---
name: "digestbuf"
section: "3bobcat"
platform: "linux"
package: "libbobcat-dev"
version: "6.07.01-2"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/digestbuf"
---

# digestbuf(3bobcat)

Computes Message Digests from information inserted into a std::ostream

## NAME

FBB::DigestBuf - Computes Message Digests from information inserted into a std::ostream

## SYNOPSIS

**#include \<bobcat/digestbuf\>**\
 Linking option: *-lbobcat -lcrypto*

## DESCRIPTION

**FBB::DigestBuf** objects are **std::streambuf** objects that can be used to initialize *std::ostream* objects with.

The message digest of all information inserted into such an *std::ostream* is computed.

All the message digests defined by the OpenSSL library that can be selected by name may be used in combination with *DigestBuf* objects.

The currently supported message digest algorithms are shown by the *openssl list -digest-commands*. The names of the digest algorithms can be used to select a digest algorithms for the class’s constructor, below.

## NAMESPACE

**FBB**\
 All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace **FBB**.

## INHERITS FROM

**std::streambuf**

## CONSTRUCTORS

- o

  **DigestBuf(char const \*digestName, size\_t bufsize = 1024)**:\
   This constructor initializes the streambuf, setting it up for the message digest algorithm specified with *type*. The message digest algorithms specified in the **DESCRIPTION** section may be used here. E.g., to use the sha256 algorithm specify *"sha256"*.

-

  The *bufsize* argument specifies the size of the internal buffer used by *DigestBuf* to store incoming characters temporarily. The provided default argument should be OK in all normal cases.

The copy and move constructors and assignment operators are not available.

## OVERLOADED OPERATOR

- o

  **std::ostream &operator\<\<(std::ostream &out,** **DigestBuf const &digestbuf)**:\
   The insertion operator is a free function defined in the namespace *FBB*. It inserts a hash value as a series of hexadecimally displayed values into the provided *ostream*. See the example below for an illustration.

## MEMBER FUNCTIONS

All members of **std::streambuf** are available, as **FBB::DigestBuf** inherits from this class.

- o

  **void eoi()**:\
   This member finishes the computation of the  message digest computation. It is needed as the *DigestBuf* object has no external means for deciding whether all information to compute the digest for has yet been received or not. The general approach to follow when computing a message digest is therefore:

  ```
      1. create a DigestBuf object
      2. use it to create a std::ostream object
      3. insert information into the ostream object
      4. call the DigestBuf object’s eoi() member or insert the
          eoi manipulator into the ostream (see below)
      5. obtain/process the hash value from the DigestBuf object.
  ```

-

  - o

  **std::string const &hash() const**:\
   This member returns the hash value computed by the *DigestBuf* object. Its value is only defined after having called *close()*. The hash value is returned in a *std::string* object. This string’s *length()* member contains the number of characters used by the hash value, and its *data()* member refers to the hash value’s characters. Note that a hash value’s character value may be 0 (not to be confused with *’0’*).

-

  - o

  **void reset()**:\
   This member reinitializes the message digest computation. One a message digest has been computed for, say a stream *streamA* this member can be called after which the digest for a stream *streamB* can be computed using the same *DigestBuf* object.

## MANIPULATOR

- o

  **FBB::eoi**:\
   The *eoi* manipulator can be inserted into the *ostream* to complete the digest. It performs the same actions as the *eoi* member. If inserted into a plain *std::ostream* nothing happens.

## EXAMPLE

```
#include <iostream>
#include <ostream>
#include <bobcat/digestbuf>
using namespace std;
using namespace FBB;
int main(int argc, char **argv)
try
{
    DigestBuf digestbuf(argc == 1 ? "sha256" : argv[1]);
    ostream out(&digestbuf);
    string hw("hello world\n");
    out << hw << eoi;
    cout << ">" << digestbuf << "<" << endl;
    digestbuf.reset();
    out.write(hw.c_str(), hw.length()) << eoi;
    cout << ">" << digestbuf << "<" << endl;
}
catch(exception const &err)
{
    cout << err.what() << endl;
    return 1;
}
```

## FILES

*bobcat/digestbuf* - defines the class interface

## SEE ALSO

[**bobcat**(7)](/bobcat/7.md), [**hmacbuf**(3bobcat)](/hmacbuf/3bobcat.md), **std::streambuf**

## BUGS

None reported

## BOBCAT PROJECT FILES

- o

  *https://fbb-git.gitlab.io/bobcat/*: gitlab project page;

Debian Bobcat project files:

- o

  *libbobcat6*: debian package containing the shared library, changelog and copyright note;

- o

  *libbobcat-dev*: debian package containing the static library, headers, manual pages, and developer info;

## BOBCAT

Bobcat is an acronym of \`Brokken’s Own Base Classes And Templates’.

## COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

## AUTHOR

Frank B. Brokken (**f.b.brokken@rug.nl**).