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+.TH LIBKECCAK_KECCAKSUM_FD 3 LIBKECCAK
+.SH NAME
+libkeccak_keccaksum_fd - Calculate a Keccak hashsum of a file
+.SH SYNOPSIS
+.LP
+.nf
+#include <libkeccak.h>
+.P
+int
+libkeccak_keccaksum_fd(int \fIfd\fP, libkeccak_state_t *\fIstate\fP,
+                       const libkeccak_spec_t *\fIspec\fP, char *\fIhashsum\fP);
+.fi
+.P
+Link with
+.IR -lkeccak .
+.SH DESCRIPTION
+The
+.BR libkeccak_keccaksum_fd ()
+function calculates a Keccak hashsum of a file, whose file
+desriptor is specified by
+.I fd
+(and should be at the beginning of the file.) The hash
+algorithm tuning is specified by
+.IR *spec .
+.PP
+The hash is stored in binary form to
+.IR hashsum .
+.I hashsum
+should have an allocation size of at least
+.RI ((( spec->output
++ 7) / 8) * sizeof(char)).
+.PP
+.I *state
+should not be initialised.
+.BR libkeccak_keccaksum_fd ()
+initialises
+.I *state
+itself. Therefore there would be a memory leak if
+.I *state
+is already initialised.
+.SH RETURN VALUES
+The
+.BR libkeccak_keccaksum_fd ()
+function returns 0 upon successful completion.
+On error, -1 is returned and
+.I errno
+is set to describe the error.
+.SH ERRORS
+The
+.BR libkeccak_keccaksum_fd ()
+function may fail for any reason, except those resulting
+in
+.I errno
+being set to
+.BR EINTR ,
+specified for the functions
+.BR read (2),
+.BR malloc (3),
+and
+.BR realloc (3).
+.SH NOTES
+Be aware,
+.BR libkeccak_keccaksum_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_keccaksum_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_keccaksum_fd ()
+does not stop if interrupted
+.RI ( read (2)
+returns
+.BR EINTR .)
+.PP
+.BR libkeccak_keccaksum_fd ()
+assumes all information is non-sensitive, and will
+therefore not perform any secure erasure of information.
+.PP
+.BR libkeccak_keccaksum_fd ()
+does not validate the tuning of the algorithm.
+.SH EXAMPLE
+This example calculates the Keccak[b = 1024, c = 576, n = 256]
+hash of the input from stdin, and prints the hash, in hexadecimal
+form, to stdout.
+.LP
+.nf
+libkeccak_state_t state;
+libkeccak_spec_t spec;
+char binhash[256 / 8];
+char hexhash[256 / 8 * 2 + 1];
+
+spec.bitrate = 1024;
+spec.capacity = 576;
+spec.output = 256;
+
+if (libkeccak_keccaksum_fd(STDIN_FILENO, &state, &spec, binhash) < 0)
+    goto fail;
+libkeccak_behex_lower(hexhash, binhash, sizeof(binhash));
+printf("%s\\n", hexhash);
+libkeccak_state_destroy(&state);
+.fi
+.SH SEE ALSO
+.BR libkeccak_behex_lower (3),
+.BR libkeccak_behex_upper (3),
+.BR libkeccak_generalised_sum_fd (3),
+.BR libkeccak_sha3sum_fd (3),
+.BR libkeccak_rawshakesum_fd (3),
+.BR libkeccak_shakesum_fd (3),
+.BR libkeccak_spec_check (3),
+.BR libkeccak_generalised_spec_initialise (3),
+.BR libkeccak_state_initialise (3)
+.SH BUGS
+Please report bugs to https://github.com/maandree/libkeccak/issues or to
+maandree@kth.se