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.

AUTHORS

Eric Blake

Richard W.M. Jones

COPYRIGHT

LICENSE

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