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 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

       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         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.1‐2017, 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         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.1‐2017.

       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.



SEE ALSO         top

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

       The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)

       The Shell and Utilities volume of POSIX.1‐2017, c99(1p)

COPYRIGHT         top

       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 .

       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               2017                       CONFSTR(3P)

Pages that refer to this page: poll.h(0p)stddef.h(0p)sys_types.h(0p)termios.h(0p)unistd.h(0p)wchar.h(0p)getconf(1p)exec(3p)fpathconf(3p)sysconf(3p)