putenv(3) — Linux manual page


putenv(3)               Library Functions Manual               putenv(3)

NAME         top

       putenv - change or add an environment variable

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <stdlib.h>

       int putenv(char *string);

   Feature Test Macro Requirements for glibc (see

               || /* glibc >= 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
       │ Interface               Attribute     Value               │
       │ putenv()                │ Thread safety │ MT-Unsafe const:env │

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr2, 4.3BSD-Reno.

       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 glibc 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, from glibc 2.0 to glibc 2.1.1, it
       differs: 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 4.3BSD-Reno version, like glibc 2.0, uses a copy; this is
       fixed in all modern BSDs.

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

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


       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

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Linux man-pages 6.9.1          2024-05-02                      putenv(3)

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