BIO_S_MEM(3) | 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

library “libcrypto”
`#include <openssl/bio.h>`

const 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`(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.

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. To find out whether a memory BIO is read only, BIO_test_flags(3) can be called with an argument of `BIO_FLAGS_MEM_RDONLY`.

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`() 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.

`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_get_mem_ptr`() places the underlying BUF_MEM structure in *pp.

`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.

Writes to memory BIOs will always succeed if memory is available: their size can grow indefinitely.

BIO_ctrl(3) cmd arguments correspond to macros as follows:

cmd constant | corresponding macro
`BIO_C_GET_BUF_MEM_PTR` | `BIO_get_mem_ptr`()
`BIO_C_SET_BUF_MEM` | `BIO_set_mem_buf`()
`BIO_C_SET_BUF_MEM_EOF_RETURN` | `BIO_set_mem_eof_return`()
`BIO_CTRL_EOF` | BIO_eof(3)
`BIO_CTRL_GET_CLOSE` | BIO_get_close(3)
`BIO_CTRL_INFO` | `BIO_get_mem_data`()
`BIO_CTRL_PENDING` | BIO_pending(3)
`BIO_CTRL_RESET` | BIO_reset(3)
`BIO_CTRL_SET_CLOSE` | BIO_set_close(3)
`BIO_CTRL_WPENDING` | BIO_wpending(3)

# RETURN VALUES

`BIO_s_mem`() returns a pointer to a static object.

When called on a memory BIO object, BIO_method_type(3) returns the constant `BIO_TYPE_MEM` and BIO_method_name(3) returns a pointer to the static string "memory buffer".

`BIO_set_mem_eof_return`(), `BIO_get_mem_data`(), `BIO_set_mem_buf`(), and `BIO_get_mem_ptr`() return 1 on success or a value less than or equal to 0 if an error occurred.

`BIO_new_mem_buf`() returns a newly allocated BIO object 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;
    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);

# SEE ALSO

BIO_new(3), BUF_MEM_new(3)

# HISTORY

`BIO_s_mem`() first appeared in SSLeay 0.6.0. `BIO_set_mem_buf`() and `BIO_get_mem_ptr`() first appeared in SSLeay 0.6.5. These functions have been available since OpenBSD 2.4.

`BIO_set_mem_eof_return`() and `BIO_get_mem_data`() first appeared in SSLeay 0.9.1 and have been available since OpenBSD 2.6.

`BIO_new_mem_buf`() first appeared in OpenSSL 0.9.5 and has been available since OpenBSD 2.7.

# CAVEATS

Do not manually switch a writable memory BIO to read-only mode: calling BIO_set_flags(3) with an argument of `BIO_FLAGS_MEM_RDONLY` will ultimately result in a memory leak when the BIO object is finally handed to BIO_free(3). It might also cause security issues because it prevents BIO_reset(3) from clearing the data.

# 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.

June 8, 2025 | Debian