FBB::Process(3bobcat) Running Child Processes FBB::Process(3bobcat)
NAME
FBB::Process - Runs external programs
SYNOPSIS
#include <bobcat/process>
Linking option: -lbobcat
DESCRIPTION
The FBB::Process class offers an elaborate interface to external programs and/or scripts from a C++ program (so-called child-processes).
The class offers an easy to use, stream-based interface to the child process's standard input, standard output and standard error streams.
Objects of the FBB::Process class use standard process-executing functions, like sh(1) and members of the execl(1) family to execute the
child process, and allow shell-scripts to be executed as well.
The standard input, output and error streams of executed child processes may be accessed via their Process parent objects. Input expected
by the child process may be inserted into the Process object, output generated by the child process may be extracted from the Process
object. Process objects cannot use these streams to communicate with the child process's standard streams when the child processes them-
selves redirect their standard streams.
When using (output) redirection with the USE_SHELL path specification (see below for the path and IOMode specifications) the IGNORE_COUT
IOMode (and possibly IGNORE_CERR) will normally be specified as well. See also section PIPING below.
The same Process object may be used repeatedly to execute the same or different child processes in sequence. If a previously started child
process is still active it will first be terminated. It is also possible (using the stop member) to end a child process explicitly.
Programs to call as child processes may be specified using Process's constructors. The child processes are not started by Process construc-
tors. To start a child process the start members or the assignment operator may be used.
Some child processes continue until their standard input streams are exhausted. The close member is provided to close such streams, thus
ending such child processes.
The class Process cannot be used to construct daemons as Process's destructor forcefully terminates its child process. To create daemon
processes the Fork(3bobcat) class can be used.
Command line arguments passed to child processes may be surrounded by double or single quotes. Arguments surrounded by double quotes will
have their double quotes removed, interpreting any escape-sequences that may have been used within. Arguments surrounded by single quotes
will have their single quotes removed, accepting their contents unmodified.
Child processes may be allowed a limited amount of time (in seconds) to complete. Alternatively, child processes may have no time limit
imposed upon then. A child process is forcefully terminated when its parent Process object goes out of scope. Process makes FBB::Fork's
waitForChild member available, which may be used to prevent a premature termination of a child process.
If waitForChild is not called but information sent to the child which could not be fully processed by the child process because the child
process terminated as a result of the Process object going out of scope, then the operating system issues a Broken pipe message, indicating
that information in a pipe was lost.
By default the standard input and output streams of child processes are accessed from their Process parent processes: information inserted
into the Process object is forwarded to the child process's standard input stream, information sent by the child process to its standard
output stream can be extracted from its parent Process object.
NAMESPACE
FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.
INHERITS FROM
FBB::Fork(3bobcat) (private),
FBB:IOStream(3bobcat)
ENUMERATIONS
enum ProcessType:
This enumeration has the following values:
o NO_PATH:
This value indicates that the external program should be called as specified, without searching the elements in the PATH environment
variable.
o USE_PATH:
This value indicates that the PATH environment variable should be used to locate the external program to be executed.
o USE_SHELL:
This value indicates that the external program should be called as a command (using its -c option) to /bin/sh. When (output) redi-
rection is used with the specified command the IGNORE_COUT IOMode (and possibly the IGNORE_CERR IOMode as well) should be specified.
enum IOMode:
The IOMode specification is used to define which of the standard streams used by child processes are accessed through the Process object.
Sensible combinations may be formed using the bit_or operator. If no IOMode is specified, then CIN | COUT | CERR is used (see below). This
is also used by Process's default constructor.
This enumeration defines the following values:
o CIN:
Information inserted into the Process object is forwarded to its child process. If this is not required then the CIN specification
can be omitted (but note IGNORE_ALL below).
o CERR:
Information written by the child process to its standard error stream is accessible through the cerr member. If this is not required
then the CERR specification can be omitted (but note IGNORE_ALL below).
o COUT:
Information extracted from the Process object was written by the child process to its standard output stream. If this is not
required then the COUT specification can be omitted (but note IGNORE_ALL below).
o DIRECT:
When starting the child process (see below at the member start) the current process is replaced by the child process, inheriting the
current process's standard input and output streams. When this mode is specified in combination with other IOMode values (except for
STD, see below) an std::invalid_argument exception is thrown.
o IGNORE_CERR:
Information written by the child process to its standard error stream is sent to /dev/null. This specification is silently ignored
if either CERR or MERGE_COUT_CERR were specified.
o IGNORE_COUT:
Information written by the child process to its standard output stream is sent to /dev/null. This specification is silently ignored
when either COUT or MERGE_COUT_CERR were specified.
o MERGE_COUT_CERR:
Information extracted from the Process object is written by the child process to its standard output and standard error streams.
o STD:
The Process object will not extract information from or insert information into the standard streams of its child process. The child
process read from the same standard input stream and write to the same standard output streams as used by its parent Process object.
