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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 will contain the <space>-separated list of variable=value environment variable
pairs required by the implementation to create 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, 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.1‐2008, 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.1‐2008.
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.1‐2008, <unistd.h>
The Shell and Utilities volume of POSIX.1‐2008, c99
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/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 .