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3. Generalities

3.1 Controlling the library  Controlling Libgcrypt's behaviour.
3.2 Modules  Description of extension modules.
3.3 Error Handling  Error codes and such.


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3.1 Controlling the library

Function: gcry_error_t gcry_control (enum gcry_ctl_cmds cmd, ...)

This function can be used to influence the general behaviour of Libgcrypt in several ways. Depending on cmd, more arguments can or have to be provided.


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3.2 Modules

Libgcrypt supports the use of `extension modules', which implement algorithms in addition to those already built into the library directly.

Data type: gcry_module_t
This data type represents a `module'.

Functions registering modules provided by the user take a `module specification structure' as input and return a value of gcry_module_t and an ID that is unique in the modules' category. This ID can be used to reference the newly registered module. After registering a module successfuly, the new functionality should be able to be used through the normal functions provided by Libgcrypt until it is unregistered again.


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3.3 Error Handling

Many functions in Libgcrypt can return an error if they fail. For this reason, the application should always catch the error condition and take appropriate measures, for example by releasing the resources and passing the error up to the caller, or by displaying a descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the operation, but the result of an operation that failed properly. For example, if you try to decrypt a tempered message, the decryption will fail. Another error value actually means that the end of a data buffer or list has been reached. The following descriptions explain for many error codes what they mean usually. Some error values have specific meanings if returned by a certain functions. Such cases are described in the documentation of those functions.

Libgcrypt uses the libgpg-error library. This allows to share the error codes with other components of the GnuPG system, and thus pass error values transparently from the crypto engine, or some helper application of the crypto engine, to the user. This way no information is lost. As a consequence, Libgcrypt does not use its own identifiers for error codes, but uses those provided by libgpg-error. They usually start with GPG_ERR_.

However, Libgcrypt does provide aliases for the functions defined in libgpg-error, which might be preferred for name space consistency.

Most functions in Libgcrypt return an error code in the case of failure. For this reason, the application should always catch the error condition and take appropriate measures, for example by releasing the resources and passing the error up to the caller, or by displaying a descriptive message to the user and canceling the operation.

Some error values do not indicate a system error or an error in the operation, but the result of an operation that failed properly.

GnuPG components, including libgcrypt, use an extra library named libgpg-error to provide a common error handling scheme. For more information on libgpg-error, see the according manual.

3.3.1 Error Values  The error value and what it means.
3.3.2 Error Sources  A list of important error sources.
3.3.3 Error Codes  A list of important error codes.
3.3.4 Error Strings  How to get a descriptive string from a value.


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3.3.1 Error Values

Data type: gcry_err_code_t
The gcry_err_code_t type is an alias for the libgpg-error type gpg_err_code_t. The error code indicates the type of an error, or the reason why an operation failed.

A list of important error codes can be found in the next section.

Data type: gcry_err_source_t
The gcry_err_source_t type is an alias for the libgpg-error type gpg_err_source_t. The error source has not a precisely defined meaning. Sometimes it is the place where the error happened, sometimes it is the place where an error was encoded into an error value. Usually the error source will give an indication to where to look for the problem. This is not always true, but it is attempted to achieve this goal.

A list of important error sources can be found in the next section.

Data type: gcry_error_t
The gcry_error_t type is an alias for the libgpg-error type gpg_error_t. An error value like this has always two components, an error code and an error source. Both together form the error value.

Thus, the error value can not be directly compared against an error code, but the accessor functions described below must be used. However, it is guaranteed that only 0 is used to indicate success (GPG_ERR_NO_ERROR), and that in this case all other parts of the error value are set to 0, too.

Note that in Libgcrypt, the error source is used purely for diagnostical purposes. Only the error code should be checked to test for a certain outcome of a function. The manual only documents the error code part of an error value. The error source is left unspecified and might be anything.

Function: gcry_err_code_t gcry_err_code (gcry_error_t err)
The static inline function gcry_err_code returns the gcry_err_code_t component of the error value err. This function must be used to extract the error code from an error value in order to compare it with the GPG_ERR_* error code macros.

Function: gcry_err_source_t gcry_err_source (gcry_error_t err)
The static inline function gcry_err_source returns the gcry_err_source_t component of the error value err. This function must be used to extract the error source from an error value in order to compare it with the GPG_ERR_SOURCE_* error source macros.

Function: gcry_error_t gcry_err_make (gcry_err_source_t source, gcry_err_code_t code)
The static inline function gcry_err_make returns the error value consisting of the error source source and the error code code.

This function can be used in callback functions to construct an error value to return it to the library.

Function: gcry_error_t gcry_error (gcry_err_code_t code)
The static inline function gcry_error returns the error value consisting of the default error source and the error code code.

For GCRY applications, the default error source is GPG_ERR_SOURCE_USER_1. You can define GCRY_ERR_SOURCE_DEFAULT before including `gcrypt.h' to change this default.

This function can be used in callback functions to construct an error value to return it to the library.

The libgpg-error library provides error codes for all system error numbers it knows about. If err is an unknown error number, the error code GPG_ERR_UNKNOWN_ERRNO is used. The following functions can be used to construct error values from system errnor numbers.

Function: gcry_error_t gcry_err_make_from_errno (gcry_err_source_t source, int err)
The function gcry_err_make_from_errno is like gcry_err_make, but it takes a system error like errno instead of a gcry_err_code_t error code.

Function: gcry_error_t gcry_error_from_errno (int err)
The function gcry_error_from_errno is like gcry_error, but it takes a system error like errno instead of a gcry_err_code_t error code.

Sometimes you might want to map system error numbers to error codes directly, or map an error code representing a system error back to the system error number. The following functions can be used to do that.

