NAME
libexpect - programmed dialogue with interactive programs -
C functions
DESCRIPTION
This library contains functions that allow Expect to be used
as a Tcl extension or to be used directly from C or C++
(without Tcl). Adding Expect as a Tcl extension is very
short and simple, so that will be covered first.
SYNOPSIS
#include expect_tcl.h
Exp_Init(interp);
cc files... -lexpect -ltcl -lm
The Exp_Init function adds expect commands to the named
interpreter. It avoids overwriting commands that already
exist, however aliases beginning with "exp_" are always
created for expect commands. So for example, "send" can be
used as "exp_send".
Generally, you should only call Expect commands via
Tcl_Eval. Certain auxiliary functions may be called
directly. They are summarized below. They may be useful in
constructing your own main. Look at the file exp_main_exp.c
in the Expect distribution as a prototype main. Another
prototype is tclAppInit.c in the Tcl source distribution. A
prototype for working with Tk is in exp_main_tk.c in the
Expect distribution.
int exp_cmdlinecmds;
int exp_interactive;
FILE *exp_cmdfile;
char *exp_cmdfilename;
int exp_tcl_debugger_available;
void exp_parse_argv(Tcl_Interp *,int argc,char **argv);
int exp_interpreter(Tcl_Interp *);
void exp_interpret_cmdfile(Tcl_Interp *,FILE *);
void exp_interpret_cmdfilename(Tcl_Interp *,char *);
void exp_interpret_rcfiles(Tcl_Interp *,int my_rc,int sys_rc);
char * exp_cook(char *s,int *len);
void (*exp_app_exit)EXP_PROTO((Tcl_Interp *);
void exp_exit(Tcl_Interp *,int status);
void exp_exit_handlers(Tcl_Interp *);
void exp_error(Tcl_Interp,char *,...);
exp_cmdlinecmds is 1 if Expect has been invoked with
commands on the program command-line (using "-c" for
example). exp_interactive is 1 if Expect has been invoked
with the -i flag or if no commands or script is being
invoked. exp_cmdfile is a stream from which Expect will
read commands. exp_cmdfilename is the name of a file which
Expect will open and read commands from.
exp_tcl_debugger_available is 1 if the debugger has been
armed.
exp_parse_argv reads the representation of the command line.
Based on what is found, any of the other variables listed
here are initialized appropriately. exp_interpreter
interactively prompts the user for commands and evaluates
them. exp_interpret_cmdfile reads the given stream and
evaluates any commands found. exp_interpret_cmdfilename
opens the named file and evaluates any commands found.
exp_interpret_rcfiles reads and evalutes the .rc files. If
my_rc is zero, then ~/.expectrc is skipped. If sys_rc is
zero, then the system-wide expectrc file is skipped.
exp_cook returns a static buffer containing the argument
reproduced with newlines replaced by carriage-return
linefeed sequences. The primary purpose of this is to allow
messages to be produced without worrying about whether the
terminal is in raw mode or cooked mode. If length is zero,
it is computed via strlen. exp_error is a printf-like
function that to interp->result.
SYNOPSIS
#include <expect.h>
int
exp_spawnl(file, arg0 [, arg1, ..., argn] (char *)0);
char *file;
char *arg0, *arg1, ... *argn;
int
exp_spawnv(file,argv);
char *file, *argv[ ];
int
exp_spawnfd(fd);
int fd;
FILE *
exp_popen(command);
char *command;
extern int exp_pid;
extern int exp_ttyinit;
extern int exp_ttycopy;
extern int exp_console;
extern char *exp_stty_init;
extern void (*exp_close_in_child)();
extern void (*exp_child_exec_prelude)();
extern void exp_close_tcl_files();
cc files... -lexpect -ltcl -lm
DESCRIPTION
exp_spawnl and exp_spawnv fork a new process so that its
stdin, stdout, and stderr can be written and read by the
current process. file is the name of a file to be executed.
The arg pointers are null-terminated strings. Following the
style of execve(), arg0 (or argv[0]) is customarily a
duplicate of the name of the file.