When this mode is specified in combination with other IOMode values it is silently ignored. The IOMode enumeration also defines the
enumeration values IN_PIPE, OUT_PIPE and CLOSE_ON_EXEC. These enumeration values are ignored when specified by users of the class
Process and are for internal use only.
enum ChildOutput:
The ChildOutput enumeration defines values returned by the available member (see below) indicating to which standard stream the child
process has written information. This enumeration defines the following values:
o NOTHING_AVAILABLE:
This value indicates that the child process did not (yet) write any information to its standard streams;
o CHILD_COUT:
This value indicates that the child process did write information to its standard output stream which is waiting for extraction.
o CHILD_CERR:
This value indicates that the child process did write information to its standard error stream which is waiting for extraction. The
latter two values may be combined using the bit_or operator indicating that information on both standard streams is available.
TYPE
o IOMode:
Combinations of values of the enum IOMode may be used as value of a variable of this type. In functions expecting an IOMode argument
the empty set should not normally be specified. To specify multiple IOMode values the bit_or operator should be used.
PROCESS PARAMETERS
When running a child process three process parameters may be specified: the child streams to access from the Process object (as an IOMode
value); the way to locate or start the child program (as a ProcessType value); and the maximum time (in seconds) the child program is
allowed to run.
Unless specified otherwise, all the child's standard streams (standard input, output and error) are accessible from the Parent process; the
PATH environment variable will not be used to locate the child program to be executed (often resulting in the requirement to provide an
absolute path to the intended program) and the child processes will be allowed unlimited time to run.
Following the construction of a Process object all default parameter values may be modified. Process parameters may be altered for a single
process or the general defaults may be modified. The setXXX members (see below) may be used to change the default process parameters. When
process parameters are specified otherwise they will be active for the next process only.
CONSTRUCTORS
The command that may be provided to the following constructors may be the (initial part of the) specification of an external program to
run. When the program is eventually started it may start and end with a back-tick (`). The back-ticks will be removed just before the spec-
ified program is executed.
A child process is not started automatically following the object construction. A start member or the assignment operator (see below) can
be used to start the specified child process.
o Process(std::string const &cmd = ""):
This constructor can also be used as default constructor. It can be used to specify the (initial part of a) command to execute from
a Process object. Standard default values are used for the process parameters (see section PROCESS PARAMETERS).
o Process(IOMode mode, std::string const &cmd = ""):
This constructor requires the specification of the object's initial default IOMode setting and it can be used to specify the (ini-
tial part of a) command to execute from a Process object. Standard default values are used for the remaining two process parameters
((NO_PATH) and no time limit imposed on the child process).
o Process(IOMode mode, ProcessType type, std::string const &cmd = ""):
This constructor requires the specification of the object's initial default IOMode setting and ProcessType and it can be used to
specify the (initial part of a) command to execute from a Process object. By default no time limit will be imposed upon the child
process.
o Process(IOMode mode, ProcessType type, size_t timeLimit, std::string const &cmd = ""):
This constructor requires the specification of the object's initial default IOMode setting, ProcessType and time limit imposed upon
child processes it can be used to specify the (initial part of a) command to execute from a Process object. To prevent a time limit
from being imposed upon the child process specify a time limit of 0 (zero).
The class Process does not offer copy and move constructors.
