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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ATTRIBUTES | STANDARDS | HISTORY | NOTES | SEE ALSO | COLOPHON |
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clearenv(3) Library Functions Manual clearenv(3)
clearenv - clear the environment
Standard C library (libc, -lc)
#include <stdlib.h>
int clearenv(void);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
clearenv():
/* glibc >= 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
The clearenv() function clears the environment of all name-value
pairs and sets the value of the external variable environ to NULL.
After this call, new variables can be added to the environment
using putenv(3) and setenv(3).
The clearenv() function returns zero on success, and a nonzero
value on failure.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────┬───────────────┬─────────────────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────┼───────────────┼─────────────────────┤
│ clearenv() │ Thread safety │ MT-Unsafe const:env │
└──────────────────────────┴───────────────┴─────────────────────┘
putenv()
POSIX.1-2008.
clearenv()
None.
putenv()
glibc 2.0. POSIX.1-2001.
clearenv()
glibc 2.0.
Various UNIX variants (DG/UX, HP-UX, QNX, ...). POSIX.9 (bindings
for FORTRAN77). POSIX.1-1996 did not accept clearenv() and
putenv(3), but changed its mind and scheduled these functions for
some later issue of this standard (see §B.4.6.1). However,
POSIX.1-2001 adds only putenv(3), and rejected clearenv().
On systems where clearenv() is unavailable, the assignment
environ = NULL;
will probably do.
The clearenv() function may be useful in security-conscious
applications that want to precisely control the environment that
is passed to programs executed using exec(3). The application
would do this by first clearing the environment and then adding
select environment variables.
Note that the main effect of clearenv() is to adjust the value of
the pointer environ(7); this function does not erase the contents
of the buffers containing the environment definitions.
The DG/UX and Tru64 man pages write: If environ has been modified
by anything other than the putenv(3), getenv(3), or clearenv()
functions, then clearenv() will return an error and the process
environment will remain unchanged.
getenv(3), putenv(3), setenv(3), unsetenv(3), environ(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 clearenv(3)
Pages that refer to this page: getenv(3), putenv(3), setenv(3), environ(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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