Four interfaces are available, exp_spawnl is useful when the
number of arguments is known at compile time. exp_spawnv is
useful when the number of arguments is not known at compile
time. exp_spawnfd is useful when an open file descriptor is
already available as a source. exp_popen is explained later
on.
If the process is successfully created, a file descriptor is
returned which corresponds to the process's stdin, stdout
and stderr. A stream may be associated with the file
descriptor by using fdopen(). (This should almost certainly
be followed by setbuf() to unbuffer the I/O.)
Closing the file descriptor will typically be detected by
the process as an EOF. Once such a process exits, it should
be waited upon (via wait) in order to free up the kernel
process slot. (Some systems allow you to avoid this if you
ignore the SIGCHLD signal).
exp_popen is yet another interface, styled after popen().
It takes a Bourne shell command line, and returns a stream
that corresponds to the process's stdin, stdout and stderr.
The actual implementation of exp_popen below demonstrates
exp_spawnl.
FILE *
exp_popen(program)
char *program;
{
FILE *fp;
int ec;
if (0 > (ec = exp_spawnl("sh","sh","-c",program,(char *)0)))
return(0);
if (NULL == (fp = fdopen(ec,"r+")) return(0);
setbuf(fp,(char *)0);
return(fp);
}
After a process is started, the variable exp_pid is set to
the process-id of the new process. The variable
exp_pty_slave_name is set to the name of the slave side of
the pty.
The spawn functions uses a pty to communicate with the
process. By default, the pty is initialized the same way as
the user's tty (if possible, i.e., if the environment has a
controlling terminal.) This initialization can be skipped
by setting exp_ttycopy to 0.
The pty is further initialized to some system wide defaults
if exp_ttyinit is non-zero. The default is generally
comparable to "stty sane".
The tty setting can be further modified by setting the
variable exp_stty_init. This variable is interpreted in the
style of stty arguments. For example, exp_stty_init =
"sane"; repeats the default initialization.
On some systems, it is possible to redirect console output
to ptys. If this is supported, you can force the next spawn
to obtain the console output by setting the variable
exp_console to 1.
Between the time a process is started and the new program is
given control, the spawn functions can clean up the
environment by closing file descriptors. By default, the
only file descriptors closed are ones internal to Expect and
any marked "close-on-exec".
If needed, you can close additional file descriptors by
creating an appropriate function and assigning it to
exp_close_in_child. The function will be called after the
fork and before the exec. (This also modifies the behavior
of the spawn command in Expect.)
If you are also using Tcl, it may be convenient to use the
function exp_close_tcl_files which closes all files between
the default standard file descriptors and the highest
descriptor known to Tcl. (Expect does this.)
The function exp_child_exec_prelude is the last function
called prior to the actual exec in the child. You can
redefine this for effects such as manipulating the uid or
the signals.
IF YOU WANT TO ALLOCATE YOUR OWN PTY
extern int exp_autoallocpty;
extern int exp_pty[2];
The spawn functions use a pty to communicate with the
process. By default, a pty is automatically allocated each
time a process is spawned. If you want to allocate ptys
yourself, before calling one of the spawn functions, set
exp_autoallocpty to 0, exp_pty[0] to the master pty file
descriptor and exp_pty[1] to the slave pty file descriptor.
The expect library will not do any pty initializations
(e.g., exp_stty_init will not be used). The slave pty file
descriptor will be automatically closed when the process is
spawned. After the process is started, all further
communication takes place with the master pty file
descriptor.
exp_spawnl and exp_spawnv duplicate the shell's actions in
searching for an executable file in a list of directories.
The directory list is obtained from the environment.
