libexpect(3) — Linux manual page


LIBEXPECT(3)            Library Functions Manual            LIBEXPECT(3)

NAME         top

       libexpect - programmed dialogue library with interactive programs

DESCRIPTION         top

       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         top

       #include expect_tcl.h

       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

       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.

SYNOPSIS         top

       #include <expect.h>

       exp_spawnl(file, arg0 [, arg1, ..., argn] (char *)0);
       char *file;
       char *arg0, *arg1, ... *argn;

       char *file, *argv[ ];

       int fd;

       FILE *
       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         top

       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 *
       char *program;
            FILE *fp;
            int ec;

            if (0 > (ec = exp_spawnl("sh","sh","-c",program,(char *)0)))
            if (NULL == (fp = fdopen(ec,"r+")) return(0);
            setbuf(fp,(char *)0);

       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

       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

       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

       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 fd;
       enum exp_type type;
       char *pattern1, *pattern2, ...;
       regexp *re1, *re2, ...;
       int value1, value2, ...;

       FILE *fp;
       enum exp_type type;
       char *pattern1, *pattern2, ...;
       regexp *re1, *re2, ...;
       int value1, value2, ...;

       enum exp_type {

       struct exp_case {
       char *pattern;
       regexp *re;
       enum exp_type type;
       int value;

       int fd;
       struct exp_case *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

       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

       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,"invalid password",ABORT,
                 exp_end)) {
            case CONN:     /* logged in successfully */
            case BUSY:     /* couldn't log in at the moment */
            case EXP_TIMEOUT:
            case ABORT:    /* can't log in at any moment! */
            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

MULTIPLEXING         top

       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         top

       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

       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         top

       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         top

       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         top

       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         top

       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

CAVEATS         top

       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         top

       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         top

       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         top

       Don Libes,, National Institute of Standards and


       Thanks to John Ousterhout (UCBerkeley) for supplying the pattern

       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.

COLOPHON         top

       This page is part of the expect (programmed dialogue with
       interactive programs) project.  Information about the project can
       be found at ⟨⟩.  If you have a
       bug report for this manual page, see
       ⟨⟩.  This page was obtained
       from the tarball expect5.45.3.tar.gz fetched from
       ⟨⟩ on
       2024-06-14.  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

                            12 December 1991                LIBEXPECT(3)

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