nbdkit_error, nbdkit_verror, nbdkit_set_error - return an error from nbdkit plugins
#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);If thre 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.
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;
}
//...
}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.
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.
nbdkit_error and nbdkit_verror were present in nbdkit 0.1.0. nbdkit_set_error was added in nbdkit 1.2.
nbdkit(1), nbdkit_debug(3), nbdkit-plugin(3), nbdkit-filter(3).
Eric Blake
Richard W.M. Jones
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