In my C++ program (in linux), I can open a pipe for writing and set values for Gnuplot program.
FILE *pipe = NULL;
#ifdef WIN32
pipe = _popen("pgnuplot -persist", "w");
#else
pipe = popen("gnuplot", "w");
#endif
if(pipe == NULL)
error("Could not open pipe for write!");
// set title name
fprintf(pipe, "set title 'Sample Points' \n");
Now I need to get the Gnuplot version. The show version command does this but how I can send this command and then read the value. Opening a pipe for reading seems to not work for me and the code stuck in the while loop without getting any data.
FILE* pipe = popen(command, "r");
if (!pipe)
{
std::cout << "failed! (can not open pipe)" << endl;
return;
}
char buffer[128];
std::string result = "";
while(!feof(pipe))
{
if(fgets(buffer, 128, pipe) != NULL)
result += buffer;
}
pclose(pipe);
Since on my Debian/Linux/Sid/x86-64 the command gnuplot --version is outputting to stdout the following line:
gnuplot 5.0 patchlevel 1
I would simply recommend
FILE* pipversion = popen("gnuplot --version", "r");
if (!pipversion) { perror("popen gnuplot"); exit(EXIT_FAILURE); };
char lineversion[128];
memset (lineversion, 0, sizeof(lineversion));
if (!fgets(lineversion, sizeof(lineversion), pipversion) {
perror("fgets"); exit(EXIT_FAILURE);
}
/// lineversion is like: gnuplot 5.0 patchlevel 1
int majvers=0, minvers=0, pos= -1;
char* restvers = NULL;
if (sscanf(lineversion, "gnuplot %d.%d %n", &majvers, &minvers, &pos) >= 2) {
assert (pos>=0);
restvers = lineversion+pos;
};
pclose(pipversion);
pipversion = NULL;
After that, majvers contains the major version of gnuplot (e.g. 5 in my case) and minvers contains the minor version (e.g. 0), with restvers being a suffix string (e.g. "patchlevel 1" without the quotes).
There might be a potential race condition in the unusual and unlikely case that gnuplot is updated between this popen and the next one pipe = popen("gnuplot", "w");. BTW, naming a variable pipe is poor taste, since POSIX and Linux have the pipe(2) system call. But I don't think it is worth caring about that race condition.
BTW, you very probably want to replace your second pipe = popen("gnuplot", "w"); with an explicit double invocation of pipe(2) (followed by appropriate fork(2) & execvp(3) ...) to have both input and output pipes to gnuplot, and manage them in your own event loop (probably around poll(2) ... see this & that answers).
(if you application has or uses its own event loop, in particular if it is a GUI application above Qt or GTK, you want to use the same event loop for the pipes; details are specific to the library providing that event loop: g_spawn_async_with_pipes & g_source_add_unix_fd for GTK, QProcess for Qt ... )
I don't have time to explain how to do that (double piping into command + event loop) in details, but the Advanced Linux Programming book (available online) has several chapters on that. Be aware that you need some event loop.
Related
In our C++ project under MacOS X, we are using stdio for interaction with clients. Yet, the dylib we are using is also using stdio to print "open log files". This breaks down all communication between our clients and the application. I looked at the examples of redirecting stdio to a file or temporarily disabling stdio. Yet, we could not succeed in it.
So, how can we disable or redirect stdio temporarily while interacting with the dylib.
void RedirectStandardStreamsToDEVNULL(int *_piOriginalSTDIN_FILENO, int *_piOriginalSTDOUT_FILENO, int *_piOriginalSTDERR_FILENO)
{
//flushing pending things before redirection.
//fflush(stdin);
fflush(stdout);
fflush(stderr);
*_piOriginalSTDIN_FILENO = dup(STDIN_FILENO);
*_piOriginalSTDOUT_FILENO = dup(STDOUT_FILENO);
*_piOriginalSTDERR_FILENO = dup(STDERR_FILENO);
int devnull = open("/dev/null", O_RDWR);
dup2(devnull, STDIN_FILENO);
dup2(devnull, STDOUT_FILENO);
dup2(devnull, STDERR_FILENO);
close(devnull);
}
void RestoreStandardStreams(int *_piOriginalSTDIN_FILENO, int *_piOriginalSTDOUT_FILENO, int *_piOriginalSTDERR_FILENO)
{
//flushing pending things before restoring.
