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Guidelines for writing ksh-93 built-in commands
David G. Korn
One of the features of ksh93, the latest version of ksh,
is the ability to add built-in commands at run time.
This feature only works on operating systems that have the ability
to load and link code into the current process at run time.
Some examples of the systems that have this feature
are Linux, System V Release 4, Solaris, Sun OS, HP-UX Release 8 and above,
AIX 3.2 and above, and Microsoft Windows systems.
This memo describes how to write and compile programs
that can be loaded into ksh at run time as built-in
commands.
A built-in command is executed without creating a separate process.
Instead, the command is invoked as a C function by ksh.
If this function has no side effects in the shell process,
then the behavior of this built-in is identical to that of
the equivalent stand-alone command. The primary difference
in this case is performance. The overhead of process creation
is eliminated. For commands of short duration, the effect
can be dramatic. For example, on SUN OS 4.1, the time to
run wc on a small file of about 1000 bytes, runs
about 50 times faster as a built-in command.
In addition, built-in commands may have side effects on the
shell environment.
This is usually done to extend the application domain for
shell programming. For example, there is a group of X-windows extension
built-ins that make heavy use of the shell variable namespace.
These built-ins are added at run time and
result in a windowing shell that can be used to write
X-windows applications.
While there are definite advantages to adding built-in
commands, there are some disadvantages as well.
Since the built-in command and ksh share the same
address space, a coding error in the built-in program
may affect the behavior of ksh; perhaps causing
it to core dump or hang.
Debugging is also more complex since your code is now
a part of a larger entity.
The isolation provided by a separate process
guarantees that all resources used by the command
will be freed when the command completes.
Resources used by a built-in must be meticulously maintained and freed.
Also, since the address space of ksh will be larger when built-in are loaded,
it may increase the time it takes ksh to fork() and
exec() non-built-in commands.
It makes no sense to add a built-in command that takes
a long time to run or that is run only once, since the performance
benefits will be negligible.
Built-ins that have side effects in the current shell
environment have the disadvantage of increasing the
coupling between the built-in and ksh, making
the overall system less modular and more monolithic.
Despite these drawbacks, in many cases extending
ksh by adding built-in
commands makes sense and allows reuse of the shell
scripting ability in an application specific domain.
This memo describes how to write ksh extensions.
There is a development kit available for writing ksh
built-ins as part of the AST (AT&T Software Technology) Toolkit.
The development kit has three directories,
include, lib, and bin.
It is best to set the value of the environment variable
PACKAGE_ast to the pathname of the directory
containing the development kit.
The include directory contains a sub-directory
named ast that contains interface prototypes
for functions that you can call from built-ins. The lib
directory contains the ast library
and a library named cmd that contains a version
of several of the standard POSIX[1]
utilities that can be made run time built-ins.
The lib/ksh directory contains shared libraries
that implement other ksh built-ins.
The bin directory contains build tools such as nmake[2].
To add built-ins at runtime, it is necessary to build a shared library
containing one or more built-ins that you wish to add.
The built-ins are then added by running builtin -f shared_lib.
Since the procedure for building share libraries is system dependent,
it is best to use
nmake
using the sample nmake makefile below as a prototype.
The AST Toolkit also contains some examples of built-in libraries under
the src/cmd/kshlib directory.
There are two ways to code adding built-ins. One method is to replace
the function main with a function
b_name, where name is the name
of the built-in you wish to define.
A built-in command has a calling convention similar to
the main function of a program,
int main(int argc, char *argv[]).
except that it takes a third argument of type Shbltin_t* which can
be passed as NULL if it is not used. The definition for
Shbltin_t* is in <ast/shcmd.h>.
Instead of exit, you need to use return
to terminate your command.
The return value will become the exit status of the command.
The open built-in, installed in lib/ksh in the AST Toolkit, uses this method.
The Shbltin_t structure contains a field named shp which is
a pointer the the shell data that is needed for shell library callbacks.
It also contains the fields, shrun, shtrap, shexit,
and shbltin
that are function pointers to the shell library functions sh_run, sh_trap
sh_exit, and sh_addbuiltin, respectively. These functions
can be invoked without the need for runtime symbol lookup when the
shell is statically linked with libshell.
The alternative method is to create a function lib_init and
use the Shbltin_t.shbltin() function to add one or more built-ins.
The lib_init function will be called with two arguments. The
first argument will be 0 when the library is loaded and the second
argument will be of type Shbltin_t*.
The dbm_t and dss shell built-ins use this method.
No matter which way you add built-ins you should add the line
SHLIB(identifier) as the last line of one
of the built-in source file, where identifier is any C identifier.
