libcrypto/man/BIO_s_mem.3

.\" $OpenBSD: BIO_s_mem.3,v 1.14 2019/06/06 01:06:58 schwarze Exp $
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.Dd $Mdocdate: June 6 2019 $
.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.
.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.
It is implemented as a macro.
.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 .
.Fn BIO_set_mem_buf
is a macro.
.Pp
.Fn BIO_get_mem_ptr
places the underlying
.Vt BUF_MEM
structure in
.Pf * Fa pp .
It is a macro.
.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
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.
.Sh RETURN VALUES
.Fn BIO_s_mem
returns a pointer to a static object.
.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 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.
.Pp
The copying operation should not occur after every small read
of a large BIO to improve efficiency.