libcrypto/man/BIO_read.3

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.Dd $Mdocdate: December 18 2022 $
.Dt BIO_READ 3
.Os
.Sh NAME
.Nm BIO_read ,
.Nm BIO_number_read ,
.Nm BIO_gets ,
.Nm BIO_write ,
.Nm BIO_puts ,
.Nm BIO_indent ,
.Nm BIO_number_written
.Nd BIO I/O functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft int
.Fo BIO_read
.Fa "BIO *b"
.Fa "void *buf"
.Fa "int len"
.Fc
.Ft unsigned long
.Fo BIO_number_read
.Fa "BIO *b"
.Fc
.Ft int
.Fo BIO_gets
.Fa "BIO *b"
.Fa "char *buf"
.Fa "int size"
.Fc
.Ft int
.Fo BIO_write
.Fa "BIO *b"
.Fa "const void *buf"
.Fa "int len"
.Fc
.Ft int
.Fo BIO_puts
.Fa "BIO *b"
.Fa "const char *string"
.Fc
.Ft int
.Fo BIO_indent
.Fa "BIO *b"
.Fa "int indent"
.Fa "int max"
.Fc
.Ft unsigned long
.Fo BIO_number_written
.Fa "BIO *b"
.Fc
.Sh DESCRIPTION
.Fn BIO_read
attempts to read
.Fa len
bytes from
.Fa b
and places the data in
.Fa buf .
.Pp
.Fn BIO_number_read
returns the grand total of bytes read from
.Fa b
using
.Fn BIO_read
so far.
Bytes read with
.Fn BIO_gets
do
.Sy not
count.
.Xr BIO_new 3
and
.Xr BIO_set 3
initialize the counter to 0.
When reading very large amounts of data,
the counter will eventually wrap around from
.Dv ULONG_MAX
to 0.
.Pp
.Fn BIO_gets
performs the BIOs "gets" operation and places the data in
.Fa buf .
Usually this operation will attempt to read a line of data
from the BIO of maximum length
.Fa size No \- 1 .
There are exceptions to this however, for example
.Fn BIO_gets
on a digest BIO will calculate and return the digest
and other BIOs may not support
.Fn BIO_gets
at all.
The returned string is always NUL-terminated.
.Pp
.Fn BIO_write
attempts to write
.Fa len
bytes from
.Fa buf
to
.Fa b .
.Pp
.Fn BIO_puts
attempts to write the NUL-terminated
.Fa string
to
.Fa b .
.Pp
.Fn BIO_indent
attempts to write
.Fa indent
space characters to
.Fa b ,
but not more than
.Fa max
characters.
.Pp
.Fn BIO_number_written
returns the grand total of bytes written to
.Fa b
using
.Fn BIO_write ,
.Fn BIO_puts ,
and
.Fn BIO_indent
so far.
.Xr BIO_new 3
and
.Xr BIO_set 3
initialize the counter to 0.
When writing very large amounts of data,
the counter will eventually wrap around from
.Dv ULONG_MAX
to 0.
.Pp
One technique sometimes used with blocking sockets
is to use a system call (such as
.Xr select 2 ,
.Xr poll 2
or equivalent) to determine when data is available and then call
.Xr read 2
to read the data.
The equivalent with BIOs (that is call
.Xr select 2
on the underlying I/O structure and then call
.Fn BIO_read
to read the data) should
.Em not
be used because a single call to
.Fn 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
.Xr select 2
(or equivalent) should be combined with non-blocking I/O
so successive reads will request a retry instead of blocking.
.Pp
See
.Xr BIO_should_retry 3
for details of how to determine the cause of a retry and other I/O issues.
.Pp
If the
.Fn BIO_gets
function is not supported by a BIO then it is possible to
work around this by adding a buffering BIO
.Xr BIO_f_buffer 3
to the chain.
.Sh RETURN VALUES
.Fn BIO_indent
returns 1 if successful, even if nothing was written,
or 0 if writing fails.
.Pp
.Fn BIO_number_read
and
.Fn BIO_number_written
return a number of bytes or 0 if
.Fa b
is a
.Dv NULL
pointer.
.Pp
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
.Fn BIO_gets .
.Pp
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.
.Sh SEE ALSO
.Xr BIO_meth_new 3 ,
.Xr BIO_new 3 ,
.Xr BIO_should_retry 3
.Sh HISTORY
.Fn BIO_read ,
.Fn BIO_gets ,
.Fn BIO_write ,
and
.Fn BIO_puts
first appeared in SSLeay 0.6.0.
.Fn BIO_number_read
and
.Fn BIO_number_written
first appeared in SSLeay 0.6.5.
These functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_indent
first appeared in OpenSSL 0.9.7 and has been available since
.Ox 3.4 .