man/man3/0intro.3

.TH INTRO 3
.SH NAME
intro \- introduction to library functions
.SH SYNOPSIS
.nf
.B #include <u.h>
.PP
.B #include \fIany Unix headers\fR
.PP
.B #include <libc.h>
.PP
.B #include <auth.h>
.PP
.B #include <bio.h>
.PP
.B #include <draw.h>
.PP
.B #include <fcall.h>
.PP
.B #include <frame.h>
.PP
.B #include <mach.h>
.PP
.B #include <regexp.h>
.PP
.B #include <thread.h>
.fi
.SH DESCRIPTION
This section describes functions
in various libraries.
For the most part, each library is defined by a single C include
file, such as those listed above, and a single archive file containing
the library proper.  The name of the archive is
.BI \*9/lib/lib x .a \f1,
where
.I x
is the base of the include file name, stripped of a leading
.B lib
if present.
For example,
.B <draw.h>
defines the contents of library
.BR \*9/lib/libdraw.a ,
which may be abbreviated when named to the loader as
.BR -ldraw .
In practice, each include file contains a magic pragma
that directs the loader to pick up the associated archive
automatically, so it is rarely necessary to tell the loader
which
libraries a program needs;
see
.IR 9c (1).
.PP
The library to which a function belongs is defined by the
header file that defines its interface.
The `C library',
.IR libc ,
contains most of the basic subroutines such
as
.IR strlen .
Declarations for all of these functions are
in
.BR <libc.h> ,
which must be preceded by
.RI ( needs )
an include of
.BR <u.h> .
The graphics library,
.IR draw ,
is defined by
.BR <draw.h> ,
which needs
.B <libc.h>
and
.BR <u.h> .
The Buffered I/O library,
.IR libbio ,
is defined by
.BR <bio.h> ,
which needs
.B <libc.h>
and
.BR <u.h> .
The ANSI C Standard I/O library,
.IR libstdio ,
is defined by
.BR <stdio.h> ,
which needs
.BR <u.h> .
There are a few other, less commonly used libraries defined on
individual pages of this section.
.PP
The include file
.BR <u.h> ,
a prerequisite of several other include files,
declares the architecture-dependent and -independent types, including:
.IR uchar ,
.IR ushort ,
and
.IR ulong ,
the unsigned integer types;
.IR schar ,
the signed char type;
.I vlong
and
.IR uvlong ,
the signed and unsigned very long integral types;
.IR Rune ,
the Unicode character type;
.IR u8int ,
.IR u16int ,
.IR u32int ,
and
.IR u64int ,
the unsigned integral types with specific widths;
.IR jmp_buf ,
the type of the argument to
.I setjmp
and 
.IR longjmp ,
plus macros that define the layout of
.IR jmp_buf
(see
.IR setjmp (3));
.\" definitions of the bits in the floating-point control register
.\" as used by
.\" .IR getfcr (2);
and
the macros
.B va_arg
and friends for accessing arguments of variadic functions (identical to the
macros defined in
.B <stdarg.h>
in ANSI C).
.PP
Plan 9 and Unix use many similarly-named functions for different purposes:
for example, Plan 9's
.I dup
is closer to (but not exactly) Unix's
.IR dup2 .
To avoid name conflicts,
.B <libc.h>
defines many of these names as preprocessor macros to add a
.I p9
prefix,
so that
.I dup
becomes
.IR p9dup .
To disable this renaming,
.B #define
.B NOPLAN9DEFINES
before including
.BR <libc.h> .
If Unix headers must be included in a program,
they should be included after
.BR <u.h> ,
which sets important preprocessor directives
(for example, to enable 64-bit file offsets),
but before
.BR <libc.h> ,
to avoid renaming problems.
.SS "Name space
Files are collected into a hierarchical organization called a
.I "file tree
starting in a
.I directory
called the
.IR root .
File names, also called
.IR paths ,
consist of a number of
.BR / -separated
.I "path elements"
with the slashes corresponding to directories.
A path element must contain only printable
characters (those outside the control spaces of
.SM ASCII
and Latin-1).
A path element cannot contain a slash.
.PP
When a process presents a file name to Plan 9, it is
.I evaluated
by the following algorithm.
Start with a directory that depends on the first
character of the path:
.L /
means the root of the main hierarchy,
and anything else means the process's current working directory.
Then for each path element, look up the element
in the directory, advance to that directory,
do a possible translation (see below), and repeat.
The last step may yield a directory or regular file.
.SS "File I/O"
Files are opened for input or output
by
.I open
or
.I create
(see
.IR open (3)).
These calls return an integer called a
.IR "file descriptor"
which identifies the file
to subsequent I/O calls,
notably
.IR read (3)
and
.IR write .
