POSIX.2 defines a way to get string-valued parameters from the operating
system with the function confstr
:
The normal return value from confstr
is the length of the string
value that you asked for. If you supply a null pointer for buf,
then confstr
does not try to store the string; it just returns
its length. A value of 0
indicates an error.
If the string you asked for is too long for the buffer (that is, longer
than len - 1
), then confstr
stores just that much
(leaving room for the terminating null character). You can tell that
this has happened because confstr
returns a value greater than or
equal to len.
The following errno
error conditions are defined for this function:
EINVAL
Currently there is just one parameter you can read with confstr
:
_CS_PATH
_CS_LFS_CFLAGS
_LARGEFILE_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS_LDFLAGS
_LARGEFILE_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS_LIBS
_LARGEFILE_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS_LINTFLAGS
_LARGEFILE_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS64_CFLAGS
_LARGEFILE64_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS64_LDFLAGS
_LARGEFILE64_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS64_LIBS
_LARGEFILE64_SOURCE
feature select macro; see section Feature Test Macros.
_CS_LFS64_LINTFLAGS
_LARGEFILE64_SOURCE
feature select macro; see section Feature Test Macros.
The way to use confstr
without any arbitrary limit on string size
is to call it twice: first call it to get the length, allocate the
buffer accordingly, and then call confstr
again to fill the
buffer, like this:
char * get_default_path (void) { size_t len = confstr (_CS_PATH, NULL, 0); char *buffer = (char *) xmalloc (len); if (confstr (_CS_PATH, buf, len + 1) == 0) { free (buffer); return NULL; } return buffer; }
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