BIO_read, BIO_write, BIO_gets, BIO_puts - BIO I/O functions
int len ); int BIO_gets(
int size ); int BIO_write(
const void *buf,
int len ); int BIO_puts(
const char *buf );
The BIO_read() function attempts to read len bytes from BIO b and
places the data in buf.
The BIO_gets() function 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 len. However, there are exceptions to
this. For example, BIO_gets() on a digest BIO will calculate and return
the digest, and other BIOs might not support BIO_gets().
The BIO_write() function attempts to write len bytes from buf to BIO b.
The BIO_puts() function attempts to write a null terminated string buf
to BIO b
A 0 or -1 return might indicate an error. However, when the source/sink
is non-blocking or of a certain type, it might be an indication that no
data is available and that the application should retry the operation
One technique sometimes used with blocking sockets is to use a system
call (such as select(), poll(), or equivalent) to determine when data
is available, and then call read() to read the data. The equivalent
with BIOs (that is, call select() 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() (or equivalent) should be combined with non‐
blocking I/O so successive reads will request a retry instead of block‐
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 possi‐
ble to work around this by adding a buffering BIO, BIO_f_buffer(), to
All these functions return either the amount of data successfully read
or written (if the return value is positive) or that no data was suc‐
cessfully 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.