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_XBS5_ILP32_OFF32_CFLAGS ( LEGACY)
_CS_XBS5_ILP32_OFF32_LDFLAGS ( LEGACY)
_CS_XBS5_ILP32_OFF32_LIBS ( LEGACY)
_CS_XBS5_ILP32_OFF32_LINTFLAGS ( LEGACY)
_CS_XBS5_ILP32_OFFBIG_CFLAGS ( LEGACY)
_CS_XBS5_ILP32_OFFBIG_LDFLAGS ( LEGACY)
_CS_XBS5_ILP32_OFFBIG_LIBS ( LEGACY)
_CS_XBS5_ILP32_OFFBIG_LINTFLAGS ( LEGACY)
_CS_XBS5_LP64_OFF64_CFLAGS ( LEGACY)
_CS_XBS5_LP64_OFF64_LDFLAGS ( LEGACY)
_CS_XBS5_LP64_OFF64_LIBS ( LEGACY)
_CS_XBS5_LP64_OFF64_LINTFLAGS ( LEGACY)
_CS_XBS5_LPBIG_OFFBIG_CFLAGS ( LEGACY)
_CS_XBS5_LPBIG_OFFBIG_LDFLAGS ( LEGACY)
_CS_XBS5_LPBIG_OFFBIG_LIBS ( LEGACY)
_CS_XBS5_LPBIG_OFFBIG_LINTFLAGS ( LEGACY)
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
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 IEEE Std 1003.1-2001, 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
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.
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 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 too
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 IEEE Std 1003.1-2001.
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
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 original
Standard can be obtained online at http://www.opengroup.org/unix/online.html