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Title "BIO_s_mem 3"
way too many mistakes in technical documents.
.nh
A memory \s-1BIO\s0 is a source/sink \s-1BIO\s0 which uses memory for its I/O. Data written to a memory \s-1BIO\s0 is stored in a \s-1BUF_MEM\s0 structure which is extended as appropriate to accommodate the stored data.
\fBBIO_s_secmem() is like BIO_s_mem() except that the secure heap is used for buffer storage.
Any data written to a memory \s-1BIO\s0 can be recalled by reading from it. Unless the memory \s-1BIO\s0 is read only any data read from it is deleted from the \s-1BIO.\s0
Memory BIOs support BIO_gets() and BIO_puts().
If the \s-1BIO_CLOSE\s0 flag is set when a memory \s-1BIO\s0 is freed then the underlying \s-1BUF_MEM\s0 structure is also freed.
Calling BIO_reset() on a read write memory \s-1BIO\s0 clears any data in it if the flag \s-1BIO_FLAGS_NONCLEAR_RST\s0 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 \s-1BIO\s0 it similarly restores the \s-1BIO\s0 to its original state and the read only data can be read again.
\fBBIO_eof() is true if no data is in the \s-1BIO.\s0
\fBBIO_ctrl_pending() returns the number of bytes currently stored.
\fBBIO_set_mem_eof_return() sets the behaviour of memory \s-1BIO\s0 b when it is empty. If the v is zero then an empty memory \s-1BIO\s0 will return \s-1EOF\s0 (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.
\fBBIO_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.
\fBBIO_set_mem_buf() sets the internal \s-1BUF_MEM\s0 structure to bm and sets the close flag to c, that is c should be either \s-1BIO_CLOSE\s0 or \s-1BIO_NOCLOSE.\s0 It is a macro.
\fBBIO_get_mem_ptr() places the underlying \s-1BUF_MEM\s0 structure in *pp. It is a macro.
\fBBIO_new_mem_buf() creates a memory \s-1BIO\s0 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 \s-1BIO\s0 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 \s-1BIO.\s0 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 \s-1BIO\s0 is freed.
Every write after partial read (not all data in the memory buffer was read) to a read write memory \s-1BIO\s0 will have to move the unread data with an internal copy operation, if a \s-1BIO\s0 contains a lot of data and it is read in small chunks intertwined with writes the operation can be very slow. Adding a buffering \s-1BIO\s0 to the chain can speed up the process.
Calling BIO_set_mem_buf() on a \s-1BIO\s0 created with BIO_new_secmem() will give undefined results, including perhaps a program crash.
Switching the memory \s-1BIO\s0 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 \s-1BIO\s0 creation and set the \s-1BIO\s0 as read only.
The other supported sequence is to start with read write \s-1BIO\s0 then temporarily switch it to read only and call BIO_reset() on the read only \s-1BIO\s0 immediately before switching it back to read write. Before the \s-1BIO\s0 is freed it must be switched back to the read write mode.
Calling BIO_get_mem_ptr() on read only \s-1BIO\s0 will return a \s-1BUF_MEM\s0 that contains only the remaining data to be read. If the close status of the \s-1BIO\s0 is set to \s-1BIO_NOCLOSE,\s0 before freeing the \s-1BUF_MEM\s0 the data pointer in it must be set to \s-1NULL\s0 as the data pointer does not point to an allocated memory.
Calling BIO_reset() on a read write memory \s-1BIO\s0 with \s-1BIO_FLAGS_NONCLEAR_RST\s0 flag set can have unexpected outcome when the reads and writes to the \s-1BIO\s0 are intertwined. As documented above the \s-1BIO\s0 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 \s-1BIO_FLAGS_NONCLEAR_RST\s0 set has the same effect as a write operation.
\fBBIO_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.
\fBBIO_get_mem_data() returns the total number of bytes available on success, 0 if b is \s-1NULL,\s0 or a negative value in case of other errors.
\fBBIO_new_mem_buf() returns a valid \s-1BIO\s0 structure on success or \s-1NULL\s0 on error.
.Vb 1 BIO *mem = BIO_new(BIO_s_mem()); \& BIO_puts(mem, "Hello World\en"); .Ve
Create a read only memory \s-1BIO:\s0
.Vb 2 char data[] = "Hello World"; BIO *mem = BIO_new_mem_buf(data, -1); .Ve
Extract the \s-1BUF_MEM\s0 structure from a memory \s-1BIO\s0 and then free up the \s-1BIO:\s0
.Vb 1 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); .Ve
Licensed under the OpenSSL license (the \*(L"License\*(R"). You may not use this file except in compliance with the License. You can obtain a copy in the file \s-1LICENSE\s0 in the source distribution or at <https://www.openssl.org/source/license.html>.
Indexes created Wed Apr 22 00:22:44 UTC 2026