This line provides version information to the shell builtin command
that it uses to verify compatibility between the built-in and ksh
implementation versions. builtin fails with a diagnostic on version
mismatch. The diagnostic helps determine whether ksh is out of
date and requires an upgrade or the built-in is out of date and requires
recompilation.
The steps necessary to create and add a run time built-in are
illustrated in the following simple example.
Suppose you wish to add a built-in command named hello
which requires one argument and prints the word hello followed
by its argument. First, write the following program in the file
hello.c:
#include <stdio.h>
int b_hello(int argc, char *argv[], void *context)
{
if(argc != 2)
{
fprintf(stderr,"Usage: hello arg\n");
return(2);
}
printf("hello %s\n",argv[1]);
return(0);
}
SHLIB(hello)
Next, the program needs to be compiled.
If you are building with AT&T nmake use the following Makefile:
:PACKAGE: --shared ast
hello plugin=ksh :LIBRARY: hello.c
and run nmake install to compile, link, and install the built-in shared library
in lib/ksh/ under PACKAGE_ast.
If the built-in extension uses several .c files, list all of these on
the :LIBRARY: line.
Otherwise you will have to compile hello.c with an option
to pick up the AST include directory
(since the AST <stdio.h> is required for ksh compatibility)
and options required for generating shared libraries.
For example, on Linux use this to compile:
cc -fpic -I$PACKAGE_ast/include/ast -c hello.c
and use the appropriate link line.
It really is best to use nmake because the 2 line Makefile above
will work on all systems that have ksh installed.
If you have several built-ins, it is desirable
to build a shared library that contains them all.
The final step is using the built-in.
This can be done with the ksh command builtin.
To load the shared library libhello.so from the current directory
and add the built-in hello, invoke the command,
builtin -f ./libhello.so hello
The shared library prefix (lib here) and suffix (.so here) be omitted;
the shell will add an appropriate suffix
for the system that it is loading from.
If you install the shared library in lib/ksh/, where ../lib/ksh/ is
a directory on $PATH, the command
will automatically find, load and install the built-in on any system.
Once this command has been invoked, you can invoke hello
as you do any other command.
If you are using lib_init method to add built-ins then no arguments
follow the -f option.
It is often desirable to make a command built-in
the first time that it is referenced. The first
time hello is invoked, ksh should load and execute it,
whereas for subsequent invocations ksh should just execute the built-in.
This can be done by creating a file named hello
with the following contents:
function hello
{
unset -f hello
builtin -f hello hello
hello "$@"
}
This file hello needs to be placed in a directory that is
in your FPATH variable, and the built-in shared library
should be installed in lib/ksh/, as described above.
As mentioned above, the entry point for built-ins must either be of
the form b_name or else be loaded from a function named
lib_init.
Your built-ins can call functions from the standard C library,
the ast library, interface functions provided by ksh,
and your own functions.
You should avoid using any global symbols beginning with
sh_,
nv_,
and
ed_
since these are used by ksh itself.
#define constants in ksh interface
files use symbols beginning with SH_ and NV_,
so avoid using names beginning with these too.
The development kit provides a portable interface
to the C library and to libast.
The header files in the development kit are compatible with
K&R C[3],
ANSI-C[4],
and C++[5].
The best thing to do is to include the header file <shell.h>.
This header file causes the <ast.h> header, the
<error.h> header and the <stak.h>
header to be included as well as defining prototypes
for functions that you can call to get shell
services for your builtins.
The header file <ast.h>
provides prototypes for many libast functions
and all the symbol and function definitions from the
ANSI-C headers, <stddef.h>,
<stdlib.h>, <stdarg.h>, <limits.h>,
and <string.h>.
It also provides all the symbols and definitions for the
POSIX[6]
headers <sys/types.h>, <fcntl.h>, and
<unistd.h>.
You should include <ast.h> instead of one or more of
these headers.
The <error.h> header provides the interface to the error
and option parsing routines defined below.
The <stak.h> header provides the interface to the memory
allocation routines described below.
Programs that want to use the information in <sys/stat.h>
should include the file <ls.h> instead.
This provides the complete POSIX interface to stat()
related functions even on non-POSIX systems.
ksh uses sfio,
the Safe/Fast I/O library[7],
to perform all I/O operations.
The sfio library, which is part of libast,
provides a superset of the functionality provided by the standard
I/O library defined in ANSI-C.
If none of the additional functionality is required,
and if you are not familiar with sfio and
you do not want to spend the time learning it,
then you can use sfio via the stdio library
interface. The development kit contains the header <stdio.h>
which maps stdio calls to sfio calls.
In most instances the mapping is done
by macros or inline functions so that there is no overhead.
The man page for the sfio library is in an Appendix.
However, there are some very nice extensions and
performance improvements in sfio
and if you plan any major extensions I recommend
that you use it natively.
