Updated: 2021/Apr/14

BIO_s_mem(3)                        OpenSSL                       BIO_s_mem(3)

       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

       libcrypto, -lcrypto

        #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);

       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.

       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.

       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

       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.

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

       BIO_s_mem() and BIO_s_secmem() return a valid memory BIO_METHOD

       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

       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

        BUF_MEM *bptr;

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

       Copyright 2000-2019 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (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

1.1.1i                            2020-01-23                      BIO_s_mem(3)