BIO_S_MEM(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

🧠💾 BIO_S_MEM(3) 💾🧠

okay so bestie, this little icon is from the Library Functions Manual and it is literally all about the memory BIO. grab a strawberry 🍓 and let’s get into it.

✨ 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. iconic.

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. it’s giving “infinite lil storage era” ✨

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. lowkey rewind magic 🪄

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. no cap, do not touch that buffer until the BIO is gone 🫶

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:

🎀 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_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_get_mem_data() returns the number of bytes available for reading at *ptr on success, or a value less than or equal to zero if an error occurs. Error conditions include that b is NULL, no data is available, or any error of BIO_ctrl(3), including error returns of a callback called with a BIO_CB_CTRL or BIO_CB_CTRL|BIO_CB_RETURN operation.
  • 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 ⚠️

bestie, 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. like… do not do it. 💀

🙈 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.

served fresh by Debian on March 10, 2026 — page BIO_S_MEM(3) 💅✨

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