NAME

       nbdkit_error, nbdkit_verror, nbdkit_set_error - return an error from nbdkit plugins

SYNOPSIS

        #include <nbdkit-plugin.h>

        void nbdkit_error (const char *fs, ...);
        void nbdkit_verror (const char *fs, va_list args);

        void nbdkit_set_error (int err);

DESCRIPTION

       If there is an error in your plugin, the plugin should call "nbdkit_error" to report an
       error message.  Then the callback should return the appropriate error indication, eg.
       "NULL" or -1.

       "nbdkit_error" works like printf(3).  "nbdkit_verror" works like vprintf(3).

       For convenience, "nbdkit_error" preserves the value of "errno", and also supports the
       glibc extension of a single %m in a format string expanding to strerror(errno), even on
       platforms that don't support that natively.

   Example
        int
        my_pread (void *handle, void *buf, uint32_t count, uint64_t offset,
                  uint32_t flags)
        {
          char *bounce_buffer = malloc (SECTOR_SIZE);
          if (bounce_buffer == NULL) {
            nbdkit_error ("malloc: %m");
            return -1;
          }
          //...
        }

   Setting errno
       For callbacks which serve data to the client, the plugin may call "nbdkit_set_error" to
       influence the errno that will be sent back to the client.

       If the call to "nbdkit_set_error" is omitted while serving data, then the global variable
       "errno" may be used.  For plugins which have ".errno_is_preserved != 0" the server will
       use "errno".  In plugins written in non-C languages, we usually cannot trust that "errno"
       will be preserved when returning from that language to C.  In that case, either the plugin
       must call "nbdkit_set_error" or hard-coded "EIO" is used.

LANGUAGE BINDINGS

       Most language bindings do not expose these functions explicitly.  Instead you are expected
       to throw an exception on error which is implicitly turned into a call to "nbdkit_error".
       Read the documentation for the language binding to find out how errors are handled.

HISTORY

       "nbdkit_error" and "nbdkit_verror" were present in nbdkit 0.1.0.  "nbdkit_set_error" was
       added in nbdkit 1.2.

SEE ALSO

       nbdkit(1), nbdkit_debug(3), nbdkit-plugin(3), nbdkit-filter(3).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

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