EXPECT PROCESSING
While it is possible to use read() to read information from
a process spawned by exp_spawnl or exp_spawnv, more
convenient functions are provided. They are as follows:
int
exp_expectl(fd,type1,pattern1,[re1,],value1,type2,...,exp_end);
int fd;
enum exp_type type;
char *pattern1, *pattern2, ...;
regexp *re1, *re2, ...;
int value1, value2, ...;
int
exp_fexpectl(fp,type1,pattern1,[re1,]value1,type2,...,exp_end);
FILE *fp;
enum exp_type type;
char *pattern1, *pattern2, ...;
regexp *re1, *re2, ...;
int value1, value2, ...;
enum exp_type {
exp_end,
exp_glob,
exp_exact,
exp_regexp,
exp_compiled,
exp_null,
};
struct exp_case {
char *pattern;
regexp *re;
enum exp_type type;
int value;
};
int
exp_expectv(fd,cases);
int fd;
struct exp_case *cases;
int
exp_fexpectv(fp,cases);
FILE *fp;
struct exp_case *cases;
extern int exp_timeout;
extern char *exp_match;
extern char *exp_match_end;
extern char *exp_buffer;
extern char *exp_buffer_end;
extern int exp_match_max;
extern int exp_full_buffer;
extern int exp_remove_nulls;
The functions wait until the output from a process matches
one of the patterns, a specified time period has passed, or
an EOF is seen.
The first argument to each function is either a file
descriptor or a stream. Successive sets of arguments
describe patterns and associated integer values to return
when the pattern matches.
The type argument is one of four values. exp_end indicates
that no more patterns appear. exp_glob indicates that the
pattern is a glob-style string pattern. exp_exact indicates
that the pattern is an exact string. exp_regexp indicates
that the pattern is a regexp-style string pattern.
exp_compiled indicates that the pattern is a regexp-style
string pattern, and that its compiled form is also provided.
exp_null indicates that the pattern is a null (for debugging
purposes, a string pattern must also follow).
If the compiled form is not provided with the functions
exp_expectl and exp_fexpectl, any pattern compilation done
internally is thrown away after the function returns. The
functions exp_expectv and exp_fexpectv will automatically
compile patterns and will not throw them away. Instead,
they must be discarded by the user, by calling free on each
pattern. It is only necessary to discard them, the last
time the cases are used.
Regexp subpatterns matched are stored in the compiled
regexp. Assuming "re" contains a compiled regexp, the
matched string can be found in re->startp[0]. The match
substrings (according to the parentheses) in the original
pattern can be found in re->startp[1], re->startp[2], and so
on, up to re->startp[9]. The corresponding strings ends are
re->endp[x] where x is that same index as for the string
start.
The type exp_null matches if a null appears in the input.
The variable exp_remove_nulls must be set to 0 to prevent
nulls from being automatically stripped. By default,
exp_remove_nulls is set to 1 and nulls are automatically
stripped.
exp_expectv and exp_fexpectv are useful when the number of
patterns is not known in advance. In this case, the sets
are provided in an array. The end of the array is denoted
by a struct exp_case with type exp_end. For the rest of
this discussion, these functions will be referred to
generically as expect.
If a pattern matches, then the corresponding integer value
is returned. Values need not be unique, however they should
be positive to avoid being mistaken for EXP_EOF,
EXP_TIMEOUT, or EXP_FULLBUFFER. Upon EOF or timeout, the
value EXP_EOF or EXP_TIMEOUT is returned. The default
timeout period is 10 seconds but may be changed by setting
the variable exp_timeout. A value of -1 disables a timeout
from occurring. A value of 0 causes the expect function to
return immediately (i.e., poll) after one read(). However
it must be preceded by a function such as select, poll, or
an event manager callback to guarantee that there is data to
be read.
If the variable exp_full_buffer is 1, then EXP_FULLBUFFER is
returned if exp_buffer fills with no pattern having matched.
When the expect function returns, exp_buffer points to the
buffer of characters that was being considered for matching.
exp_buffer_end points to one past the last character in
exp_buffer. If a match occurred, exp_match points into
exp_buffer where the match began. exp_match_end points to
one character past where the match ended.
