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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
      diagnostic 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 errno 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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