BIO_read, BIO_write, BIO_gets, BIO_puts — BIO I/O functions
*b, void *buf, int len);
int BIO_gets(BIO *b,char *buf,
int BIO_write(BIO *b, const void *buf, int
int BIO_puts(BIO *b,const char *buf);
BIO_read() attempts to read len bytes
from BIO b and places the data in buf.
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 len.
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.
BIO_write() attempts to write len bytes
from buf to BIO b.
BIO_puts() attempts to write a null terminated string buf 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 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.
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.
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 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 possible to work around this by adding a buffering BIO BIO_f_buffer(3) to