For error messages it is best to use the ast library
function errormsg() rather that sending output to
stderr or the equivalent sfstderr directly.
Using errormsg() will make error message appear
more uniform to the user.
Furthermore, using errormsg() should make it easier
to do error message translation for other locales
in future versions of ksh.
The first argument to
errormsg() specifies the dictionary in which the string
will be searched for translation.
The second argument to errormsg() contains that error type
and value. The third argument is a printf style format
and the remaining arguments are arguments to be printed
as part of the message. A new-line is inserted at the
end of each message and therefore, should not appear as
part of the format string.
The second argument should be one of the following:
- ERROR_exit(n):
If n is not-zero, the builtin will exit value n after
printing the message.
- ERROR_system(n):
Exit builtin with exit value n after printing the message.
The message will display the message corresponding to errno
enclosed within [ ] at the end of the message.
- ERROR_usage(n):
Will generate a usage message and exit. If n is non-zero,
the exit value will be 2. Otherwise the exit value will be 0.
- ERROR_debug(n):
Will print a level n debugging message and will then continue.
- ERROR_warn(n):
Prints a warning message. n is ignored.
The first thing that a built-in should do is to check
the arguments for correctness and to print any usage
messages on standard error.
For consistency with the rest of ksh, it is best
to use the libast functions optget() and
optusage()for this
purpose.
The header <error.h> includes prototypes for
these functions.
The optget() function is similar to the
System V C library function getopt(),
but provides some additional capabilities.
Built-ins that use optget() provide a more
consistent user interface.
The optget() function is invoked as
int optget(char *argv[], const char *optstring)
where argv is the argument list and optstring
is a string that specifies the allowable arguments and
additional information that is used to format usage
messages.
In fact a complete man page in troff or html
can be generated by passing a usage string as described
by the getopts command.
Like getopt(),
single letter options are represented by the letter itself,
and options that take a string argument are followed by the :
character.
Option strings have the following special characters:
- :
-
Used after a letter option to indicate that the option
takes an option argument.
The variable opt_info.arg will point to this
value after the given argument is encountered.
- #
-
Used after a letter option to indicate that the option
can only take a numerical value.
The variable opt_info.num will contain this
value after the given argument is encountered.
- ?
-
Used after a : or # (and after the optional ?)
to indicate the the
preceding option argument is not required.
- [...]
After a : or #, the characters contained
inside the brackets are used to identify the option
argument when generating a usage message.
- space
The remainder of the string will only be used when generating
usage messages.
The optget() function returns the matching option letter if
one of the legal option is matched.
Otherwise, optget() returns
- ':'
-
If there is an error. In this case the variable opt_info.arg
contains the error string.
- 0
-
Indicates the end of options.
The variable opt_info.index contains the number of arguments
processed.
- '?'
-
A usage message has been required.
You normally call optusage() to generate and display
the usage message.
The following is an example of the option parsing portion
of the wc utility.
#include <shell.h>
while(1) switch(n=optget(argv,"xf:[file]"))
{
case 'f':
file = opt_info.arg;
break;
case ':':
error(ERROR_exit(0), opt_info.arg);
break;
case '?':
error(ERROR_usage(2), opt_info.arg);
break;
}
It is important that any memory used by your built-in
be returned. Otherwise, if your built-in is called frequently,
ksh will eventually run out of memory.
You should avoid using malloc() for memory that must
be freed before returning from you built-in, because by default,
ksh will terminate you built-in in the event of an
interrupt and the memory will not be freed.
The best way to allocate variable sized storage is
through calls to the stak library
which is included in libast
and which is used extensively by ksh itself.
Objects allocated with the stakalloc()
function are freed when you function completes
or aborts.
The stak library provides a convenient way to
build variable length strings and other objects dynamically.
The man page for the stak library is contained
in the Appendix.
Before ksh calls each built-in command, it saves
the current stack location and restores it after
it returns.
It is not necessary to save and restore the stack
location in the b_ entry function,
but you may want to write functions that use this stack
are restore it when leaving the function.
The following coding convention will do this in
an efficient manner:
yourfunction()
{
char *savebase;
int saveoffset;
if(saveoffset=staktell())
savebase = stakfreeze(0);
...
if(saveoffset)
stakset(savebase,saveoffset);
else
stakseek(0);
}
Some of the more interesting applications are those that extend
the functionality of ksh in application specific directions.
A prime example of this is the X-windows extension which adds
builtins to create and delete widgets.
The nval library is used to interface with the shell
name space.
The shell library is used to access other shell services.
A great deal of power is derived from the ability to use
portions of the hierarchal variable namespace provided by ksh-93
and turn these names into active objects.
The nval library is used to interface with shell
variables.
A man page for this file is provided in an Appendix.
