Add some documentation for the BIO_s_mem() datagram capability

Reviewed-by: Tomas Mraz <[email protected]>
Reviewed-by: Hugo Landau <[email protected]>
(Merged from openssl#18596)

CHANGES.md

@@ -24,6 +24,10 @@ OpenSSL 3.1
 
 ### Changes between 3.0 and 3.1 [xx XXX xxxx]
 
+ * Added a new BIO_s_dgram_mem() to read/write datagrams to memory
+
+   *Matt Caswell*
+
  * Add a mac salt length option for the pkcs12 command.
 
    *Xinping Chen*

doc/man3/BIO_s_mem.pod

@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-BIO_s_secmem,
+BIO_s_secmem, BIO_s_dgram_mem,
 BIO_s_mem, BIO_set_mem_eof_return, BIO_get_mem_data, BIO_set_mem_buf,
 BIO_get_mem_ptr, BIO_new_mem_buf - memory BIO
 
@@ -11,6 +11,7 @@ BIO_get_mem_ptr, BIO_new_mem_buf - memory BIO
  #include <openssl/bio.h>
 
  const BIO_METHOD *BIO_s_mem(void);
+ const BIO_METHOD *BIO_s_dgram_mem(void);
  const BIO_METHOD *BIO_s_secmem(void);
 
  BIO_set_mem_eof_return(BIO *b, int v);
@@ -31,6 +32,22 @@ as appropriate to accommodate the stored data.
 BIO_s_secmem() is like BIO_s_mem() except that the secure heap is used
 for buffer storage.
 
+BIO_s_dgram_mem() is a memory BIO that respects datagram semantics. A single
+call to L<BIO_write(3)> will write a single datagram to the memory BIO. A
+subsequent call to L<BIO_read(3)> will read the data in that datagram. The
+L<BIO_read(3)> call will never return more data than was written in the original
+L<BIO_write(3)> call even if there were subsequent L<BIO_write(3)> calls that
+wrote more datagrams. Each successive call to L<BIO_read(3)> will read the next
+datagram. If a L<BIO_read(3)> call supplies a read buffer that is smaller than
+the size of the datagram, then the read buffer will be completely filled and the
+remaining data from the datagram will be discarded.
+
+It is not possible to write a zero length datagram. Calling L<BIO_write(3)> in
+this case will return 0 and no datagrams will be written. Calling L<BIO_read(3)>
+when there are no datagrams in the BIO to read will return a negative result and
+the "retry" flags will be set (i.e. calling L<BIO_should_retry(3)> will return
+true). A datagram mem BIO will never return true from L<BIO_eof(3)>.
+
 Any data written to a memory BIO can be recalled by reading from it.
 Unless the memory BIO is read only any data read from it is deleted from
 the BIO.
@@ -56,6 +73,7 @@ it will return zero and BIO_should_retry(b) will be false. If B<v> is non
 zero then it will return B<v> when it is empty and it will set the read retry
 flag (that is BIO_read_retry(b) is true). To avoid ambiguity with a normal
 positive return value B<v> should be set to a negative value, typically -1.
+Calling this macro will fail for datagram mem BIOs.
 
 BIO_get_mem_data() sets *B<pp> to a pointer to the start of the memory BIOs data
 and returns the total amount of data available. It is implemented as a macro.
@@ -86,15 +104,15 @@ copy operation, if a BIO contains a lot of data and it is read in small
 chunks intertwined with writes the operation can be very slow. Adding
 a buffering BIO to the chain can speed up the process.
 
-Calling BIO_set_mem_buf() on a BIO created with BIO_new_secmem() will
-give undefined results, including perhaps a program crash.
+Calling BIO_set_mem_buf() on a secmem or dgram BIO will give undefined results,
+including perhaps a program crash.
 
-Switching the memory BIO from read write to read only is not supported and
+Switching a memory BIO from read write to read only is not supported and
 can give undefined results including a program crash. There are two notable
 exceptions to the rule. The first one is to assign a static memory buffer
 immediately after BIO creation and set the BIO as read only.
 
-The other supported sequence is to start with read write BIO then temporarily
+The other supported sequence is to start with a read write BIO then temporarily
 switch it to read only and call BIO_reset() on the read only BIO immediately
 before switching it back to read write. Before the BIO is freed it must be
 switched back to the read write mode.
@@ -114,13 +132,10 @@ preceding that write operation cannot be undone.
 Calling BIO_get_mem_ptr() prior to a BIO_reset() call with
 BIO_FLAGS_NONCLEAR_RST set has the same effect as a write operation.
 
-=head1 BUGS
-
-There should be an option to set the maximum size of a memory BIO.
-
 =head1 RETURN VALUES
 
-BIO_s_mem() and BIO_s_secmem() return a valid memory B<BIO_METHOD> structure.
+BIO_s_mem(), BIO_s_dgram_mem() and BIO_s_secmem() return a valid memory
+B<BIO_METHOD> structure.
 
 BIO_set_mem_eof_return(), BIO_set_mem_buf() and BIO_get_mem_ptr()
 return 1 on success or a value which is less than or equal to 0 if an error occurred.