BIO_s_mem.3

.\" $OpenBSD: BIO_s_mem.3,v 1.19 2023/11/16 20:19:23 schwarze Exp $
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.\" This file was written by Dr. Stephen Henson <steve@openssl.org>.
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.Dd $Mdocdate: November 16 2023 $
.Dt BIO_S_MEM 3
.Os
.Sh NAME
.Nm BIO_s_mem ,
.Nm BIO_set_mem_eof_return ,
.Nm BIO_get_mem_data ,
.Nm BIO_set_mem_buf ,
.Nm BIO_get_mem_ptr ,
.Nm BIO_new_mem_buf
.Nd memory BIO
.Sh SYNOPSIS
.In openssl/bio.h
.Ft const BIO_METHOD *
.Fo BIO_s_mem
.Fa "void"
.Fc
.Ft long
.Fo BIO_set_mem_eof_return
.Fa "BIO *b"
.Fa "int v"
.Fc
.Ft long
.Fo BIO_get_mem_data
.Fa "BIO *b"
.Fa "char **pp"
.Fc
.Ft long
.Fo BIO_set_mem_buf
.Fa "BIO *b"
.Fa "BUF_MEM *bm"
.Fa "int c"
.Fc
.Ft long
.Fo BIO_get_mem_ptr
.Fa "BIO *b"
.Fa "BUF_MEM **pp"
.Fc
.Ft BIO *
.Fo BIO_new_mem_buf
.Fa "const void *buf"
.Fa "int len"
.Fc
.Sh DESCRIPTION
.Fn BIO_s_mem
returns the memory BIO method function.
.Pp
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
.Vt BUF_MEM
structure which is extended as appropriate to accommodate the stored data.
.Pp
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.
To find out whether a memory BIO is read only,
.Xr BIO_test_flags 3
can be called with an argument of
.Dv BIO_FLAGS_MEM_RDONLY .
.Pp
Memory BIOs support
.Xr BIO_gets 3
and
.Xr BIO_puts 3 .
.Pp
If the
.Dv BIO_CLOSE
flag is set when a memory BIO is freed, the underlying
.Dv BUF_MEM
structure is also freed.
.Pp
Calling
.Xr BIO_reset 3
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.
.Pp
.Xr BIO_eof 3
is true if no data is in the BIO.
.Pp
.Xr BIO_ctrl_pending 3
returns the number of bytes currently stored.
.Pp
.Fn BIO_set_mem_eof_return
sets the behaviour of memory BIO
.Fa b
when it is empty.
If
.Fa v
is zero, then an empty memory BIO will return EOF:
it will return zero and
.Fn BIO_should_retry
will be false.
If
.Fa v
is non-zero then it will return
.Fa v
when it is empty and it will set the read retry flag:
.Fn BIO_read_retry
is true.
To avoid ambiguity with a normal positive return value
.Fa v
should be set to a negative value, typically -1.
.Pp
.Fn BIO_get_mem_data
sets
.Pf * Fa pp
to a pointer to the start of the memory BIO's data
and returns the total amount of data available.
.Pp
.Fn BIO_set_mem_buf
sets the internal BUF_MEM structure to
.Fa bm
and sets the close flag to
.Fa c .
That is,
.Fa c
should be either
.Dv BIO_CLOSE
or
.Dv BIO_NOCLOSE .
.Pp
.Fn BIO_get_mem_ptr
places the underlying
.Vt BUF_MEM
structure in
.Pf * Fa pp .
.Pp
.Fn BIO_new_mem_buf
creates a memory BIO using
.Fa len
bytes of data at
.Fa buf .
If
.Fa len
is -1, then
.Fa buf
is assumed to be NUL terminated and its length is determined by
.Xr strlen 3 .
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
.Em not
copied first, so the supplied area of memory must be unchanged
until the BIO is freed.
.Pp
Writes to memory BIOs will always succeed if memory is available:
their size can grow indefinitely.
