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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 /usr/local/plan9/lib/libx.a,
where x is the base of the include file name, stripped of a leading
lib if present. For
example, <draw.h> defines the contents of library /usr/local/plan9/lib/libdraw.a,
which may be abbreviated when named to the loader as −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 9c(1).
The library to which a function belongs is defined by the header
file that defines its interface. The ‘C library’, libc, contains
most of the basic subroutines such as strlen. Declarations for
all of these functions are in <libc.h>, which must be preceded by
(needs) an include of <u.h>. The graphics library, draw, is defined
by <draw.h>, which needs
<libc.h> and <u.h>. The Buffered I/O library, libbio, is defined by
<bio.h>, which needs <libc.h> and <u.h>. The ANSI C Standard I/O library,
libstdio, is defined by <stdio.h>, which needs <u.h>. There are a
few other, less commonly used libraries defined on individual
pages of this section.
The include file <u.h>, a prerequisite of several other include
files, declares the architecture-dependent and -independent types,
including: uchar, ushort, and ulong, the unsigned integer types;
schar, the signed char type; vlong and uvlong, the signed and
unsigned very long integral types; Rune, the Unicode character
type; u8int, u16int, u32int, and
u64int, the unsigned integral types with specific widths; jmp_buf,
the type of the argument to setjmp and longjmp, plus macros that
define the layout of jmp_buf (see setjmp(3)); and the macros va_arg
and friends for accessing arguments of variadic functions (identical
to the macros defined in <stdarg.h> in ANSI C).
Plan 9 and Unix use many similarly-named functions for different
purposes: for example, Plan 9’s dup is closer to (but not exactly)
Unix’s dup2. To avoid name conflicts, <libc.h> defines many of these
names as preprocessor macros to add a p9 prefix, so that dup becomes
p9dup. To disable this renaming, #define NOPLAN9DEFINES before
including <libc.h>. If Unix headers must be included in a program,
they should be included after <u.h>, which sets important preprocessor
directives (for example, to enable 64-bit file offsets), but before
<libc.h>, to avoid renaming problems.
Name space
Files are collected into a hierarchical organization called a
file tree starting in a directory called the root. File names,
also called paths, consist of a number of /-separated path elements
with the slashes corresponding to directories. A path element
must contain only printable characters (those outside the control
spaces of ASCII and Latin-1). A path
element cannot contain a slash.
When a process presents a file name to Plan 9, it is evaluated
by the following algorithm. Start with a directory that depends
on the first character of the path: / 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.
File I/O
Files are opened for input or output by open or create (see open(3)).
These calls return an integer called a file descriptor which identifies
the file to subsequent I/O calls, notably read(3) and 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
dup(3). File descriptors are indices into a kernel resident file
descriptor table. Each process has an associated file descriptor
table. In threaded programs (see thread(3)), the file descriptor
table is shared by all the procs.
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.
Files are normally read or written in sequential order. The I/O
position in the file is called the file offset and may be set
arbitrarily using the seek(3) system call.
Directories may be opened like regular files. Instead of reading
them with read(3), use the Dir structure-based routines described
in dirread(3). The entry corresponding to an arbitrary file can
be retrieved by dirstat (see stat(3)) or dirfstat; dirwstat and
dirfwstat write back entries, thus changing the properties of
a file.
New files are made with create (see open(3)) and deleted with
remove(3). Directories may not directly be written; create, remove,
wstat, and fwstat alter them.
Pipe(3) creates a connected pair of file descriptors, useful for
bidirectional local communication.
Process execution and control
A new process is created when an existing one calls 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; exec(3)
will bring in a different one.
Each process has a unique integer process id; a set of open files,
indexed by file descriptor; and a current working directory (changed
by chdir(2)).
Each process has a set of attributes -- memory, open files, name
space, etc. -- that may be shared or unique. Flags to rfork control
the sharing of these attributes.
A process terminates by calling exits(3). A parent process may
call wait(3) to wait for some child to terminate. A bit of status
information may be passed from exits to 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.
A process can go to sleep for a specified time by calling sleep(3).
There is a notification mechanism for telling a process about
events such as address faults, floating point faults, and messages
from other processes. A process uses notify(3) to register the
function to be called (the notification handler) when such events
occur.
Multithreading
Where possible according to the ANSI C standard, the main C library
works properly in multiprocess programs; malloc, print, and the
other routines use locks (see lock(3)) to synchronize access to
their data structures. The graphics library defined in <draw.h>
is also multi-process capable; details are in graphics(3). In
general, though, multiprocess
programs should use some form of synchronization to protect shared
data.
The thread library, defined in <thread.h>, provides support for
multiprocess programs. It includes a data structure called a Channel
that can be used to send messages between processes, and coroutine-like
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.
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 ioproc(3) or mouse(3).
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