Each time new input arrives, it is compared to each pattern
in the order they are listed. Thus, you may test for
absence of a match by making the last pattern something
guaranteed to appear, such as a prompt. In situations where
there is no prompt, you must check for EXP_TIMEOUT (just
like you would if you were interacting manually). More
philosophy and strategies on specifying expect patterns can
be found in the documentation on the expect program itself.
See SEE ALSO below.
Patterns are the usual C-shell-style regular expressions.
For example, the following fragment looks for a successful
login, such as from a telnet dialogue.
switch (exp_expectl(
exp_glob,"connected",CONN,
exp_glob,"busy",BUSY,
exp_glob,"failed",ABORT,
exp_glob,"invalid password",ABORT,
exp_end)) {
case CONN: /* logged in successfully */
break;
case BUSY: /* couldn't log in at the moment */
break;
case EXP_TIMEOUT:
case ABORT: /* can't log in at any moment! */
break;
default: /* problem with expect */
}
Asterisks (as in the example above) are a useful shorthand
for omitting line-termination characters and other detail.
Patterns must match the entire output of the current process
(since the previous read on the descriptor or stream). More
than 2000 bytes of output can force earlier bytes to be
"forgotten". This may be changed by setting the variable
exp_match_max. Note that excessively large values can slow
down the pattern matcher.
RUNNING IN THE BACKGROUND
extern int exp_disconnected;
int exp_disconnect();
It is possible to move a process into the background after
it has begun running. A typical use for this is to read
passwords and then go into the background to sleep before
using the passwords to do real work.
To move a process into the background, fork, call
exp_disconnect() in the child process and exit() in the
parent process. This disassociates your process from the
controlling terminal. If you wish to move a process into
the background in a different way, you must set the variable
exp_disconnected to 1. This allows processes spawned after
this point to be started correctly.
MULTIPLEXING
By default, the expect functions block inside of a read on a
single file descriptor. If you want to wait on patterns
from multiple file descriptors, use select, poll, or an
event manager. They will tell you what file descriptor is
ready to read.
When a file descriptor is ready to read, you can use the
expect functions to do one and only read by setting timeout
to 0.
SLAVE CONTROL
void
exp_slave_control(fd,enable)
int fd;
int enable;
Pty trapping is normally done automatically by the expect
functions. However, if you want to issue an ioctl, for
example, directly on the slave device, you should temporary
disable trapping.
Pty trapping can be controlled with exp_slave_control. The
first argument is the file descriptor corresponding to the
spawned process. The second argument is a 0 if trapping is
to be disabled and 1 if it is to be enabled.
ERRORS
All functions indicate errors by returning -1 and setting
errno.
Errors that occur after the spawn functions fork (e.g.,
attempting to spawn a non-existent program) are written to
the process's stderr, and will be read by the first expect.
SIGNALS
extern int exp_reading;
extern jmp_buf exp_readenv;
expect uses alarm() to timeout, thus if you generate alarms
during expect, it will timeout prematurely.
Internally, expect calls read() which can be interrupted by
signals. If you define signal handlers, you can choose to
restart or abort expect's internal read. The variable,
exp_reading, is true if (and only if) expect's read has been
interrupted. longjmp(exp_readenv,EXP_ABORT) will abort the
read. longjmp(exp_readenv,EXP_RESTART) will restart the
read.
LOGGING
extern int exp_loguser;
extern int exp_logfile_all
extern FILE *exp_logfile;
If exp_loguser is nonzero, expect sends any output from the
spawned process to stdout. Since interactive programs
typically echo their input, this usually suffices to show
both sides of the conversation. If exp_logfile is also
nonzero, this same output is written to the stream defined
by exp_logfile. If exp_logfile_all is non-zero, exp_logfile
is written regardless of the value of exp_loguser.
DEBUGGING
While I consider the library to be easy to use, I think that
the standalone expect program is much, much, easier to use
than working with the C compiler and its usual edit,
compile, debug cycle. Unlike typical C programs, most of
the debugging isn't getting the C compiler to accept your
programs - rather, it is getting the dialogue correct.
Also, translating scripts from expect to C is usually not
necessary. For example, the speed of interactive dialogues
is virtually never an issue. So please try the standalone
'expect' program first. I suspect it is a more appropriate
solution for most people than the library.
