BIO_READ(3) | Library Functions Manual | BIO_READ(3)

# NAME

`BIO_read`, `BIO_number_read`, `BIO_gets`, `BIO_write`, `BIO_puts`, `BIO_indent`, `BIO_number_written` — BIO I/O functions

# SYNOPSIS

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

int
`BIO_read`(BIO *b, void *buf, int len);

unsigned long
`BIO_number_read`(BIO *b);

int
`BIO_gets`(BIO *b, char *buf, int size);

int
`BIO_write`(BIO *b, const void *buf, int len);

int
`BIO_puts`(BIO *b, const char *string);

int
`BIO_indent`(BIO *b, int indent, int max);

unsigned long
`BIO_number_written`(BIO *b);

# DESCRIPTION

`BIO_read`() attempts to read len bytes from b and places the data in buf.

`BIO_number_read`() returns the grand total of bytes read from b using `BIO_read`() so far. Bytes read with `BIO_gets`() do not count. BIO_new(3) and BIO_set(3) initialize the counter to 0. When reading very large amounts of data, the counter will eventually wrap around from `ULONG_MAX` to 0.

`BIO_gets`() performs the BIOs "gets" operation and places the data in buf. Usually this operation will attempt to read a line of data from the BIO of maximum length size - 1. There are exceptions to this however, for example `BIO_gets`() on a digest BIO will calculate and return the digest and other BIOs may not support `BIO_gets`() at all. The returned string is always NUL-terminated.

`BIO_write`() attempts to write len bytes from buf to b.

`BIO_puts`() attempts to write the NUL-terminated string to b.

`BIO_indent`() attempts to write indent space characters to b, but not more than max characters.

`BIO_number_written`() returns the grand total of bytes written to b using `BIO_write`(), `BIO_puts`(), and `BIO_indent`() so far. BIO_new(3) and BIO_set(3) initialize the counter to 0. When writing very large amounts of data, the counter will eventually wrap around from `ULONG_MAX` to 0.

One technique sometimes used with blocking sockets is to use a system call (such as select(2), poll(2) or equivalent) to determine when data is available and then call read(2) to read the data. The equivalent with BIOs (that is call select(2) on the underlying I/O structure and then call `BIO_read`() to read the data) should not be used because a single call to `BIO_read`() can cause several reads (and writes in the case of SSL BIOs) on the underlying I/O structure and may block as a result. Instead select(2) (or equivalent) should be combined with non-blocking I/O so successive reads will request a retry instead of blocking.

See BIO_should_retry(3) for details of how to determine the cause of a retry and other I/O issues.

If the `BIO_gets`() function is not supported by a BIO then it is possible to work around this by adding a buffering BIO BIO_f_buffer(3) to the chain.

# RETURN VALUES

`BIO_indent`() returns 1 if successful, even if nothing was written, or 0 if writing fails.

`BIO_number_read`() and `BIO_number_written`() return a number of bytes or 0 if b is a `NULL` pointer.

The other functions return either the amount of data successfully read or written (if the return value is positive) or that no data was successfully read or written if the result is 0 or -1. If the return value is -2, then the operation is not implemented in the specific BIO type. The trailing NUL is not included in the length returned by `BIO_gets`().

A 0 or -1 return is not necessarily an indication of an error. In particular when the source/sink is non-blocking or of a certain type it may merely be an indication that no data is currently available and that the application should retry the operation later.

# SEE ALSO

BIO_meth_new(3), BIO_new(3), BIO_should_retry(3)

# HISTORY

`BIO_read`(), `BIO_gets`(), `BIO_write`(), and `BIO_puts`() first appeared in SSLeay 0.6.0. `BIO_number_read`() and `BIO_number_written`() first appeared in SSLeay 0.6.5. These functions have been available since OpenBSD 2.4.

`BIO_indent`() first appeared in OpenSSL 0.9.7 and has been available since OpenBSD 3.4.

June 8, 2025 | Debian