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NAME | DESCRIPTION | SYNOPSIS | SYNOPSIS | DESCRIPTION | IF YOU WANT TO ALLOCATE YOUR OWN PTY | EXPECT PROCESSING | RUNNING IN THE BACKGROUND | MULTIPLEXING | SLAVE CONTROL | ERRORS | SIGNALS | LOGGING | DEBUGGING | CAVEATS | BUGS | SEE ALSO | AUTHOR | ACKNOWLEDGEMENTS | COLOPHON |
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LIBEXPECT(3) Library Functions Manual LIBEXPECT(3)
libexpect - programmed dialogue library with interactive programs
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.
#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 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 writes the result to
interp->result.
#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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Don Libes, libes@nist.gov, National Institute of Standards and
Technology
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.
This page is part of the expect (programmed dialogue with
interactive programs) project. Information about the project can
be found at ⟨https://core.tcl.tk/expect/index⟩. If you have a bug
report for this manual page, see
⟨https://sourceforge.net/p/expect/bugs/⟩. This page was obtained
from the tarball expect5.45.3.tar.gz fetched from
⟨http://sourceforge.net/projects/expect/files/Expect/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
12 December 1991 LIBEXPECT(3)
Pages that refer to this page: expect(1)
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