You need to include the header <nval.h>
to access the functions defined in the nval library.
All the functions provided by the nval library begin
with the prefix nv_.
Each shell variable is an object in an associative table
that is referenced by name.
The type Namval_t* is pointer to a shell variable.
To operate on a shell variable, you first get a handle
to the variable with the nv_open() function
and then supply the handle returned as the first
argument of the function that provides an operation
on the variable.
You must call nv_close() when you are finished
using this handle so that the space can be freed once
the value is unset.
The two most frequent operations are to get the value of
the variable, and to assign value to the variable.
The nv_getval() returns a pointer the the
value of the variable.
In some cases the pointer returned is to a region that
will be overwritten by the next nv_getval() call
so that if the value isn't used immediately, it should
be copied.
Many variables can also generate a numeric value.
The nv_getnum() function returns a numeric
value for the given variable pointer, calling the
arithmetic evaluator if necessary.
The nv_putval() function is used to assign a new
value to a given variable.
The second argument to putval() is the value
to be assigned
and the third argument is a flag which
is used in interpreting the second argument.
Each shell variable can have one or more attributes.
The nv_isattr() is used to test for the existence
of one or more attributes.
See the appendix for a complete list of attributes.
By default, each shell variable passively stores the string you
give with with nv_putval(), and returns the value
with getval(). However, it is possible to turn
any node into an active entity by assigning functions
to it that will be called whenever nv_putval()
and/or nv_getval() is called.
In fact there are up to five functions that can
associated with each variable to override the
default actions.
The type Namfun_t is used to define these functions.
Only those that are non-NULL override the
default actions.
To override the default actions, you must allocate an
instance of Namfun_t, and then assign
the functions that you wish to override.
The putval()
function is called by the nv_putval() function.
A NULL for the value argument
indicates a request to unset the variable.
The type argument might contain the NV_INTEGER
bit so you should be prepared to do a conversion if
necessary.
The getval()
function is called by nv_getval()
value and must return a string.
The getnum()
function is called by by the arithmetic evaluator
and must return double.
If omitted, then it will call nv_getval() and
convert the result to a number.
The functionality of a variable can further be increased
by adding discipline functions that
can be associated with the variable.
A discipline function allows a script that uses your
variable to define functions whose name is
varname.discname
where varname is the name of the variable, and discname
is the name of the discipline.
When the user defines such a function, the settrap()
function will be called with the name of the discipline and
a pointer to the parse tree corresponding to the discipline
function.
The application determines when these functions are actually
executed.
By default, ksh defines get,
set, and unset as discipline functions.
In addition, it is possible to provide a data area that
will be passed as an argument to
each of these functions whenever any of these functions are called.
To have private data, you need to define and allocate a structure
that looks like
struct yours
{
Namfun_t fun;
your_data_fields;
};
There are several functions that are used by ksh itself
that can also be called from built-in commands.
The man page for these routines are in the Appendix.
The sh_addbuiltin() function can be used to add or delete
builtin commands. It takes the name of the built-in, the
address of the function that implements the built-in, and
a void* pointer that will be passed to this function
as the third argument whenever it is invoked.
If the function address is NULL, the specified built-in
will be deleted. However, special built-in functions cannot
be deleted or modified.
The sh_fmtq() function takes a string and returns
a string that is quoted as necessary so that it can
be used as shell input.
This function is used to implement the %q option
of the shell built-in printf command.
The sh_parse() function returns a parse tree corresponding
to a give file stream. The tree can be executed by supplying
it as the first argument to
the sh_trap() function and giving a value of 1 as the
second argument.
Alternatively, the sh_trap() function can parse and execute
a string by passing the string as the first argument and giving 0
as the second argument.
The sh_isoption() function can be used to set to see whether one
or more of the option settings is enabled.
- [1]
-
POSIX - Part 2: Shell and Utilities,
IEEE Std 1003.2-1992, ISO/IEC 9945-2:1993.
- [2]
-
Glenn Fowler,
A Case for make,
Software - Practice and Experience, Vol. 20 No. S1, pp. 30-46, June 1990.
- [3]
-
Brian W. Kernighan and Dennis M. Ritchie,
The C Programming Language,
Prentice Hall, 1978.
- [4]
-
American National Standard for Information Systems - Programming
Language - C, ANSI X3.159-1989.
- [5]
-
Bjarne Stroustroup,
C++,
Addison Wesley, xxxx
- [6]
-
POSIX - Part 1: System Application Program Interface,
IEEE Std 1003.1-1990, ISO/IEC 9945-1:1990.
- [7]
-
David Korn and Kiem-Phong Vo,
SFIO - A Safe/Fast Input/Output library,
Proceedings of the Summer Usenix,
pp. , 1991.
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