv2:
       the pointer string given to putenv() is used.  In particular,
       this string becomes part of the environment; changing it later
       will change the environment.  (Thus, it is an error to call
       putenv() with an automatic variable as the argument, then return
       from the calling function while string is still part of the
       environment.)  However, glibc versions 2.0 to 2.1.1 differ: a
       copy of the string is used.  On the one hand this causes a memory
       leak, and on the other hand it violates SUSv2.

       The 4.4BSD version, like glibc 2.0, uses a copy.

       SUSv2 removes the const from the prototype, and so does glibc
       2.1.3.

       The GNU C library implementation provides a nonstandard
       extension.  If string does not include an equal sign:

           putenv("NAME");

       then the named variable is removed from the caller's environment.

SEE ALSO         top

       clearenv(3), getenv(3), setenv(3), unsetenv(3), environ(7)

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                      PUTENV(3)

Pages that refer to this page: clearenv(3)getenv(3)pam_putenv(3)setenv(3)proc(5)environ(7)