OVERLOADED OPERATORS
o Process &operator<<(Type value):
This operator inserts value into the child's standard input stream. I.e., the child process reads value from its standard input. A
value of any type that can be inserted into an ostream can be inserted into a Process object. Nothing happens if the member is used
when the child process has terminated. The behavior of this operator is undefined unless IOMode CIN was specified.
o Process &operator>>(Type value):
This operator extracts value from the child's standard output stream and optionally (if IOMode MERGE_COUT_CERR was specified) from
the child's error stream. I.e., value may be extracted from Process objects. A value of any type that can be extracted from an
istream can be extracted from a Process object. Nothing happens if the member is used when the child process has terminated. The
behavior of this operator is undefined unless IOMode COUT or MERGE_COUT_CERR was specified.
o Process &operator+=(std::string const &):
This operator adds the provided std::string object to the command specification currenly stored in a Process object. The currently
stored command specification may be redefined using the member setCommand (see below). The member operator+= does not add a sepa-
rating blank space between the currently stored command specification and the text to append. It merely adds its right-hand side
string to the command stored so far. It does not affect a currently running child process.
o int operator=(std::string const &cmd):
The operator= member defines cmd as the stored command in a Process object. If the command starts and ends with a back-tick (`)
then the back-ticks are removed, and the resulting contents are interpreted as a command to execute.
Next it will call stop (see below) to end any ongoing process followed by calling start to execute the newly defined command using
the current default process parameters. It returns stop's return value.
o Process &operator()(iomode mode):
This operator changes the IOMode of the next child process. It returns the Process object allowing constructions like
process(COUT) = "/bin/cat";
to start a new child process with the specified IOMode.
o Process &operator()(iomode mode, ProcessType type):
This operator changes the IOMode and ProcessType process parameters of the next child process.
o Process &operator()(iomode mode, ProcessType type, size_t timeLimit):
This operator changes all three process parameters of the next child process. Time limit 0 prevents a time limit from being imposed
upon the next child process.
o Process &operator|(Process &lhs, Process &rhs):
This operator implements piping: information sent to the lhs's standard output is passed on to the rhs's standard input. The opera-
tor returns rhs. This operator mimics the piping-operator supported by most command-shell programs and should not be confused with
the binary-or operator. Before returning, the lhs's child process is started, but the rhs's child process isn't.
Since the operator is left-associative and rhs is returned piping can be chained, allowing constructions like p1 | p2 | p3, where
p1, p2 and p3 are Process objects.
The following idiom can be used to start the execution of the chain of processes: (p1 | p2 | p3).start(). Alternatively, the follow-
ing two-step procedure can be used:
p1 | p2 | p3;
p3.start();
If p1 specifies Process::CIN then this IOMode is forwared to the final process of the chain of processes. It is not necessary to
specify Process::CIN for p3. In fact, all IOMode flags of processes passed to operator| are ignored and possibly modified except for
Process::IGNORE_CERR and Process::CERR (for all processes), Process::CIN (for the first process of the chain) and Process::COUT (for
the last process of the chain).
The following example illustrates how input can be inserted into the first process from a main process and sent to the standard out-
put stream by the final process. Note that, due to the forwarding of Process::CIN information must actually be inserted into the
final process, p3:
using namespace std;
using namespace FBB;
Process p1(Process::CIN, "/bin/cat");
Process p2("/bin/cat");
Process p3(Process::STD, "/bin/cat");
p1 | p2 | p3;
p3.start();
string line;
while (getline(cin, line) && not line.empty())
p3 << line << endl;
p3.close();
p3.waitForChild();
The default overloaded assignment operator is not available.
MEMBERS
o bool active():
This member returns true if the child process is currently running and false if not. It is automatically called from the insertion
and extraction operators and from the cerr member but is available as a public member as well. It may terminate the child process if
the child process has timed out.
o size_t available():
This member returns immediately. Its return value indicates whether any information can be obtained from the child process as
value(s) from the enum ChildOutput. NOTHING_AVAILABLE is returned if no information is ready for extraction. CHILD_COUT is returned
if information from the child process's standard output stream is available; CHILD_CERR is returned if information from the child
process's standard error stream is available; CHILD_COUT | CHILD_CERR is returned if information from both the standard output and
standard error streams is available.
o std::istream &cerr():
This member may be used to extract information written by the child process to its standard error stream. This member should only
be used when IOMode CERR was specified for the currently running child process; otherwise its behavior is undefined.
o void close():
This member may be called from the parent process to close the child's input stream. In situations where the child continuously
reads information from its standard input stream this member must be used to inform the child process that input has terminated.
