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PROLOG
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.
NAME
confstr — get configurable variables
SYNOPSIS
#include <unistd.h>
size_t confstr(int name, char *buf, size_t len);
DESCRIPTION
The confstr() function shall return configuration-defined string values. Its use and
purpose are similar to sysconf(), but it is used where string values rather than numeric
values are returned.
The name argument represents the system variable to be queried. The implementation shall
support the following name values, defined in <unistd.h>. It may support others:
_CS_PATH
_CS_POSIX_V7_ILP32_OFF32_CFLAGS
_CS_POSIX_V7_ILP32_OFF32_LDFLAGS
_CS_POSIX_V7_ILP32_OFF32_LIBS
_CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LIBS
_CS_POSIX_V7_LP64_OFF64_CFLAGS
_CS_POSIX_V7_LP64_OFF64_LDFLAGS
_CS_POSIX_V7_LP64_OFF64_LIBS
_CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LIBS
_CS_POSIX_V7_THREADS_CFLAGS
_CS_POSIX_V7_THREADS_LDFLAGS
_CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
_CS_V7_ENV
_CS_POSIX_V6_ILP32_OFF32_CFLAGS
_CS_POSIX_V6_ILP32_OFF32_LDFLAGS
_CS_POSIX_V6_ILP32_OFF32_LIBS
_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LIBS
_CS_POSIX_V6_LP64_OFF64_CFLAGS
_CS_POSIX_V6_LP64_OFF64_LDFLAGS
_CS_POSIX_V6_LP64_OFF64_LIBS
_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LIBS
_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
_CS_V6_ENV
If len is not 0, and if name has a configuration-defined value, confstr() shall copy that
value into the len-byte buffer pointed to by buf. If the string to be returned is longer
than len bytes, including the terminating null, then confstr() shall truncate the string
to len-1 bytes and null-terminate the result. The application can detect that the string
was truncated by comparing the value returned by confstr() with len.
If len is 0 and buf is a null pointer, then confstr() shall still return the integer value
as defined below, but shall not return a string. If len is 0 but buf is not a null
pointer, the result is unspecified.
After a call to:
confstr(_CS_V7_ENV, buf, sizeof(buf))
the string stored in buf shall contain a <space>-separated list of the variable=value
environment variable pairs an implementation requires as part of specifying a conforming
environment, as described in the implementations' conformance documentation.
If the implementation supports the POSIX shell option, the string stored in buf after a
call to:
confstr(_CS_PATH, buf, sizeof(buf))
can be used as a value of the PATH environment variable that accesses all of the standard
utilities of POSIX.1‐2008, that are provided in a manner accessible via the exec family of
functions, if the return value is less than or equal to sizeof(buf).
RETURN VALUE
If name has a configuration-defined value, confstr() shall return the size of buffer that
would be needed to hold the entire configuration-defined value including the terminating
null. If this return value is greater than len, the string returned in buf is truncated.
If name is invalid, confstr() shall return 0 and set errno to indicate the error.
If name does not have a configuration-defined value, confstr() shall return 0 and leave
errno unchanged.
ERRORS
The confstr() function shall fail if:
EINVAL The value of the name argument is invalid.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
An application can distinguish between an invalid name parameter value and one that
corresponds to a configurable variable that has no configuration-defined value by checking
if errno is modified. This mirrors the behavior of sysconf().
The original need for this function was to provide a way of finding the configuration-
defined default value for the environment variable PATH. Since PATH can be modified by
the user to include directories that could contain utilities replacing the standard
utilities in the Shell and Utilities volume of POSIX.1‐2017, applications need a way to
determine the system-supplied PATH environment variable value that contains the correct
search path for the standard utilities.
An application could use:
confstr(name, (char *)NULL, (size_t)0)
to find out how big a buffer is needed for the string value; use malloc() to allocate a
buffer to hold the string; and call confstr() again to get the string. Alternately, it
could allocate a fixed, static buffer that is big enough to hold most answers (perhaps 512
or 1024 bytes), but then use malloc() to allocate a larger buffer if it finds that this is
too small.
RATIONALE
Application developers can normally determine any configuration variable by means of
reading from the stream opened by a call to:
popen("command -p getconf variable", "r");
The confstr() function with a name argument of _CS_PATH returns a string that can be used
as a PATH environment variable setting that will reference the standard shell and
utilities as described in the Shell and Utilities volume of POSIX.1‐2017.
The confstr() function copies the returned string into a buffer supplied by the
application instead of returning a pointer to a string. This allows a cleaner function in
some implementations (such as those with lightweight threads) and resolves questions about
when the application must copy the string returned.
FUTURE DIRECTIONS
None.
SEE ALSO
exec, fpathconf(), sysconf()
The Base Definitions volume of POSIX.1‐2017, <unistd.h>
The Shell and Utilities volume of POSIX.1‐2017, c99
COPYRIGHT
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 .