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+.TH LIBKECCAK_SHA3SUM_FD 3 LIBKECCAK-%VERSION%
+.SH NAME
+libkeccak_sha3sum_fd - Calculate a SHA-3 hashsum of a file
+.SH SYNOPSIS
+.LP
+.nf
+#include <libkeccak.h>
+.P
+int libkeccak_sha3sum_fd(int \fIfd\fP, libkeccak_state_t *\fIstate\fP,
+                         long \fIoutput\fP, char *\fIhashsum\fP);
+.fi
+.P
+Link with \fI-lkeccak\fP.
+.SH DESCRIPTION
+The
+.BR libkeccak_sha3sum_fd ()
+calculates a SHA-3 hashsum of a file, whose file desriptor is specified
+by \fIfd\fP (and should be at the beginning of the file.) The
+hash algorithm is tuned by the \fIoutput\fP parameter; it specifies
+the output size, in bits.
+.PP
+The hash is stored in binary form to \fIhashsum\fP. \fIhashsum\fP
+should have an allocation size of at least
+(((\fIoutput\fP + 7) / 8) * sizeof(char)).
+.PP
+\fI*state\fP should not be initialised.
+.BR libkeccak_sha3sum_fd ()
+initialises \fI*state\fP itself. Therefore there would be a
+memory leak if \fI*state\fP is already initialised.
+.SH RETURN VALUES
+The
+.BR libkeccak_sha3sum_fd ()
+function return 0 upon successful completion.
+On error, -1 is returned and \fIerrno\fP is set to describe
+the error.
+.SH ERRORS
+The
+.BR libkeccak_sha3sum_fd ()
+function may fail for any reason, except those resulting
+in \fIerrno\fP being set to \fBEINTR\fP, specified for the
+functions
+.BR read (2),
+.BR malloc (3),
+and
+.BR realloc (3).
+.SH NOTES
+Be aware,
+.BR libkeccak_sha3sum_fd ()
+hashes the file until the end has been reached. For pipes
+and sockets and this means until the file has been closed.
+But for character devices, this usually means never.
+Attempting to hash files in /dev is therefore usually a
+bad idea.
+.BR libkeccak_sha3sum_fd ()
+does not check for the file length or file type before
+hashing as this could limit what you can do, and make
+the library more complex.
+.PP
+.BR libkeccak_sha3sum_fd ()
+does not stop if interrupted (\fBread\fP(2) returns
+\fBEINTR\fP.)
+.PP
+.BR libkeccak_sha3sum_fd ()
+assumes all information is non-sensitive, and will
+therefore not perform any secure erasure of information.
+.PP
+.BR libkeccak_sha3sum_fd ()
+does not validate the tuning of the algorithm.
+.SH EXAMPLES
+This example calculates the SHA3-256 hash of the input
+from stdin, and prints the hash, in hexadecimal form, to stdout.
+.LP
+.nf
+libkeccak_state_t state;
+if (libkeccak_sha3sum_fd(STDIN_FILNO, &state, 256, binhash) < 0)
+    goto fail;
+libkeccak_behex_lower(hexhash, binhash, sizeof(binhash));
+printf("%s\\n", hexhash);
+.fi
+.SH SEE ALSO
+.BR libkeccak_behex_lower (3),
+.BR libkeccak_behex_upper (3),
+.BR libkeccak_generalised_sum_fd (3),
+.BR libkeccak_keccaksum_fd (3),
+.BR libkeccak_rawshakesum_fd (3),
+.BR libkeccak_shakesum_fd (3),
+.BR libkeccak_spec_sha3 (3),
+.BR libkeccak_spec_check (3),
+.BR libkeccak_generalised_spec_initialise (3)
+.SH AUTHORS
+Principal author, Mattias Andrée.  See the LICENSE file for the full
+list of authors.
+.SH BUGS
+Please report bugs to https://github.com/maandree/bus/issues or to
+maandree@member.fsf.org