xref: /freebsd-11-stable/crypto/openssl/doc/crypto/BIO_s_mem.pod

=pod

=head1 NAME

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

=head1 SYNOPSIS

 #include <openssl/bio.h>

 BIO_METHOD *	BIO_s_mem(void);

 BIO_set_mem_eof_return(BIO *b,int v)
 long BIO_get_mem_data(BIO *b, char **pp)
 BIO_set_mem_buf(BIO *b,BUF_MEM *bm,int c)
 BIO_get_mem_ptr(BIO *b,BUF_MEM **pp)

 BIO *BIO_new_mem_buf(const void *buf, int len);

=head1 DESCRIPTION

BIO_s_mem() return the memory BIO method function. 

A memory BIO is a source/sink BIO which uses memory for its I/O. Data
written to a memory BIO is stored in a BUF_MEM structure which is extended
as appropriate to accommodate the stored data.

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.

Memory BIOs support BIO_gets() and BIO_puts().

If the BIO_CLOSE flag is set when a memory BIO is freed then the underlying
BUF_MEM structure is also freed.

Calling BIO_reset() on a read write memory BIO clears any data in it. On a
read only BIO it restores the BIO to its original state and the read only
data can be read again.

BIO_eof() is true if no data is in the BIO.

BIO_ctrl_pending() returns the number of bytes currently stored.

BIO_set_mem_eof_return() sets the behaviour of memory BIO B<b> when it is
empty. If the B<v> is zero then an empty memory BIO will return EOF (that is
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.

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.

BIO_set_mem_buf() sets the internal BUF_MEM structure to B<bm> and sets the
close flag to B<c>, that is B<c> should be either BIO_CLOSE or BIO_NOCLOSE.
It is a macro.

BIO_get_mem_ptr() places the underlying BUF_MEM structure in *B<pp>. It is
a macro.

BIO_new_mem_buf() creates a memory BIO using B<len> bytes of data at B<buf>,
if B<len> is -1 then the B<buf> is assumed to be nul terminated and its
length is determined by B<strlen>. The BIO is set to a read only state and
as a result cannot be written to. This is useful when some data needs to be
made available from a static area of memory in the form of a BIO. The
supplied data is read directly from the supplied buffer: it is B<not> copied
first, so the supplied area of memory must be unchanged until the BIO is freed.

=head1 NOTES

Writes to memory BIOs will always succeed if memory is available: that is
their size can grow indefinitely.

Every read from a read write memory BIO will remove the data just read with
an internal copy operation, if a BIO contains a lot of data and it is
read in small chunks the operation can be very slow. The use of a read only
memory BIO avoids this problem. If the BIO must be read write then adding
a buffering BIO to the chain will speed up the process.

=head1 BUGS

There should be an option to set the maximum size of a memory BIO.

There should be a way to "rewind" a read write BIO without destroying
its contents.

The copying operation should not occur after every small read of a large BIO
to improve efficiency.

=head1 EXAMPLE

Create a memory BIO and write some data to it:

 BIO *mem = BIO_new(BIO_s_mem());
 BIO_puts(mem, "Hello World\n"); 

Create a read only memory BIO:

 char data[] = "Hello World";
 BIO *mem;
 mem = BIO_new_mem_buf(data, -1);

Extract the BUF_MEM structure from a memory BIO and then free up the BIO:

 BUF_MEM *bptr;
 BIO_get_mem_ptr(mem, &bptr);
 BIO_set_close(mem, BIO_NOCLOSE); /* So BIO_free() leaves BUF_MEM alone */
 BIO_free(mem);
 

=head1 SEE ALSO

TBA