This member should only be used when IOMode CIN was specified for the currently running child process; otherwise its behavior is
undefined.
o iomode ioMode() const:
This member returns the current default IOMode. Note that the actual IOMode that will be used may be different from the default
value as the actual value may have been altered using a function call operator (see operator() above).
o ProcessType processType() const:
This member returns the current default ProcessType. Note that the actual ProcessType that will be used may be different from the
default value as the actual value may have been altered using a function call operator (see operator() above).
o size_t timeLimit() const:
This member returns the current default child process time limit. Note that the actual time limit that will be used may be differ-
ent from the default value as the actual value may have been altered using a function call operator (see operator() above).
o void setCommand(std::string const &cmd):
The setCommand member (re)defines the (initial part of a) command specification currently stored in an Process object. When the pro-
gram is eventually started it may start and end with a back-tick (`). The back-ticks will be removed just before the specified pro-
gram is executed.
The setCommand member will not start the specified command and operator+= may be used to append the command specification before the
command is eventually executed. Also, this member does not affect a currently running child process.
o void setIOMode(iomode mode):
This member will change the current default IOMode process parameter of child processes.
o void setProcessType(ProcessType type):
This member will change the current default ProcessType process parameter of child processes.
o void setTimeLimit(size_t timeLimit):
This member will change the current default time limit process parameter of child processes. No time limit will be imposed upon
child processes if timeLimit 0 is specified.
o int start():
The currently specified command is executed using the currently active process parameters. These process parameters may be modified
(either as new defaults or for the next process only) using overloaded start members, the system members, function call operator or
various set-members.
Having specified a command to execute, the first white-space delimited element of the specified command is used as the name of the
program to execute. If the program should be called as a command to be executed by sh(1), the USE_SHELL ProcessType or a system
member should be used.
If a child process does not terminate by itself (within its allotted amount of time) it is terminated when it has run for its
alloted time; when the Process object's start or stop members are called; when the object's assignment operator is used; or when the
object goes out of scope.
o int start(IOMode mode):
The currently specified command is executed using the specified IOMode rather than the currently specified default but otherwise
using the currently specified default process parameters. The specified IOMode will only be used for the process executed by this
start member.
o int start(IOMode mode, Program program):
The currently specified command is executed using the specified IOMode and ProcessType as well as the currently specified child
process time limit. The specified arguments will only be used for the process executed by this start member.
o int start(IOMode mode, Program program, size_t timeLimit):
The currently specified command is executed using the specified arguments for the process parameters. The specified arguments will
only be used for the process executed by this start member.
o std::string const &str() const:
This member returns the text of the command currently stored in the Process object. It shows the command as it will be executed by
start, system or the assignment operator. If the command's first and last characters are back-ticks (`) then those back-ticks will
be removed when the command is actually executed.
o void system():
This member executes the currently stored command as a command to sh(1) (therefore executing the command as a shell command). When
using system redirections can be included in the command itself (although this might render the redirected streams implied by the
current IOMode) useless. The currently set process parameters are used when sh(1) is executed. Calling system implies calling
start.
o void system(IOMode mode):
This member executes the currently stored command as a command to sh(1) (cf. system above) using the specified IOMode rather than
the current default IOMode setting. The currently specified default time limit, however, will be used when executing the sh(1)
process.
o void system(IOMode mode, size_t timeLimit):
This member executes the currently stored command as a command to sh(1) (cf. system above) using the specified IOMode and time limit
values rather than their current default settings.
o int stop():
This member terminates a currently active child process. The child process is twice sent a SIG_TERM signal, followed by a SIG_KILL
signal. This member's return value is the exit-value of the child process that was stopped. It is underfined if it was called with-
out a running child process.
Following stop a new command may be called using start, system or the assignment operator (see earlier). Those members will first
call stop so when the intention is to start another child process calling stop can be skipped. Also, stop is called when the Process
object goes out of scope.
o int waitForChild():
This member is inherited from the class FBB::Fork and will wait for a child process to finish. It can be used to prevent premature
termination of a child process before calling stop. It is not always required to use waitForChild. E.g., when a process writes to
its standard output stream and all output has been read then the child process can be stopped without calling waitForChild.
USING PIPING IN COMMANDS
When specifying multiple commands using the piping operator ('|') USE_SHELL must be specified as piping using the piping operator is a
shell-feature.
The additional shell process may be avoided by explicitly setting up the stdin-stdout piping using Process's operator|.
