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LIBEXPECT(3) LIBEXPECT(3)
NAME
libexpect - programmed dialogue library with interactive programs
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
Expect_Init(interp);
cc files... -lexpect5.20 -ltcl7.5 -lm
Note: library versions may differ in the actual release.
The Expect_Init function adds expect commands to the named interpreter. It avoids over-
writing commands that already exist, however aliases beginning with "exp_" are always cre-
ated 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 inter-
actively 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 writes the result 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 com-
pile 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 descrip-
tor 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 vari-
able 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_con-
sole 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 sig-
nals.
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 auto-
matically allocated each time a process is spawned. If you want to allocate ptys your-
self, 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 speci-
fied 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 pat-
tern 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 func-
tions 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_TIME-
OUT, 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 typi-
cal 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 com-
piler to accept your programs - rather, it is getting the dialogue correct. Also, trans-
lating 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 avail-
able. 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 ver-
sions 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 inter-
mix 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 con-
trol 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, Col-
orado, 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 distribu-
tion which use the expect library.
AUTHOR
Don Libes, libes AT 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.
12 December 1991 LIBEXPECT(3)
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