sbuf(9): Add NOWAIT dynamic buffer extension mode

[FreeBSD/FreeBSD.git] / share / man / man9 / sbuf.9

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.Dd July 12, 2021
.Dt SBUF 9
.Os
.Sh NAME
.Nm sbuf ,
.Nm sbuf_new ,
.Nm sbuf_new_auto ,
.Nm sbuf_new_for_sysctl ,
.Nm sbuf_clear ,
.Nm sbuf_get_flags ,
.Nm sbuf_set_flags ,
.Nm sbuf_clear_flags ,
.Nm sbuf_setpos ,
.Nm sbuf_bcat ,
.Nm sbuf_bcopyin ,
.Nm sbuf_bcpy ,
.Nm sbuf_cat ,
.Nm sbuf_copyin ,
.Nm sbuf_cpy ,
.Nm sbuf_printf ,
.Nm sbuf_vprintf ,
.Nm sbuf_putc ,
.Nm sbuf_set_drain ,
.Nm sbuf_trim ,
.Nm sbuf_error ,
.Nm sbuf_finish ,
.Nm sbuf_data ,
.Nm sbuf_len ,
.Nm sbuf_done ,
.Nm sbuf_delete ,
.Nm sbuf_start_section ,
.Nm sbuf_end_section ,
.Nm sbuf_hexdump ,
.Nm sbuf_putbuf
.Nd safe string composition
.Sh SYNOPSIS
.In sys/types.h
.In sys/sbuf.h
.Ft typedef\ int ( sbuf_drain_func ) ( void\ *arg, const\ char\ *data, int\ len ) ;
.Pp
.Ft struct sbuf *
.Fo sbuf_new
.Fa "struct sbuf *s"
.Fa "char *buf"
.Fa "int length"
.Fa "int flags"
.Fc
.Ft struct sbuf *
.Fo sbuf_new_auto
.Fa "void"
.Fc
.Ft void
.Fo sbuf_clear
.Fa "struct sbuf *s"
.Fc
.Ft int
.Fo sbuf_get_flags
.Fa "struct sbuf *s"
.Fc
.Ft void
.Fo sbuf_set_flags
.Fa "struct sbuf *s"
.Fa "int flags"
.Fc
.Ft void
.Fo sbuf_clear_flags
.Fa "struct sbuf *s"
.Fa "int flags"
.Fc
.Ft int
.Fo sbuf_setpos
.Fa "struct sbuf *s"
.Fa "int pos"
.Fc
.Ft int
.Fo sbuf_bcat
.Fa "struct sbuf *s"
.Fa "const void *buf"
.Fa "size_t len"
.Fc
.Ft int
.Fo sbuf_bcpy
.Fa "struct sbuf *s"
.Fa "const void *buf"
.Fa "size_t len"
.Fc
.Ft int
.Fo sbuf_cat
.Fa "struct sbuf *s"
.Fa "const char *str"
.Fc
.Ft int
.Fo sbuf_cpy
.Fa "struct sbuf *s"
.Fa "const char *str"
.Fc
.Ft int
.Fo sbuf_printf
.Fa "struct sbuf *s"
.Fa "const char *fmt" "..."
.Fc
.Ft int
.Fo sbuf_vprintf
.Fa "struct sbuf *s"
.Fa "const char *fmt"
.Fa "va_list ap"
.Fc
.Ft int
.Fo sbuf_putc
.Fa "struct sbuf *s"
.Fa "int c"
.Fc
.Ft void
.Fo sbuf_set_drain
.Fa "struct sbuf *s"
.Fa "sbuf_drain_func *func"
.Fa "void *arg"
.Fc
.Ft int
.Fo sbuf_trim
.Fa "struct sbuf *s"
.Fc
.Ft int
.Fo sbuf_error
.Fa "struct sbuf *s"
.Fc
.Ft int
.Fo sbuf_finish
.Fa "struct sbuf *s"
.Fc
.Ft char *
.Fo sbuf_data
.Fa "struct sbuf *s"
.Fc
.Ft ssize_t
.Fo sbuf_len
.Fa "struct sbuf *s"
.Fc
.Ft int
.Fo sbuf_done
.Fa "struct sbuf *s"
.Fc
.Ft void
.Fo sbuf_delete
.Fa "struct sbuf *s"
.Fc
.Ft void
.Fo sbuf_start_section
.Fa "struct sbuf *s"
.Fa "ssize_t *old_lenp"
.Fc
.Ft ssize_t
.Fo sbuf_end_section
.Fa "struct sbuf *s"
.Fa "ssize_t old_len"
.Fa "size_t pad"
.Fa "int c"
.Fc
.Ft void
.Fo sbuf_hexdump
.Fa "struct sbuf *sb"
.Fa "void *ptr"
.Fa "int length"
.Fa "const char *hdr"
.Fa "int flags"
.Fc
.Ft void
.Fo sbuf_putbuf
.Fa "struct sbuf *s"
.Fc
.Fd #ifdef _KERNEL
.In sys/types.h
.In sys/sbuf.h
.Ft int
.Fo sbuf_bcopyin
.Fa "struct sbuf *s"
.Fa "const void *uaddr"
.Fa "size_t len"
.Fc
.Ft int
.Fo sbuf_copyin
.Fa "struct sbuf *s"
.Fa "const void *uaddr"
.Fa "size_t len"
.Fc
.In sys/sysctl.h
.Ft struct sbuf *
.Fo sbuf_new_for_sysctl
.Fa "struct sbuf *s"
.Fa "char *buf"
.Fa "int length"
.Fa "struct sysctl_req *req"
.Fc
.Fd #endif      /* _KERNEL */
.Sh DESCRIPTION
The
.Nm
family of functions allows one to safely allocate, compose and
release strings in kernel or user space.
