Manual Reference Pages - 0INTRO (3)
intro - introduction to library functions
#include any Unix headers
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
x is the base of the include file name, stripped of a leading
lib if present.
<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
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
strlen. Declarations for all of these functions are
<libc.h>, which must be preceded by
(needs) an include of
<u.h>. The graphics library,
draw, is defined by
<draw.h>, which needs
<u.h>. The Buffered I/O library,
libbio, is defined by
<bio.h>, which needs
<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:
ulong, the unsigned integer types;
schar, the signed char type;
uvlong, the signed and unsigned very long integral types;
Rune, the Unicode character type;
u64int, the unsigned integral types with specific widths;
jmp_buf, the type of the argument to
longjmp, plus macros that define the layout of
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 9s
dup is closer to (but not exactly) Unixs
dup2. To avoid name conflicts,
<libc.h> defines many of these names as preprocessor macros to add a
p9dup. To disable this renaming,
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),
<libc.h>, to avoid renaming problems.
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
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 processs 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.
Files are opened for input or output
These calls return an integer called a
file descriptor which identifies the file
to subsequent I/O calls,
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
File descriptors are indices into a
file descriptor table. Each process has an associated file descriptor table.
In threaded programs
the file descriptor table is shared by all the procs.
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
Directories may be opened like regular files.
Instead of reading them with
routines described in
corresponding to an arbitrary file can be retrieved by
dirfwstat write back entries, thus changing the properties of a file.
New files are made with
and deleted with
Directories may not directly be written;
fwstat alter them.
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
The new (child) process starts out with
copies of the address space and most other attributes
of the old (parent) process.
the child starts out running
the same program as the parent;
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
Each process has a set of attributes memory, open files,
name space, etc. that may be shared or unique.
rfork control the sharing of these attributes.
A process terminates by calling
A parent process may call
to wait for some child to terminate.
A bit of status information
may be passed from
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
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
to register the function to be called (the
notification handler) when such events occur.
Where possible according to the ANSI C standard,
the main C library works properly in multiprocess programs;
print, and the other routines use locks (see
to synchronize access to their data structures.
The graphics library defined in
<draw.h> is also multi-process capable; details are in
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,
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 shouldnt 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
Math functions in
special values when the function is undefined for the
given arguments or when the value is not representable
Some of the functions in
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;
recovers a string describing the error.
Some user-level library functions also use the
errstr mechanism to report errors.
Functions that may affect the value of the error string are said to set
errstr; it is understood that the error string is altered only if an error occurs.
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