BIO_F_BUFFER(3) | Library Functions Manual | BIO_F_BUFFER(3)

# NAME

`BIO_f_buffer`, `BIO_get_buffer_num_lines`, `BIO_set_read_buffer_size`, `BIO_set_write_buffer_size`, `BIO_set_buffer_size`, `BIO_set_buffer_read_data` — buffering BIO

# SYNOPSIS

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

const BIO_METHOD *
`BIO_f_buffer`(void);

long
`BIO_get_buffer_num_lines`(BIO *b);

long
`BIO_set_read_buffer_size`(BIO *b, long size);

long
`BIO_set_write_buffer_size`(BIO *b, long size);

long
`BIO_set_buffer_size`(BIO *b, long size);

long
`BIO_set_buffer_read_data`(BIO *b, void *buf, long num);

# DESCRIPTION

`BIO_f_buffer`() returns the buffering BIO method.

Data written to a buffering BIO is buffered and periodically written to the next BIO in the chain. Data read from a buffering BIO comes from an internal buffer which is filled from the next BIO in the chain. Both BIO_gets(3) and BIO_puts(3) are supported.

Calling BIO_reset(3) on a buffering BIO clears any buffered data.

`BIO_get_buffer_num_lines`() returns the number of lines currently buffered.

`BIO_set_read_buffer_size`(), `BIO_set_write_buffer_size`(), and `BIO_set_buffer_size`() set the read, write or both read and write buffer sizes to size. The initial buffer size is `DEFAULT_BUFFER_SIZE`, currently 4096. Any attempt to reduce the buffer size below `DEFAULT_BUFFER_SIZE` is ignored. Any buffered data is cleared when the buffer is resized.

`BIO_set_buffer_read_data`() clears the read buffer and fills it with num bytes of buf. If num is larger than the current buffer size, the buffer is expanded.

Buffering BIOs implement BIO_gets(3) by using BIO_read(3) operations on the next BIO in the chain. By prepending a buffering BIO to a chain it is therefore possible to provide the functionality of BIO_gets(3) if the following BIOs do not support it (for example SSL BIOs).

Data is only written to the next BIO in the chain when the write buffer fills or when BIO_flush(3) is called. It is therefore important to call BIO_flush(3) whenever any pending data should be written such as when removing a buffering BIO using BIO_pop(3). BIO_flush(3) may need to be retried if the ultimate source/sink BIO is non-blocking.

When a chain containing a buffering BIO is copied with BIO_dup_chain(3), `BIO_set_read_buffer_size`() and `BIO_set_write_buffer_size`() are called internally to automatically copy both buffer sizes from the original BIO object to the new one.

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

cmd constant | corresponding macro
`BIO_C_GET_BUFF_NUM_LINES` | `BIO_get_buffer_num_lines`()
`BIO_C_SET_BUFF_READ_DATA` | `BIO_set_buffer_read_data`()
`BIO_C_SET_BUFF_SIZE` | `BIO_set_buffer_size`()
`BIO_CTRL_FLUSH` | BIO_flush(3)
`BIO_CTRL_PENDING` | BIO_pending(3)
`BIO_CTRL_RESET` | BIO_reset(3)
`BIO_CTRL_WPENDING` | BIO_wpending(3)

The cmd constant `BIO_C_SET_BUFF_SIZE` is special. It is also used for BIO_int_ctrl(3) with the following iarg arguments:

cmd constant | iarg | corresponding macro
`BIO_C_SET_BUFF_SIZE` | 0 | `BIO_set_read_buffer_size`()
| 1 | `BIO_set_write_buffer_size`()

# RETURN VALUES

`BIO_f_buffer`() returns the buffering BIO method.

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

`BIO_get_buffer_num_lines`() returns the number of lines buffered (may be 0).

`BIO_set_read_buffer_size`(), `BIO_set_write_buffer_size`(), and `BIO_set_buffer_size`() return 1 if the buffer was successfully resized or 0 for failure.

`BIO_set_buffer_read_data`() returns 1 if the data was set correctly or 0 if there was an error.

# SEE ALSO

BIO_ctrl(3), BIO_flush(3), BIO_new(3), BIO_pop(3), BIO_reset(3)

# HISTORY

`BIO_f_buffer`() first appeared in SSLeay 0.6.0. `BIO_get_buffer_num_lines`() and `BIO_set_buffer_size`() first appeared in SSLeay 0.6.5. `BIO_set_read_buffer_size`() and `BIO_set_write_buffer_size`() first appeared in SSLeay 0.8.0. `BIO_set_buffer_read_data`() first appeared in SSLeay 0.9.0. All these functions have been available since OpenBSD 2.4.

June 8, 2025 | Debian