BIO_S_MEM(3) BSD Library Functions Manual BIO_S_MEM(3)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
SYNOPSIS
#include <openssl/bio.h>
BIO_METHOD *
BIO_s_mem(void);
long
BIO_set_mem_eof_return(BIO *b, int v);
long
BIO_get_mem_data(BIO *b, char **pp);
long
BIO_set_mem_buf(BIO *b, BUF_MEM *bm, int c);
long
BIO_get_mem_ptr(BIO *b, BUF_MEM **pp);
BIO *
BIO_new_mem_buf(void *buf, int len);
DESCRIPTIONBIO_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.
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(3) and BIO_puts(3).
If the BIO_CLOSE flag is set when a memory BIO is freed, the underlying
BUF_MEM structure is also freed.
Calling 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.
BIO_eof(3) is true if no data is in the BIO.
BIO_ctrl_pending(3) returns the number of bytes currently stored.
BIO_set_mem_eof_return(3) sets the behaviour of memory BIO b when it is
empty. If v is zero, then an empty memory BIO will return EOF: It will
return zero and BIO_should_retry() 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:
BIO_read_retry() 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 BIO's
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 bm and sets the
close flag to c, that is c should be either BIO_CLOSE or BIO_NOCLOSE.
BIO_set_mem_buf() 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 buf is assumed to be NUL terminated and its length is
determined by 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 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: 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.
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;
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);
/* Make sure BIO_free() leaves BUF_MEM alone. */
BIO_set_close(mem, BIO_NOCLOSE);
BIO_free(mem);
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.
BSD June 8, 2024 BSD