The system allocates the numbers by selecting the lowest unused descriptor.
They are allocated dynamically; there is no visible limit to the number of file
descriptors a process may have open.
They may be reassigned using
.IR dup (3).
File descriptors are indices into a
kernel resident
.IR "file descriptor table" .
Each process has an associated file descriptor table.
In threaded programs
(see
.IR thread (3)),
the file descriptor table is shared by all the procs.
.PP
By convention,
file descriptor 0 is the standard input,
1 is the standard output,
and 2 is the standard error output.
With one exception, the operating system is unaware of these conventions;
it is permissible to close file 0,
or even to replace it by a file open only for writing,
but many programs will be confused by such chicanery.
The exception is that the system prints messages about broken processes
to file descriptor 2.
.PP
Files are normally read or written in sequential order.
The I/O position in the file is called the
.IR "file offset"
and may be set arbitrarily using the
.IR seek (3)
system call.
.PP
Directories may be opened like regular files.
Instead of reading them with
.IR read (3),
use the
.B Dir
structure-based
routines described in
.IR dirread (3).
The entry
corresponding to an arbitrary file can be retrieved by
.IR dirstat
(see
.IR stat (3))
or
.IR dirfstat ;
.I dirwstat
and
.I dirfwstat
write back entries, thus changing the properties of a file.
.PP
New files are made with
.I create
(see
.IR open (3))
and deleted with
.IR remove (3).
Directories may not directly be written;
.IR create ,
.IR remove ,
.IR wstat ,
and
.I fwstat
alter them.
.PP
.IR Pipe (3)
creates a connected pair of file descriptors,
useful for bidirectional local communication.
.SS "Process execution and control"
A new process is created
when an existing one calls
.IR fork (2).
The new (child) process starts out with
copies of the address space and most other attributes
of the old (parent) process.
In particular,
the child starts out running
the same program as the parent;
.IR exec (3)
will bring in a different one.
.PP
Each process has a unique integer process id;
a set of open files, indexed by file descriptor;
and a current working directory
(changed by
.IR chdir (2)).
.PP
Each process has a set of attributes \(em memory, open files,
name space, etc. \(em that may be shared or unique.
Flags to
.IR rfork
control the sharing of these attributes.
.PP
A process terminates by calling
.IR exits (3).
A parent process may call
.IR wait (3)
to wait for some child to terminate.
A bit of status information
may be passed from
.I exits
to
.IR wait .
On Plan 9, the status information is an arbitrary text string,
but on Unix it is a single integer.
The Plan 9 interface persists here, although the functionality does not.
Instead, empty strings are converted to exit status 0 and non-empty strings to 1.
.PP
A process can go to sleep for a specified time by calling
.IR sleep (3).
.PP
There is a
.I notification
mechanism for telling a process about events such as address faults,
floating point faults, and messages from other processes.
A process uses
.IR notify (3)
to register the function to be called (the
.IR "notification handler" )
when such events occur.
.SS Multithreading
Where possible according to the ANSI C standard,
the main C library works properly in multiprocess programs;
.IR malloc ,
.IR print ,
and the other routines use locks (see
.IR lock (3))
to synchronize access to their data structures.
The graphics library defined in
.B <draw.h>
is also multi-process capable; details are in
.IR graphics (3).
In general, though, multiprocess programs should use some form of synchronization
to protect shared data.
.PP
The thread library, defined in
.BR <thread.h> ,
provides support for multiprocess programs.
It includes a data structure called a
.B Channel
that can be used to send messages between processes,
and coroutine-like
.IR threads ,
which enable multiple threads of control within a single process.
The threads within a process are scheduled by the library, but there is
no pre-emptive scheduling within a process; thread switching occurs
only at communication or synchronization points.
.PP
Most programs using the thread library
comprise multiple processes
communicating over channels, and within some processes,
multiple threads.  Since I/O calls may block, a system
call may block all the threads in a process.
Therefore, a program that shouldn't block unexpectedly will use a process
to serve the I/O request, passing the result to the main processes
over a channel when the request completes.
For examples of this design, see
.IR ioproc (3)
or
.IR mouse (3).
.SH SEE ALSO
.IR nm (1), 
.IR 9c (1)
.SH DIAGNOSTICS
Math functions in
.I libc
return
special values when the function is undefined for the
given arguments or when the value is not representable
(see
.IR nan (3)).
.PP
Some of the functions in
.I libc
are system calls and many others employ system calls in their implementation.
All system calls return integers,
with \-1 indicating that an error occurred;
.IR errstr (3)
recovers a string describing the error.
Some user-level library functions also use the
.I errstr
mechanism to report errors.
Functions that may affect the value of the error string are said to ``set
.IR errstr '';
it is understood that the error string is altered only if an error occurs.