CONFSTR(3P) POSIX Programmer's Manual CONFSTR(3P)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.
NAMEconfstr — get configurable variables
size_t confstr(int name, char *buf, size_t len);
The confstr() function shall return configuration-defined string val‐
ues. 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, conf‐
str() 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()
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 implementa‐
tion to create a conforming environment, as described in the implemen‐
tations' 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).
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
If name does not have a configuration-defined value, confstr() shall
return 0 and leave errno unchanged.
The confstr() function shall fail if:
EINVAL The value of the name argument is invalid.
The following sections are informative.
An application can distinguish between an invalid name parameter value
and one that corresponds to a configurable variable that has no config‐
uration-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 deter‐
mine 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 mal‐
loc() to allocate a buffer to hold the string; and call confstr() again
to get the string. Alternately, it could allocate a fixed, static buf‐
fer 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.
Application developers can normally determine any configuration vari‐
able 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 sup‐
plied 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 appli‐
cation must copy the string returned.
exec, fpathconf(), sysconf()
The Base Definitions volume of POSIX.1‐2008, <unistd.h>
The Shell and Utilities volume of POSIX.1‐2008, c99
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 Electri‐
cal 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
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IEEE/The Open Group 2013 CONFSTR(3P)