CONFSTR(3)                 POSIX Programmer's Manual                CONFSTR(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 .



IEEE/The Open Group                  2017                           CONFSTR(3)