.Pp
.Xr BIO_ctrl 3
.Fa cmd
arguments correspond to macros as follows:
.Bl -column BIO_C_SET_BUF_MEM_EOF_RETURN BIO_set_mem_eof_return() -offset 3n
.It Fa cmd No constant              Ta corresponding macro
.It Dv BIO_C_GET_BUF_MEM_PTR        Ta Fn BIO_get_mem_ptr
.It Dv BIO_C_SET_BUF_MEM            Ta Fn BIO_set_mem_buf
.It Dv BIO_C_SET_BUF_MEM_EOF_RETURN Ta Fn BIO_set_mem_eof_return
.It Dv BIO_CTRL_EOF                 Ta Xr BIO_eof 3
.It Dv BIO_CTRL_GET_CLOSE           Ta Xr BIO_get_close 3
.It Dv BIO_CTRL_INFO                Ta Fn BIO_get_mem_data
.It Dv BIO_CTRL_PENDING             Ta Xr BIO_pending 3
.It Dv BIO_CTRL_RESET               Ta Xr BIO_reset 3
.It Dv BIO_CTRL_SET_CLOSE           Ta Xr BIO_set_close 3
.It Dv BIO_CTRL_WPENDING            Ta Xr BIO_wpending 3
.El
.Sh RETURN VALUES
.Fn BIO_s_mem
returns a pointer to a static object.
.Pp
When called on a memory BIO object,
.Xr BIO_method_type 3
returns the constant
.Dv BIO_TYPE_MEM
and
.Xr BIO_method_name 3
returns a pointer to the static string
.Qq memory buffer .
.Pp
.Fn BIO_set_mem_eof_return ,
.Fn BIO_get_mem_data ,
.Fn BIO_set_mem_buf ,
and
.Fn BIO_get_mem_ptr
return 1 on success or a value less than or equal to 0 if an error occurred.
.Pp
.Fn BIO_new_mem_buf
returns a newly allocated
.Vt BIO
object on success or
.Dv NULL
on error.
.Sh EXAMPLES
Create a memory BIO and write some data to it:
.Bd -literal -offset indent
BIO *mem = BIO_new(BIO_s_mem());
BIO_puts(mem, "Hello World\en");
.Ed
.Pp
Create a read only memory BIO:
.Bd -literal -offset indent
char data[] = "Hello World";
BIO *mem;
mem = BIO_new_mem_buf(data, -1);
.Ed
.Pp
Extract the
.Vt BUF_MEM
structure from a memory BIO and then free up the BIO:
.Bd -literal -offset indent
BUF_MEM *bptr;
BIO_get_mem_ptr(mem, &bptr);
/* Make sure BIO_free() leaves BUF_MEM alone. */
BIO_set_close(mem, BIO_NOCLOSE);
BIO_free(mem);
.Ed
.Sh SEE ALSO
.Xr BIO_new 3 ,
.Xr BUF_MEM_new 3
.Sh HISTORY
.Fn BIO_s_mem
first appeared in SSLeay 0.6.0.
.Fn BIO_set_mem_buf
and
.Fn BIO_get_mem_ptr
first appeared in SSLeay 0.6.5.
These functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_set_mem_eof_return
and
.Fn BIO_get_mem_data
first appeared in SSLeay 0.9.1 and have been available since
.Ox 2.6 .
.Pp
.Fn BIO_new_mem_buf
first appeared in OpenSSL 0.9.5 and has been available since
.Ox 2.7 .
.Sh CAVEATS
Do not manually switch a writable memory BIO to read-only mode: calling
.Xr BIO_set_flags 3
with an argument of
.Dv BIO_FLAGS_MEM_RDONLY
will ultimately result in a memory leak when the BIO object is
finally handed to
.Xr BIO_free 3 .
It might also cause security issues because it prevents
.Xr BIO_reset 3
from clearing the data.
.Sh BUGS
There should be an option to set the maximum size of a memory BIO.
.Pp
There should be a way to "rewind" a read/write BIO without destroying
its contents.