confstr(3p) — Linux manual page


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

PROLOG         top

       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         top

       confstr — get configurable variables

SYNOPSIS         top

       #include <unistd.h>

       size_t confstr(int name, char *buf, size_t len);

DESCRIPTION         top

       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:


       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

       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         top

       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         top

       The confstr() function shall fail if:

       EINVAL The value of the name argument is invalid.

       The following sections are informative.

EXAMPLES         top



       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         top

       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.



SEE ALSO         top

       exec(1p), fpathconf(3p), sysconf(3p)

       The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)

       The Shell and Utilities volume of POSIX.1‐2008, c99(1p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group                 2013                         CONFSTR(3P)

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