The Annoyance Filter.pdf - Fourmilab

§133 ANNOYANCE-FILTER MAIL FOLDER 117
133. Before testing whether the input file is compressed, see if the name we were given is a symbolic
link. If so, follow the link and test the actual file. We only follow links up to 50 levels. We copy the
file name given us to jname , then attempt to interpret it as a symbolic link by calling readlink , which
will fail if the name is not, in fact, a symbolic link. If it is, we obtain the link destination as a C string,
which is copied into jname prior to the test for a compressed file extension.
〈 Check for symbolic link to compressed file 133 〉 ≡
#ifdef HAVE_READLINK
int maxSlinks = 50;
string jname = fname ;
char slbuf [1024];
while (maxSlinks −− > 0) {
int sll = readlink (jname .c str ( ), slbuf , (sizeof slbuf ) − 1);
if (sll ≥ 0) {
assert(sll < static cast〈int〉(sizeof slbuf ));
slbuf [sll ] = 0;
jname = slbuf ;
}
else {
break;
}
}
if (maxSlinks ≤ 0) {
cerr ≪ "Warning:␣probable␣symbolic␣link␣loop␣for␣\"" ≪ fname ≪ "\"" ≪ endl ;
}
#endif
This code is used in sections 129 and 137.
134. If our input file bears an extension which identifies it as a compressed file, we use popen to
create a file handle connected to a pipe to the appropriate decompression program. The pipe is then
screwed into the input stream from which we subsequently read.
〈 Open pipe to read compressed file 134 〉 ≡
string cmd (Uncompress command );
cmd += ’␣’ + fname ;
ip = popen (cmd .c str ( ), "r");
#ifdef HAVE_FDSTREAM_COMPATIBILITY
iscc.attach (fileno(ip));
is = &iscc;
#else
isc.attach (fileno(ip));
is = &isc;
#endif
This code is used in section 129.