.Pp
Instead of arrays of characters, these functions operate on structures
called
.Fa sbufs ,
defined in
.In sys/sbuf.h .
.Pp
Any errors encountered during the allocation or composition of the
string will be latched in the data structure,
making a single error test at the end of the composition
sufficient to determine success or failure of the entire process.
.Pp
The
.Fn sbuf_new
function initializes the
.Fa sbuf
pointed to by its first argument.
If that pointer is
.Dv NULL ,
.Fn sbuf_new
allocates a
.Vt struct sbuf
using
.Xr malloc 9 .
The
.Fa buf
argument is a pointer to a buffer in which to store the actual string;
if it is
.Dv NULL ,
.Fn sbuf_new
will allocate one using
.Xr malloc 9 .
The
.Fa length
is the initial size of the storage buffer.
The fourth argument,
.Fa flags ,
may be comprised of the following flags:
.Bl -tag -width ".Dv SBUF_AUTOEXTEND"
.It Dv SBUF_FIXEDLEN
The storage buffer is fixed at its initial size.
Attempting to extend the sbuf beyond this size results in an overflow condition.
.It Dv SBUF_AUTOEXTEND
This indicates that the storage buffer may be extended as necessary, so long
as resources allow, to hold additional data.
.It Dv SBUF_INCLUDENUL
This causes the final nulterm byte to be counted in the length of the data.
.It Dv SBUF_DRAINTOEOR
Treat top-level sections started with
.Fn sbuf_start_section
as a record boundary marker that will be used during drain operations to avoid
records being split.
If a record grows sufficiently large such that it fills the
.Fa sbuf
and therefore cannot be drained without being split, an error of
.Er EDEADLK
is set.
.It Dv SBUF_NOWAIT
Indicates that attempts to extend the storage buffer should fail in low memory
conditions, like
.Xr malloc 9
.Dv M_NOWAIT .
.El
.Pp
Note that if
.Fa buf
is not
.Dv NULL ,
it must point to an array of at least
.Fa length
characters.
The result of accessing that array directly while it is in use by the
sbuf is undefined.
.Pp
The
.Fn sbuf_new_auto
function is a shortcut for creating a completely dynamic
.Nm .
It is the equivalent of calling
.Fn sbuf_new
with values
.Dv NULL ,
.Dv NULL ,
.Dv 0 ,
and
.Dv SBUF_AUTOEXTEND .
.Pp
The
.Fn sbuf_new_for_sysctl
function will set up an sbuf with a drain function to use
.Fn SYSCTL_OUT
when the internal buffer fills.
Note that if the various functions which append to an sbuf are used while
a non-sleepable lock is held, the user buffer should be wired using
.Fn sysctl_wire_old_buffer .
.Pp
The
.Fn sbuf_delete
function clears the
.Fa sbuf
and frees any memory allocated for it.
There must be a call to
.Fn sbuf_delete
for every call to
.Fn sbuf_new .
Any attempt to access the sbuf after it has been deleted will fail.
.Pp
The
.Fn sbuf_clear
function invalidates the contents of the
.Fa sbuf
and resets its position to zero.
.Pp
The
.Fn sbuf_get_flags
function returns the current user flags.
The
.Fn sbuf_set_flags
and
.Fn sbuf_get_flags
functions set or clear one or more user flags, respectively.
The user flags are described under the
.Fn sbuf_new
function.
.Pp
The
.Fn sbuf_setpos
function sets the
.Fa sbuf Ns 's
end position to
.Fa pos ,
which is a value between zero and the current position in the buffer.
It can only truncate the sbuf to the new position.
