---
name: "libkeccak_fast_update"
section: "3"
platform: "linux"
package: "libkeccak"
version: "1.4.3-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux"]
url: "https://new.carta.tech/libkeccak_fast_update"
---

# libkeccak\_fast\_update(3)

Partially hash a message without erasure

## NAME

libkeccak\_fast\_update - Partially hash a message without erasure

## SYNOPSIS

```
#include <libkeccak.h>
int libkeccak_fast_update(struct libkeccak_state *state, const void *msg, size_t msglen);
```

Link with *-lkeccak*.

## DESCRIPTION

The **libkeccak\_fast\_update**() function continues (or starts) hashing a message. The current state of the hashing is stored in *\*state*, and will be updated. The message specified by the *msg* parameter with the byte-size specified by the *msglen* parameter, will be hashed.

The **libkeccak\_fast\_update**() function may reallocate the state's message chunk buffer. When doing so, it attempts to do so as quickly as possible, rather than ensuring that the information in the old allocation is securely removed if a new allocation is required.

## RETURN VALUES

The **libkeccak\_fast\_update**() function returns 0 upon successful completion. On error, -1 is returned and *errno* is set to describe the error.

## ERRORS

The **libkeccak\_fast\_update**() function may fail for any reason specified by the function [**realloc**(3)](/realloc/3.md).

## NOTES

Neither parameter by be *NULL* or 0.

## 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.

```
struct libkeccak_state state;
struct libkeccak_spec spec;
char binhash[256 / 8];
char hexhash[256 / 8 * 2 + 1];
char chunk[4 << 10];
ssize_t len;
spec.bitrate = 1024;
spec.capacity = 576;
spec.output = 256;
if (libkeccak_state_initialise(&state, &spec) < 0)
	goto fail;
for (;;) {
	len = read(STDIN_FILENO, chunk, sizeof(chunk));
	if ((len < 0) && (errno == EINTR))
		continue;
	if (len < 0)
		goto fail;
	if (len == 0)
		break;
	if (libkeccak_fast_update(&state, chunk, (size_t)len) < 0)
		goto fail;
}
if (libkeccak_fast_digest(&state, NULL, 0, 0, "", binhash) < 0)
	goto fail;
libkeccak_behex_lower(hexhash, binhash, sizeof(binhash));
printf("%s\n", hexhash);
libkeccak_state_fast_destroy(&state);
```

## NOTES

For cSHAKE, the [**libkeccak\_cshake\_initialise**(3)](/libkeccak_cshake_initialise/3.md), must be called, once, immediately after state initialisation; before the first call to the **libkeccak\_fast\_update**() function.

## SEE ALSO

[**libkeccak\_state\_initialise**(3)](/libkeccak_state_initialise/3.md), [**libkeccak\_cshake\_initialise**(3)](/libkeccak_cshake_initialise/3.md), [**libkeccak\_zerocopy\_update**(3)](/libkeccak_zerocopy_update/3.md), [**libkeccak\_update**(3)](/libkeccak_update/3.md), [**libkeccak\_fast\_digest**(3)](/libkeccak_fast_digest/3.md), [**libkeccak\_zerocopy\_digest**(3)](/libkeccak_zerocopy_digest/3.md), [**libkeccak\_digest**(3)](/libkeccak_digest/3.md)