CONFSTR(P) POSIX Programmer's Manual CONFSTR(P)
confstr - get configurable variables
size_t confstr(int name, char *buf, size_t len);
The confstr() function shall return configuration-defined string values. Its use and pur-
pose are similar to sysconf(), but it is used where string values rather than numeric val-
ues 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 unspecified.
If the implementation supports the POSIX shell option, the string stored in buf after a
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 IEEE Std 1003.1-2001, if the return value is less than or equal to sizeof(
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
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 corre-
sponds 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 utili-
ties in the Shell and Utilities volume of IEEE Std 1003.1-2001, 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
Application developers can normally determine any configuration variable by means of read-
ing 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 utili-
ties as described in the Shell and Utilities volume of IEEE Std 1003.1-2001.
The confstr() function copies the returned string into a buffer supplied by the applica-
tion 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.
pathconf() , sysconf() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>,
the Shell and Utilities volume of IEEE Std 1003.1-2001, c99
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 CONFSTR(P)