.TH LIBKECCAK_SHAKESUM_FD 3 LIBKECCAK
.SH NAME
libkeccak_shakesum_fd - Calculate a SHAKE hashsum of a file
.SH SYNOPSIS
.nf
#include <libkeccak.h>
int libkeccak_shakesum_fd(int \fIfd\fP, struct libkeccak_state *\fIstate\fP, long int \fIsemicapacity\fP, long int \fIoutput\fP, void *\fIhashsum\fP);
.fi
.PP
Link with
.IR -lkeccak .
.SH DESCRIPTION
The
.BR libkeccak_shakesum_fd ()
function calculates a SHAKE hashsum of a file, whose file
desriptor is specified by
.I fd
(and should be at the beginning of the file.) The hash
algorithm is tuned by the
.I semicapacity
and
.I output
parameters; they specify the half of the capacity and the
output size, respectively, in bits.
.PP
The hash is stored in binary form to
.IR hashsum .
.I hashsum
should have an allocation size of at least
.RI ((( output
+ 7) / 8) * sizeof(char)).
.PP
.I *state
should not be initialised.
.BR libkeccak_shakesum_fd ()
initialises
.I *state
itself. Therefore there would be a memory leak if
.I *state
is already initialised.
.SH RETURN VALUES
The
.BR libkeccak_shakesum_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_shakesum_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_shakesum_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_shakesum_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_shakesum_fd ()
does not stop if interrupted
.RB ( read (2)
returns
.BR EINTR .)
.PP
.BR libkeccak_shakesum_fd ()
assumes all information is non-sensitive, and will
therefore not perform any secure erasure of information.
.PP
.BR libkeccak_shakesum_fd ()
does not validate the tuning of the algorithm.
.SH EXAMPLE
This example calculates the SHAKE256(, 512) hash of the input
from stdin, and prints the hash, in hexadecimal form, to stdout.
.LP
.nf
struct libkeccak_state state;
if (libkeccak_shakesum_fd(STDIN_FILENO, &state, 256, 512, binhash) < 0)
goto fail;
libkeccak_behex_lower(hexhash, binhash, sizeof(binhash));
printf(\(dq%s\en\(dq, 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_keccaksum_fd (3),
.BR libkeccak_sha3sum_fd (3),
.BR libkeccak_rawshakesum_fd (3),
.BR libkeccak_spec_shake (3),
.BR libkeccak_spec_check (3),
.BR libkeccak_generalised_spec_initialise (3),
.BR libkeccak_state_initialise (3)