.Pp
The
.Fn sbuf_bcat
function appends the first
.Fa len
bytes from the buffer
.Fa buf
to the
.Fa sbuf .
.Pp
The
.Fn sbuf_bcopyin
function copies
.Fa len
bytes from the specified userland address into the
.Fa sbuf .
.Pp
The
.Fn sbuf_bcpy
function replaces the contents of the
.Fa sbuf
with the first
.Fa len
bytes from the buffer
.Fa buf .
.Pp
The
.Fn sbuf_cat
function appends the NUL-terminated string
.Fa str
to the
.Fa sbuf
at the current position.
.Pp
The
.Fn sbuf_set_drain
function sets a drain function
.Fa func
for the
.Fa sbuf ,
and records a pointer
.Fa arg
to be passed to the drain on callback.
The drain function cannot be changed while
.Fa sbuf_len
is non-zero.
.Pp
The registered drain function
.Vt sbuf_drain_func
will be called with the argument
.Fa arg
provided to
.Fn sbuf_set_drain ,
a pointer
.Fa data
to a byte string that is the contents of the sbuf, and the length
.Fa len
of the data.
If the drain function exists, it will be called when the sbuf internal
buffer is full, or on behalf of
.Fn sbuf_finish .
The drain function may drain some or all of the data, but must drain
at least 1 byte.
The return value from the drain function, if positive, indicates how
many bytes were drained.
If negative, the return value indicates the negative error code which
will be returned from this or a later call to
.Fn sbuf_finish .
The returned drained length cannot be zero.
To do unbuffered draining, initialize the sbuf with a two-byte buffer.
The drain will be called for every byte added to the sbuf.
The
.Fn sbuf_bcopyin ,
.Fn sbuf_copyin ,
.Fn sbuf_trim ,
and
.Fn sbuf_data
functions cannot be used on an sbuf with a drain.
.Pp
The
.Fn sbuf_copyin
function copies a NUL-terminated string from the specified userland
address into the
.Fa sbuf .
If the
.Fa len
argument is non-zero, no more than
.Fa len
characters (not counting the terminating NUL) are copied; otherwise
the entire string, or as much of it as can fit in the
.Fa sbuf ,
is copied.
.Pp
The
.Fn sbuf_cpy
function replaces the contents of the
.Fa sbuf
with those of the NUL-terminated string
.Fa str .
This is equivalent to calling
.Fn sbuf_cat
with a fresh
.Fa sbuf
or one which position has been reset to zero with
.Fn sbuf_clear
or
.Fn sbuf_setpos .
.Pp
The
.Fn sbuf_printf
function formats its arguments according to the format string pointed
to by
.Fa fmt
and appends the resulting string to the
.Fa sbuf
at the current position.
.Pp
The
.Fn sbuf_vprintf
function behaves the same as
.Fn sbuf_printf
except that the arguments are obtained from the variable-length argument list
.Fa ap .
.Pp
The
.Fn sbuf_putc
function appends the character
.Fa c
to the
.Fa sbuf
at the current position.
.Pp
The
.Fn sbuf_trim
function removes trailing whitespace from the
.Fa sbuf .
.Pp
The
.Fn sbuf_error
function returns any error value that the
.Fa sbuf
may have accumulated, either from the drain function, or ENOMEM if the
.Fa sbuf
overflowed.
This function is generally not needed and instead the error code from
.Fn sbuf_finish
is the preferred way to discover whether an sbuf had an error.
.Pp
The
.Fn sbuf_finish
function will call the attached drain function if one exists until all
the data in the
.Fa sbuf
is flushed.
If there is no attached drain,
.Fn sbuf_finish
NUL-terminates the
.Fa sbuf .
In either case it marks the
.Fa sbuf
as finished, which means that it may no longer be modified using
.Fn sbuf_setpos ,
.Fn sbuf_cat ,
.Fn sbuf_cpy ,
.Fn sbuf_printf
or
.Fn sbuf_putc ,
until
.Fn sbuf_clear
is used to reset the sbuf.
.Pp
The
.Fn sbuf_data
function returns the actual string;
.Fn sbuf_data
only works on a finished
.Fa sbuf .
The
.Fn sbuf_len
function returns the length of the string.
For an
.Fa sbuf
with an attached drain,
.Fn sbuf_len
returns the length of the un-drained data.
.Fn sbuf_done
returns non-zero if the
.Fa sbuf
is finished.
.Pp
The
.Fn sbuf_start_section
and
.Fn sbuf_end_section
functions may be used for automatic section alignment.
The arguments
.Fa pad
and
.Fa c
specify the padding size and a character used for padding.
The arguments
.Fa old_lenp
and
.Fa old_len
are to save and restore the current section length when nested sections
are used.
