INTRO(2) INTRO(2)
NAME
intro - introduction to library functions
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <auth.h>
#include <bio.h>
#include <draw.h>
#include <fcall.h>
#include <frame.h>
#include <mach.h>
#include <ndb.h>
#include <regexp.h>
#include <stdio.h>
#include <thread.h>
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
/$objtype/lib/libx.a, where x is the base of the include
file name, stripped of a leading lib if present. For exam-
ple, <draw.h> defines the contents of library
/$objtype/lib/libdraw.a, which may be abbreviated when named
to the loader as -ldraw. In practice, each include file
contains a #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.
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
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ANSI C Standard I/O library, libstdio, is defined by
<stdio.h>, which has no prerequisites. 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: ushort, uchar, and ulong, the
unsigned integer types; schar, the signed char type; vlong,
a very long integral type; jmp_buf, the type of the argument
to setjmp and longjmp, plus macros that define the layout of
jmp_buf (see setjmp(2)); definitions of the bits in the
floating-point control register as used by getfcr(2); the
macros va_arg and friends for accessing arguments of vari-
adic functions (identical to the macros defined in
<stdarg.h> in ANSI C); and Length, for historical reasons a
union, representing the 64-bit length of a file, declared
something like
typedef union
{
vlong length;
} Length;
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 ASCII and Latin-1 control space) that occupy no more
than NAMELEN-1 bytes. A path element cannot contain a space
or slash.
When a process presents a file name to Plan 9, it is
evaluated by the following algorithm. Start with a direc-
tory that depends on the first character of the path: `/'
means the root of the main hierarchy, `#' means the separate
root of a kernel device's file tree (see Section 3), 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 transla-
tion (see below), and repeat. The last step may yield a
directory or regular file. The collection of files reach-
able from the root is called the name space of a process.
A program can use bind or mount (see bind(2)) to say that
whenever a specified file is reached during evaluation,
evaluation instead continues from a second specified file.
Also, the same system calls create union directories, which
are concatenations of ordinary directories that are searched
sequentially until the desired element is found. Using bind
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INTRO(2) INTRO(2)
and mount to do name space adjustment affects only the cur-
rent process group (see below). Certain conventions about
the layout of the name space should be preserved; see
namespace(4).
File I/O
Files are opened for input or output by open or create (see
open(2)). These calls return an integer called a file
descriptor which identifies the file to subsequent I/O
calls, notably read(2) and write. The system allocates the
numbers by selecting the lowest unused descriptor. They are
allocated dynamically; there is no visible limit to the num-
ber of file descriptors a process may have open. They may
be reassigned using dup(2). File descriptors are indices
into a kernel resident file descriptor table. Each process
has an associated file descriptor table. In some cases (see
rfork in fork(2)) a file descriptor table may be shared by
several processes.
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 pro-
grams 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(2) system call.
Directories may be opened and read much like regular files.
They contain an integral number of records, called directory
entries, of length DIRLEN (defined in <libc.h>). Each entry
is a machine-independent representation of the information
about an existing file in the directory, including the name,
ownership, permission, access dates, and so on. The entry
corresponding to an arbitrary file can be retrieved by
stat(2) or fstat; wstat and fwstat write back entries, thus
changing the properties of a file. An entry may be trans-
lated into a more convenient, addressable form called a Dir
structure; dirstat, dirfstat, dirwstat, and dirfwstat exe-
cute the appropriate translations (see stat(2)).
New files are made with create (in open(2)) and deleted with
remove(2). Directories may not directly be written; create,
remove, wstat, and fwstat alter them.
The operating system kernel records the file name used to
access each open file or directory. If the file is opened
by a local name (one that does not begin / or #), the system
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makes the stored name absolute by prefixing the string asso-
ciated with the current directory. Similar lexical adjust-
ments are made for path names containing . (dot) or ..
(dot-dot). By this process, the system maintains a record
of the route by which each file was accessed. Although
there is a possibility for error-the name is not maintained
after the file is opened, so removals and renamings can con-
found it-this simple method usually permits the system to
return, via the fd2path(2) system call and related calls
such as getwd(2), a valid name that may be used to find a
file again. This is also the source of the names reported
in the name space listing of ns(1) or /dev/ns (see proc(3)).
Pipe(2) 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 rfork
with the RFPROC bit set, usually just by calling 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(2) will bring in a differ-
ent 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.
The memory of a process is divided into segments. Every pro-
gram has at least a text (instruction) and stack segment.
Most also have an initialized data segment and a segment of
zero-filled data called bss. Processes may segattach(2)
other segments for special purposes.
A process terminates by calling exits(2). A parent process
may call wait (in exits(2)) to wait for some child to termi-
nate. A string of status information may be passed from
exits to wait. A process can go to sleep for a specified
time by calling sleep(2).
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(2)
to register the function to be called (the notification
handler) when such events occur.
Multithreading
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By calling rfork with the RFMEM bit set, a program may cre-
ate several independently executing processes sharing the
same memory (except for the stack segment, which is unique
to each process). 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(2)) to synchronize access to their data struc-
tures. The graphics library defined in <draw.h> is also
multi-process capable; details are in graphics(2). In gen-
eral, 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 pro-
cesses, multiple threads. Since Plan 9 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 com-
pletes. For an example of this design, see mouse(2).
SEE ALSO
nm(1), 2l(1), 2c(1)
DIAGNOSTICS
Math functions in libc return special values when the func-
tion is undefined for the given arguments or when the value
is not representable (see nan(2)).
Some of the functions in libc are system calls and many oth-
ers employ system calls in their implementation. All system
calls return integers, with -1 indicating that an error
occurred; errstr(2) recovers a string describing the error.
Some user-level library functions also use the errstr mecha-
nism to report errors. Functions that may affect the value
of the error string are said to ``set errstr''; it is under-
stood that the error string is altered only if an error
occurs.
BUGS
The limitation of path elements to 27 bytes (NAMELEN-1) is a
liability in the networked world.
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