fanotify.7: Fix description of FAN_EVENT_NEXT()

Hi Heinrich,

I realized that the description of FAN_EVENT_NEXT() is not quite correct. It does not update 'meta', but instead returns a pointer to the next structure. I tried also to add a bit more detail to the description. Does the following patch look okay to you?

Cheers,

Michael

man7/fanotify.7 +10 -6

@@ -309,13 +309,24 @@ against the length of the metadata structure and the
  field of the first metadata structure in the buffer.
  .TP
  .B FAN_EVENT_NEXT(meta, len)
-This macro sets the pointer
-.I meta
-to the next metadata structure using the length indicated in the
+This macro uses the length indicated in the
  .I event_len
-field of the metadata structure and reduces the remaining length of the
-buffer
-.IR len .
+field of the metadata structure pointed to by
+.IR meta
+to calculate the address of the next metadata structure that follows
+.IR meta .
+.I len
+is the number of bytes of metadata that currently remain in the buffer.
+The macro returns a pointer to the next metadata structure that follows
+.IR meta ,
+and reduces

Hello Michael,

the proposed new description of FAN_EVENT_NEXT is correct.

Should FAN_EVENT_METADATA_LEN also be put into the section with macros?

.B FAN_EVENT_METADATA_LEN
This macro returns the size of structure
.I fanotify_event_metadata
in bytes.
This is the minimum size (and currently the only size) of any event metadata.

Quoted 46 lines

Hi Heinrich,

I realized that the description of FAN_EVENT_NEXT() is not quite correct. It does not update 'meta', but instead returns a pointer to the next structure. I tried also to add a bit more detail to the description. Does the following patch look okay to you?

Cheers,

Michael

man7/fanotify.7 +17 -6

@@ -309,13 +309,24 @@ against the length of the metadata structure and the
  field of the first metadata structure in the buffer.
  .TP
  .B FAN_EVENT_NEXT(meta, len)
-This macro sets the pointer
-.I meta
-to the next metadata structure using the length indicated in the
+This macro uses the length indicated in the
  .I event_len
-field of the metadata structure and reduces the remaining length of the
-buffer
-.IR len .
+field of the metadata structure pointed to by
+.IR meta
+to calculate the address of the next metadata structure that follows
+.IR meta .
+.I len
+is the number of bytes of metadata that currently remain in the buffer.
+The macro returns a pointer to the next metadata structure that follows
+.IR meta ,
+and reduces
+.I len
+by the number of bytes in the the metadata structure that
+has been skipped over (i.e., it subtracts
+.IR meta\->event_len
+from
+.IR len ).
+.\"
  .SS Monitoring an fanotify file descriptor for events
  When an fanotify event occurs, the fanotify file descriptor indicates as
  readable when passed to

Hi Heinrich,

Quoted 3 lines · Heinrich Schuchardt

Hello Michael,

the proposed new description of FAN_EVENT_NEXT is correct.

Thanks for checking. I've added a Reviewed-by for you.

Quoted 9 lines

Should FAN_EVENT_METADATA_LEN also be put into the section with macros?

.B FAN_EVENT_METADATA_LEN
This macro returns the size of structure
.I fanotify_event_metadata
in bytes.
This is the minimum size (and currently the only size) of any event metadata.

I'm inclined not to, but I might be wrong. Is there a reason that user-space might use it?

Cheers,

Michael

Quoted 49 lines · Michael Kerrisk (man-pages)

Quoted 46 lines

Hi Heinrich,

I realized that the description of FAN_EVENT_NEXT() is not quite correct. It does not update 'meta', but instead returns a pointer to the next structure. I tried also to add a bit more detail to the description. Does the following patch look okay to you?

Cheers,

Michael

man7/fanotify.7 +17 -6

@@ -309,13 +309,24 @@ against the length of the metadata structure and the
   field of the first metadata structure in the buffer.
   .TP
   .B FAN_EVENT_NEXT(meta, len)
-This macro sets the pointer
-.I meta
-to the next metadata structure using the length indicated in the
+This macro uses the length indicated in the
   .I event_len
-field of the metadata structure and reduces the remaining length of the
-buffer
-.IR len .
+field of the metadata structure pointed to by
+.IR meta
+to calculate the address of the next metadata structure that follows
+.IR meta .
+.I len
+is the number of bytes of metadata that currently remain in the buffer.
+The macro returns a pointer to the next metadata structure that follows
+.IR meta ,
+and reduces
+.I len
+by the number of bytes in the the metadata structure that
+has been skipped over (i.e., it subtracts
+.IR meta\->event_len
+from
+.IR len ).
+.\"
   .SS Monitoring an fanotify file descriptor for events
   When an fanotify event occurs, the fanotify file descriptor indicates as
   readable when passed to

Quoted 23 lines · Michael Kerrisk (man-pages)

Hi Heinrich,

Quoted 4 lines · Heinrich Schuchardt

Hello Michael,

the proposed new description of FAN_EVENT_NEXT is correct.

Thanks for checking. I've added a Reviewed-by for you.

Quoted 9 lines

Should FAN_EVENT_METADATA_LEN also be put into the section with macros?

.B FAN_EVENT_METADATA_LEN
This macro returns the size of structure
.I fanotify_event_metadata
in bytes.
This is the minimum size (and currently the only size) of any event metadata.

Quoted 2 lines

I'm inclined not to, but I might be wrong. Is there a reason that user-space might use it?

FAN_EVENT_METADATA_LEN is the minimum size of a buffer used to read fanotify events. Of cause the user may use sizeof to calculate it himself. Anyway there is already some mentioning of FAN_EVENT_METADATA_LEN in fanotify.7.

You remarked inotify read buffers should be 4 byte aligned on some CPUs. Shouldn't the same hold true for fanotify?