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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SETENV(3P) POSIX Programmer's Manual SETENV(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
setenv — add or change environment variable
#include <stdlib.h>
int setenv(const char *envname, const char *envval, int overwrite);
The setenv() function shall update or add a variable in the
environment of the calling process. The envname argument points to
a string containing the name of an environment variable to be
added or altered. The environment variable shall be set to the
value to which envval points. The function shall fail if envname
points to a string which contains an '=' character. If the
environment variable named by envname already exists and the value
of overwrite is non-zero, the function shall return success and
the environment shall be updated. If the environment variable
named by envname already exists and the value of overwrite is
zero, the function shall return success and the environment shall
remain unchanged.
The setenv() function shall update the list of pointers to which
environ points.
The strings described by envname and envval are copied by this
function.
The setenv() function need not be thread-safe.
Upon successful completion, zero shall be returned. Otherwise, -1
shall be returned, errno set to indicate the error, and the
environment shall be unchanged.
The setenv() function shall fail if:
EINVAL The envname argument points to an empty string or points to
a string containing an '=' character.
ENOMEM Insufficient memory was available to add a variable or its
value to the environment.
The following sections are informative.
None.
See exec() for restrictions on changing the environment in multi-
threaded applications.
Unanticipated results may occur if setenv() changes the external
variable environ. In particular, if the optional envp argument to
main() is present, it is not changed, and thus may point to an
obsolete copy of the environment (as may any other copy of
environ). However, other than the aforementioned restriction, the
standard developers intended that the traditional method of
walking through the environment by way of the environ pointer must
be supported.
It was decided that setenv() should be required by this version
because it addresses a piece of missing functionality, and does
not impose a significant burden on the implementor.
There was considerable debate as to whether the System V putenv()
function or the BSD setenv() function should be required as a
mandatory function. The setenv() function was chosen because it
permitted the implementation of the unsetenv() function to delete
environmental variables, without specifying an additional
interface. The putenv() function is available as part of the XSI
option.
The standard developers considered requiring that setenv()
indicate an error when a call to it would result in exceeding
{ARG_MAX}. The requirement was rejected since the condition might
be temporary, with the application eventually reducing the
environment size. The ultimate success or failure depends on the
size at the time of a call to exec, which returns an indication of
this error condition.
See also the RATIONALE section in getenv(3p).
None.
exec(1p), getenv(3p), putenv(3p), unsetenv(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p),
sys_types.h(0p), unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 SETENV(3P)
Pages that refer to this page: stdlib.h(0p), getenv(3p), putenv(3p), unsetenv(3p)
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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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