EXAMPLE
#ifdef BOBCAT
#include <bobcat/process>
#else
#include "process"
#endif
#include <string>
#include <iostream>
#include <climits>
using namespace std;
using namespace FBB;
void prompt(char const *task)
{
cout << "Press Enter to start " << task << endl;
cin.ignore(INT_MAX, '
');
}
int main(int argc, char **argv)
try
{
cout << "Size of Process: " << sizeof(Process) << endl;
string line;
// Nota bene: without IOMode you get CIN, COUT and CERR
Process p1(Process::CIN, "/bin/cat");
Process p2(Process::STD, "/bin/cat");
Process p3(Process::STD, "/bin/cat");
prompt("sending lines (until empty) to cat | cat | cat");
(p1 | p2 | p3).start();
while (getline(cin, line) && not line.empty())
{
cout << "Entering " << line << endl;
p3 << line << endl;
}
p3.close();
p3.waitForChild();
Process process(Process::CIN | Process::COUT,
"/usr/bin/sha1sum");
prompt("sha1sum");
process.start();
process << "Hello world
"; // input to sha1sum
process.close();
process >> line; // retrieve the value
cout << line << endl;
process.stop();
prompt("cat, ignoring its output");
process.setCommand("/bin/cat");
process.setIOMode(Process::CIN | Process::IGNORE_COUT);
process.start();
process << "Hello world
"; // input to sha1sum
process.close();
line.clear();
if (process >> line) // retrieve the value
cout << ">>>" << line << "<<<" << endl;
process.stop();
// if (argc > 1) // sending an e-mail
// {
// cout << "Sending mail to " << argv[1] << endl;
// prompt("/usr/bin/mail");
// process.setCommand("/usr/bin/mail -s 'from Process' ");
// process += argv[1];
// process.start(Process::CIN);
// process << "This mail was sent by the process drive
";
// process << "It consists of multiple lines of text
";
// process.close();
// process.waitForChild();
// }
prompt("5 seconds IO to /bin/cat");
process.setIOMode(Process::CIN | Process::COUT);
process.setTimeLimit(5); // change time limit
process = "/bin/cat";
while (process.active())
{
cout << "? ";
getline(cin, line);
process << line << endl; // to /bin/cat
line.clear();
if (!getline(process, line)) // from /bin/cat
break;
cout << "Received: " << line << endl;
}
cout << "/bin/cat forcefully terminated
";
process.setTimeLimit(0);
cout << "3 times running /bin/ls
";
for (size_t trial = 0; trial < 3; ++trial)
{
prompt("ls");
process(Process::COUT) = "/bin/ls";
cerr << process.str() << endl;
size_t count = 0;
while (getline(process, line))
cout << ++count << ": " << line << endl;
}
}
catch (Errno const &err)
{
cerr << "EXCEPTION CAUGHT: " << err.why() << endl;
return 1;
}
catch (bool)
{
return 0;
}
catch (...)
{
cerr << "Unrecognized exception in main()
";
return 0;
}
FILES
bobcat/process - defines the class interface
SEE ALSO
bobcat(7), execle(3), fork(3bobcat), iostream(3fork), sh(1)
BUGS
With the release of Bobcat 1.21.1 the class Process was completely rewritten. The new implementation, however, should not affect existing
programs other than that Process will no longer impose a limited time-to-live upon child processes. The interface was enlarged, but this
should not affect existing programs. The internal organization of the Process class has changed though, requiring recompilation of sources
defining Process class type objects and linking dynamically to the Bobcat library.
With the release of Bobcat 2.11.0 another major modification of Process was realized. Although Process's internal organization was again
modified this does not affect exeisting programs using Process objects. No recompilation of existing sources using Process is required.
DISTRIBUTION FILES
o bobcat_3.01.00-x.dsc: detached signature;
o bobcat_3.01.00-x.tar.gz: source archive;
o bobcat_3.01.00-x_i386.changes: change log;
o libbobcat1_3.01.00-x_*.deb: debian package holding the libraries;
o libbobcat1-dev_3.01.00-x_*.deb: debian package holding the libraries, headers and manual pages;
o http://sourceforge.net/projects/bobcat: public archive location;
BOBCAT
Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.
COPYRIGHT
This is free software, distributed under the terms of the GNU General Public License (GPL).
AUTHOR
Frank B. Brokken (f.b.brokken@rug.nl).
libbobcat1-dev_3.01.00-x.tar.gz 2005-2012 FBB::Process(3bobcat)