INTRO(3)

NAME
intro - introduction to library functions

SYNOPSIS
#include <u.h>

#include any Unix headers

#include <libc.h>

#include <auth.h>

#include <bio.h>

#include <draw.h>

#include <fcall.h>

#include <frame.h>

#include <mach.h>

#include <regexp.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 /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 /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)).

SEE ALSO
nm(1), 9c((1))

DIAGNOSTICS
Math functions in libc return special values when the function is undefined for the given arguments or when the value is not representable (see nan((3))).

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; errstr((3)) 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.