Updated: 2022/Sep/29
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BIO_s_mem(3) OpenSSL BIO_s_mem(3)
NAME
BIO_s_secmem, 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
LIBRARY
libcrypto, -lcrypto
SYNOPSIS
#include <openssl/bio.h>
const BIO_METHOD *BIO_s_mem(void);
const BIO_METHOD *BIO_s_secmem(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);
DESCRIPTION
BIO_s_mem() returns 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.
BIO_s_secmem() is like BIO_s_mem() except that the secure heap is used
for buffer storage.
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 if
the flag BIO_FLAGS_NONCLEAR_RST is not set, otherwise it just restores
the read pointer to the state it was just after the last write was
performed and the data can be read again. On a read only BIO it
similarly 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 when it is
empty. If the 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 v is
non zero then it will return 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 v should be set to a negative
value, typically -1.
BIO_get_mem_data() sets *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. Note the pointer returned by this call is
informative, no transfer of ownership of this memory is implied. See
notes on BIO_set_close().
BIO_set_mem_buf() sets the internal BUF_MEM structure to bm and sets
the close flag to c, that is c should be either BIO_CLOSE or
BIO_NOCLOSE. It is a macro.
BIO_get_mem_ptr() places the underlying BUF_MEM structure in *pp. It is
a macro.
BIO_new_mem_buf() creates a memory BIO using len bytes of data at buf,
if len is -1 then the buf is assumed to be nul terminated and its
length is determined by 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 not
copied first, so the supplied area of memory must be unchanged until
the BIO is freed.
NOTES
Writes to memory BIOs will always succeed if memory is available: that
is their size can grow indefinitely.
Every write after partial read (not all data in the memory buffer was
read) to a read write memory BIO will have to move the unread data with
an internal 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.
Switching the 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 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.
Calling BIO_get_mem_ptr() on read only BIO will return a BUF_MEM that
contains only the remaining data to be read. If the close status of the
BIO is set to BIO_NOCLOSE, before freeing the BUF_MEM the data pointer
in it must be set to NULL as the data pointer does not point to an
allocated memory.
Calling BIO_reset() on a read write memory BIO with
BIO_FLAGS_NONCLEAR_RST flag set can have unexpected outcome when the
reads and writes to the BIO are intertwined. As documented above the
BIO will be reset to the state after the last completed write
operation. The effects of reads 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.
Calling BIO_set_close() with BIO_NOCLOSE orphans the BUF_MEM internal
to the BIO, _not_ its actual data buffer. See the examples section for
the proper method for claiming ownership of the data pointer for a
deferred free operation.
BUGS
There should be an option to set the maximum size of a memory BIO.
RETURN VALUES
BIO_s_mem() and BIO_s_secmem() return a valid memory 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.
BIO_get_mem_data() returns the total number of bytes available on
success, 0 if b is NULL, or a negative value in case of other errors.
BIO_new_mem_buf() returns a valid BIO structure on success or NULL on
error.
EXAMPLES
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 = 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);
Extract the BUF_MEM ptr, claim ownership of the internal data and free
the BIO and BUF_MEM structure:
BUF_MEM *bptr;
char *data;
BIO_get_mem_data(bio, &data);
BIO_get_mem_ptr(bio, &bptr);
BIO_set_close(mem, BIO_NOCLOSE); /* So BIO_free orphans BUF_MEM */
BIO_free(bio);
bptr->data = NULL; /* Tell BUF_MEM to orphan data */
BUF_MEM_free(bptr);
...
free(data);
COPYRIGHT
Copyright 2000-2023 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (the "License"). You may not use
this file except in compliance with the License. You can obtain a copy
in the file LICENSE in the source distribution or at
<https://www.openssl.org/source/license.html>.
3.0.12 2023-10-25 BIO_s_mem(3)