Nonetheless, if you feel compelled to debug in C, here are
some tools to help you.
extern int exp_is_debugging;
extern FILE *exp_debugfile;
While expect dialogues seem very intuitive, trying to codify
them in a program can reveal many surprises in a program's
interface. Therefore a variety of debugging aids are
available. They are controlled by the above variables, all
0 by default.
Debugging information internal to expect is sent to stderr
when exp_is_debugging is non-zero. The debugging
information includes every character received, and every
attempt made to match the current input against the
patterns. In addition, non-printable characters are
translated to a printable form. For example, a control-C
appears as a caret followed by a C. If exp_logfile is non-
zero, this information is also written to that stream.
If exp_debugfile is non-zero, all normal and debugging
information is written to that stream, regardless of the
value of exp_is_debugging.
CAVEATS
The stream versions of the expect functions are much slower
than the file descriptor versions because there is no way to
portably read an unknown number of bytes without the
potential of timing out. Thus, characters are read one at a
time. You are therefore strongly encouraged to use the file
descriptor versions of expect (although, automated versions
of interactive programs don't usually demand high speed
anyway).
You can actually get the best of both worlds, writing with
the usual stream functions and reading with the file
descriptor versions of expect as long as you don't attempt
to intermix other stream input functions (e.g., fgetc). To
do this, pass fileno(stream) as the file descriptor each
time. Fortunately, there is little reason to use anything
but the expect functions when reading from interactive
programs.
There is no matching exp_pclose to exp_popen (unlike popen
and pclose). It only takes two functions to close down a
connection (fclose() followed by waiting on the pid), but it
is not uncommon to separate these two actions by large time
intervals, so the function seems of little value.
If you are running on a Cray running Unicos (all I know for
sure from experience), you must run your compiled program as
root or setuid. The problem is that the Cray only allows
root processes to open ptys. You should observe as much
precautions as possible: If you don't need permissions,
setuid(0) only immediately before calling one of the spawn
functions and immediately set it back afterwards.
Normally, spawn takes little time to execute. If you notice
spawn taking a significant amount of time, it is probably
encountering ptys that are wedged. A number of tests are
run on ptys to avoid entanglements with errant processes.
(These take 10 seconds per wedged pty.) Running expect with
the -d option will show if expect is encountering many ptys
in odd states. If you cannot kill the processes to which
these ptys are attached, your only recourse may be to
reboot.
BUGS
The exp_fexpect functions don't work at all under HP-UX - it
appears to be a bug in getc. Follow the advice (above)
about using the exp_expect functions (which doesn't need to
call getc). If you fix the problem (before I do - please
check the latest release) let me know.
SEE ALSO
An alternative to this library is the expect program.
expect interprets scripts written in a high-level language
which direct the dialogue. In addition, the user can take
control and interact directly when desired. If it is not
absolutely necessary to write your own C program, it is much
easier to use expect to perform the entire interaction. It
is described further in the following references:
"expect: Curing Those Uncontrollable Fits of Interactivity"
by Don Libes, Proceedings of the Summer 1990 USENIX
Conference, Anaheim, California, June 11-15, 1990.
"Using expect to Automate System Administration Tasks" by
Don Libes, Proceedings of the 1990 USENIX Large Installation
Systems Administration Conference, Colorado Springs,
Colorado, October 17-19, 1990.
expect(1), alarm(3), read(2), write(2), fdopen(3),
execve(2), execvp(3), longjmp(3), pty(4).
There are several examples C programs in the test directory
of expect's source distribution which use the expect
library.
AUTHOR
Don Libes, libes@nist.gov, National Institute of Standards
and Technology
ACKNOWLEDGEMENTS
Thanks to John Ousterhout (UCBerkeley) for supplying the
pattern matcher.
Design and implementation of the expect library was paid for
by the U.S. government and is therefore in the public
domain. However the author and NIST would like credit if
this program and documentation or portions of them are used.