Function: gcry_err_code_t gcry_err_code_from_errno (int err)
The function gcry_err_code_from_errno returns the error code for the system error err. If err is not a known system error, the function returns GPG_ERR_UNKNOWN_ERRNO.

Function: int gcry_err_code_to_errno (gcry_err_code_t err)
The function gcry_err_code_to_errno returns the system error for the error code err. If err is not an error code representing a system error, or if this system error is not defined on this system, the function returns 0.


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3.3.2 Error Sources

The library libgpg-error defines an error source for every component of the GnuPG system. The error source part of an error value is not well defined. As such it is mainly useful to improve the diagnostic error message for the user.

If the error code part of an error value is 0, the whole error value will be 0. In this case the error source part is of course GPG_ERR_SOURCE_UNKNOWN.

The list of error sources that might occur in applications using Libgctypt is:

GPG_ERR_SOURCE_UNKNOWN
The error source is not known. The value of this error source is 0.

GPG_ERR_SOURCE_GPGME
The error source is GPGME itself.

GPG_ERR_SOURCE_GPG
The error source is GnuPG, which is the crypto engine used for the OpenPGP protocol.

GPG_ERR_SOURCE_GPGSM
The error source is GPGSM, which is the crypto engine used for the OpenPGP protocol.

GPG_ERR_SOURCE_GCRYPT
The error source is libgcrypt, which is used by crypto engines to perform cryptographic operations.

GPG_ERR_SOURCE_GPGAGENT
The error source is gpg-agent, which is used by crypto engines to perform operations with the secret key.

GPG_ERR_SOURCE_PINENTRY
The error source is pinentry, which is used by gpg-agent to query the passphrase to unlock a secret key.

GPG_ERR_SOURCE_SCD
The error source is the SmartCard Daemon, which is used by gpg-agent to delegate operations with the secret key to a SmartCard.

GPG_ERR_SOURCE_KEYBOX
The error source is libkbx, a library used by the crypto engines to manage local keyrings.

GPG_ERR_SOURCE_USER_1
GPG_ERR_SOURCE_USER_2
GPG_ERR_SOURCE_USER_3
GPG_ERR_SOURCE_USER_4
These error sources are not used by any GnuPG component and can be used by other software. For example, applications using Libgcrypt can use them to mark error values coming from callback handlers. Thus GPG_ERR_SOURCE_USER_1 is the default for errors created with gcry_error and gcry_error_from_errno, unless you define GCRY_ERR_SOURCE_DEFAULT before including `gcrypt.h'.


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3.3.3 Error Codes

The library libgpg-error defines many error values. The following list includes the most important error codes.

GPG_ERR_EOF
This value indicates the end of a list, buffer or file.

GPG_ERR_NO_ERROR
This value indicates success. The value of this error code is 0. Also, it is guaranteed that an error value made from the error code 0 will be 0 itself (as a whole). This means that the error source information is lost for this error code, however, as this error code indicates that no error occured, this is generally not a problem.

GPG_ERR_GENERAL
This value means that something went wrong, but either there is not enough information about the problem to return a more useful error value, or there is no separate error value for this type of problem.

GPG_ERR_ENOMEM
This value means that an out-of-memory condition occurred.

GPG_ERR_E...
System errors are mapped to GPG_ERR_EFOO where FOO is the symbol for the system error.

GPG_ERR_INV_VALUE
This value means that some user provided data was out of range.

GPG_ERR_UNUSABLE_PUBKEY
This value means that some recipients for a message were invalid.

GPG_ERR_UNUSABLE_SECKEY
This value means that some signers were invalid.

GPG_ERR_NO_DATA
This value means that data was expected where no data was found.

GPG_ERR_CONFLICT
This value means that a conflict of some sort occurred.

GPG_ERR_NOT_IMPLEMENTED
This value indicates that the specific function (or operation) is not implemented. This error should never happen. It can only occur if you use certain values or configuration options which do not work, but for which we think that they should work at some later time.

GPG_ERR_DECRYPT_FAILED
This value indicates that a decryption operation was unsuccessful.

GPG_ERR_WRONG_KEY_USAGE
This value indicates that a key is not used appropriately.

GPG_ERR_NO_SECKEY
This value indicates that no secret key for the user ID is available.

GPG_ERR_UNSUPPORTED_ALGORITHM
This value means a verification failed because the cryptographic algorithm is not supported by the crypto backend.

GPG_ERR_BAD_SIGNATURE
This value means a verification failed because the signature is bad.

GPG_ERR_NO_PUBKEY
This value means a verification failed because the public key is not available.

GPG_ERR_USER_1
GPG_ERR_USER_2
...
GPG_ERR_USER_16
These error codes are not used by any GnuPG component and can be freely used by other software. Applications using Libgcrypt might use them to mark specific errors returned by callback handlers if no suitable error codes (including the system errors) for these errors exist already.


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3.3.4 Error Strings

Function: const char * gcry_strerror (gcry_error_t err)
The function gcry_strerror returns a pointer to a statically allocated string containing a description of the error code contained in the error value err. This string can be used to output a diagnostic message to the user.

Function: const char * gcry_strsource (gcry_error_t err)
The function gcry_strerror returns a pointer to a statically allocated string containing a description of the error source contained in the error value err. This string can be used to output a diagnostic message to the user.

The following example illustrates the use of the functions described above:

 
{
  gcry_cipher_hd_t handle;
  gcry_error_t err = 0;

  err = gcry_cipher_open (&handle, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CBC, 0);
  if (err)
    {
      fprintf (stderr, "Failure: %s/%s\n",
               gcry_strsource (err),
               gcry_strerror (err));
    }
}


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