For the top level section
.Dv NULL
and \-1 can be specified for
.Fa old_lenp
and
.Fa old_len
respectively.
.Pp
The
.Fn sbuf_hexdump
function prints an array of bytes to the supplied sbuf, along with an ASCII
representation of the bytes if possible.
See the
.Xr hexdump 3
man page for more details on the interface.
.Pp
The
.Fn sbuf_putbuf
function printfs the sbuf to stdout if in userland, and to the console
and log if in the kernel.
It does not drain the buffer or update any pointers.
.Sh NOTES
If an operation caused an
.Fa sbuf
to overflow, most subsequent operations on it will fail until the
.Fa sbuf
is finished using
.Fn sbuf_finish
or reset using
.Fn sbuf_clear ,
or its position is reset to a value between 0 and one less than the
size of its storage buffer using
.Fn sbuf_setpos ,
or it is reinitialized to a sufficiently short string using
.Fn sbuf_cpy .
.Pp
Drains in user-space will not always function as indicated.
While the drain function will be called immediately on overflow from
the
.Fa sbuf_putc ,
.Fa sbuf_bcat ,
.Fa sbuf_cat
functions,
.Fa sbuf_printf
and
.Fa sbuf_vprintf
currently have no way to determine whether there will be an overflow
until after it occurs, and cannot do a partial expansion of the format
string.
Thus when using libsbuf the buffer may be extended to allow completion
of a single printf call, even though a drain is attached.
.Sh RETURN VALUES
The
.Fn sbuf_new
function returns
.Dv NULL
if it failed to allocate a storage buffer, and a pointer to the new
.Fa sbuf
otherwise.
.Pp
The
.Fn sbuf_setpos
function returns \-1 if
.Fa pos
was invalid, and zero otherwise.
.Pp
The
.Fn sbuf_bcat ,
.Fn sbuf_cat ,
.Fn sbuf_cpy ,
.Fn sbuf_printf ,
.Fn sbuf_putc ,
and
.Fn sbuf_trim
functions
all return \-1 if the buffer overflowed, and zero otherwise.
.Pp
The
.Fn sbuf_error
function returns a non-zero value if the buffer has an overflow or
drain error, and zero otherwise.
.Pp
The
.Fn sbuf_len
function returns \-1 if the buffer overflowed.
.Pp
The
.Fn sbuf_copyin
function
returns \-1 if copying string from userland failed, and number of bytes
copied otherwise.
.Pp
The
.Fn sbuf_end_section
function returns the section length or \-1 if the buffer has an error.
.Pp
The
.Fn sbuf_finish 9
function (the kernel version) returns ENOMEM if the sbuf overflowed before
being finished,
or returns the error code from the drain if one is attached.
.Pp
The
.Fn sbuf_finish 3
function (the userland version)
will return zero for success and \-1 and set errno on error.
.Sh EXAMPLES
.Bd -literal -compact
#include <sys/types.h>
#include <sys/sbuf.h>

struct sbuf *sb;

sb = sbuf_new_auto();
sbuf_cat(sb, "Customers found:\en");
TAILQ_FOREACH(foo, &foolist, list) {
        sbuf_printf(sb, "   %4d %s\en", foo->index, foo->name);
        sbuf_printf(sb, "      Address: %s\en", foo->address);
        sbuf_printf(sb, "      Zip: %s\en", foo->zipcode);
}
if (sbuf_finish(sb) != 0) /* Check for any and all errors */
        err(1, "Could not generate message");
transmit_msg(sbuf_data(sb), sbuf_len(sb));
sbuf_delete(sb);
.Ed
.Sh SEE ALSO
.Xr hexdump 3 ,
.Xr printf 3 ,
.Xr strcat 3 ,
.Xr strcpy 3 ,
.Xr copyin 9 ,
.Xr copyinstr 9 ,
.Xr printf 9
.Sh HISTORY
The
.Nm
family of functions first appeared in
.Fx 4.4 .
.Sh AUTHORS
.An -nosplit
The
.Nm
family of functions was designed by
.An Poul-Henning Kamp Aq Mt phk@FreeBSD.org
and implemented by
.An Dag-Erling Sm\(/orgrav Aq Mt des@FreeBSD.org .
Additional improvements were suggested by
.An Justin T. Gibbs Aq Mt gibbs@FreeBSD.org .
Auto-extend support added by
.An Kelly Yancey Aq Mt kbyanc@FreeBSD.org .
Drain functionality added by
.An Matthew Fleming Aq Mt mdf@FreeBSD.org .
.Pp
This manual page was written by
.An Dag-Erling Sm\(/orgrav Aq Mt des@FreeBSD.org .