INTRO(2)                                                 INTRO(2)

     NAME
          intro - introduction to library functions

     SYNOPSIS
          #include <u.h>

          #include <libc.h>

          #include <auth.h>

          #include <bio.h>

          #include <fcall.h>

          #include <frame.h>

          #include <layer.h>

          #include <libg.h>

          #include <mach.h>

          #include <ndb.h>

          #include <panel.h>

          #include <regexp.h>

          #include <stdio.h>

     DESCRIPTION
          This section describes functions in various libraries.  For
          the most part, each library is defined by a single C include
          file, listed above, and a single archive file containing the
          library proper.  The name of the archive is
          /$objtype/lib/libx.a, where x is the base of the include
          file name, stripped of a leading lib if present.  For exam-
          ple, <libg.h> defines the contents of library
          /$objtype/lib/libg.a, which may be abbreviated when named to
          the loader as -lg.  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, libg, the graphics library.  is defined by

     INTRO(2)                                                 INTRO(2)

          <libg.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 has no prerequisites.  There are
          a few other, less commonly used libraries defined on indi-
          vidual 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); and
          Length, a union giving different views of the 64-bit length
          of a file, declared something like

               typedef union
               {
                     char   clength[8];
                     vlong  vlength;
                     struct
                     {
                            long hlength;   /* high order */
                            long length;    /* low order */
                     };
               } 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

     INTRO(2)                                                 INTRO(2)

          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
          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. File descriptors range
          from 0 to 99 in the current system.  The system allocates
          the numbers by selecting the lowest unused descriptor.  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.

     INTRO(2)                                                 INTRO(2)

          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.

        Alef
          Most of the functions in this section are available in the
          same form from Alef, with byte substituted for char and
          uchar and int for long, and with adjustment for Alef having
          only one floating-point type, called float, holding double-
          precision values.  The main exceptions are that the long-
          valued functions such as strtoul have their final l changed
          to an i to reflect the different type structure of the lan-
          guage; that the Bio library has a different organization
          (see Bio(2) for details); and for various reasons some
          things are missing, notably ctype and the Stdio, IP, Layer,
          Lock, Mach, Ndb, and Panel libraries.  Also, there is no
          <u.h>; instead <alef.h> replaces both it and <libc.h>.  The
          machine-dependent definitions in Alef, which are only needed

     INTRO(2)                                                 INTRO(2)

          for getfcr(2) and relatives, are in <arch.h>.

          Within this manual, only explicit differences in the Alef
          libraries are documented, the Alef functions are not all
          indexed, and the substitutions for <libc.h> as well as char,
          uchar, etc.  are assumed.  The sources to the Alef libraries
          all live under /sys/src/alef/lib.

          NOTE: Because the languages have different calling conven-
          tions, Alef programs cannot be linked with C libraries.

     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.