//fflush(stdin);
fflush(stdout);
fflush(stderr);
dup2(*_piOriginalSTDIN_FILENO, STDIN_FILENO);
dup2(*_piOriginalSTDOUT_FILENO, STDOUT_FILENO);
dup2(*_piOriginalSTDERR_FILENO, STDERR_FILENO);
}
void myfunction()
{
int iOriginalSTDIN_FILENO = -1;
int iOriginalSTDOUT_FILENO = -1;
int iOriginalSTDERR_FILENO = -1;
RedirectStandardStreamsToDEVNULL(&iOriginalSTDIN_FILENO, &iOriginalSTDOUT_FILENO, &iOriginalSTDERR_FILENO);
//all of your code which prints to stdout or stderr will be directed to /dev/null
RestoreStandardStreams(&iOriginalSTDIN_FILENO, &iOriginalSTDOUT_FILENO, &iOriginalSTDERR_FILENO);
}
Important point is to identify the functions which are called inside dylib from your code. Now, surround such functions with Redirect and Restore functions described above.
OSX is a POSIX systems, and like all POSIX systems standard output is file descriptor STDOUT_FILENO (which is a macro defined as 1).
What you can do is duplicate STDOUT_FILENO to another file descriptor, open a temporary file and duplicating (using dup2) the temporary file as STDOUT_FILENO. Then whenever there is output to standard out (using plain write, C printf or C++ std::cout) it will be put in the temporary file.
When done with the temporary "redirection" you simply duplicate the saved standard output (from the first dup call) back into STDOUT_FILENO. and close and remove the temporary file.
Something like the following:
int saved_stdout = dup(STDOUT_FILENO);
int temp_file = open("/tmp/temp_stdout", O_WRONLY, 0600);
dup2(temp_file, STDOUT_FILENO); // Replace standard out
// Code here to write to standard output
// It should all end up in the file /tmp/temp_stdout
dup2(saved_stdout, STDOUT_FILENO); // Restore old standard out
close(temp_file)
unlink("/tmp/temp_stdout"); // Remove file
I want to get a Linux command's output string as well as command output status in a C++ program. I am executing Linux commands in my application.
for example:
Command:
rmdir abcd
Command output string:
rmdir: failed to remove `abcd': No such file or directory
Command Status:
1 (Which means command has been failed)
I tried using Linux function system() which gives the output status, and function popen() which gives me output string of a command, but neither function gives me both
the output string and output status of a Linux command.
The output string is in standard output or standard error descriptor (1 or 2, respectively).
You have to redirect these streams (take a look at dup and dup2 function) to a place, where you can read them (for example - a POSIX pipe).
In C I'd do something like this:
int pd[2];
int retValue;
char buffer[MAXBUF] = {0};
pipe(pd);
dup2(pd[1],1);
retValue = system("your command");
read(pd[0], buffer, MAXBUF);
Now, you have (a part of) your output in buffer and the return code in retValue.
Alternatively, you can use a function from exec (i.e. execve) and get the return value with wait or waitpid.
Update: this will redirect only standard output. To redirect standard error, use dup2(pd[1],1).
The simplest solution is to use system, and to redirect standard out and standard error to a temporarly file, which you can delete later.
Unfortunately there's no easy and simple way in C on Linux to do this. Here's an example how to read/write stdout/stderr/stdin of child process correctly.
And when you want to receive exit code you have to use waitpid (complete example is provided on the bottom of the provided page):
endID = waitpid(childID, &status, WNOHANG|WUNTRACED);
Now you just have to join those two together :)
There's also a great free book named Advanced Linux Programming (ALP) containing detailed information about these kinds of problem available here.
Building on Piotr Zierhoffer answer above, here's a function that does just that, and also restores stdout and stderr their original state.
// Execute command <cmd>, put its output (stdout and stderr) in <output>,
// and return its status
int exec_command(string& cmd, string& output) {
// Save original stdout and stderr to enable restoring
int org_stdout = dup(1);
int org_stderr = dup(2);
int pd[2];
pipe(pd);
// Make the read-end of the pipe non blocking, so if the command being
// executed has no output the read() call won't get stuck
int flags = fcntl(pd[0], F_GETFL);
flags |= O_NONBLOCK;
if(fcntl(pd[0], F_SETFL, flags) == -1) {
throw string("fcntl() failed");
}
// Redirect stdout and stderr to the write-end of the pipe
dup2(pd[1], 1);
dup2(pd[1], 2);
int status = system(cmd.c_str());
int buf_size = 1000;
char buf[buf_size];
// Read from read-end of the pipe
long num_bytes = read(pd[0], buf, buf_size);
if(num_bytes > 0) {
output.clear();
output.append(buf, num_bytes);
}
// Restore stdout and stderr and release the org* descriptors
dup2(org_stdout, 1);
dup2(org_stderr, 2);
close(org_stdout);
close(org_stderr);
return status;
}
you can use popen system call, it will redirect output to a file and from file you can redirect output to a string. like :
char buffer[MAXBUF] = {0};
FILE *fd = popen("openssl version -v", "r");
if (NULL == fd)
{
printf("Error in popen");
return;
}
fread(buffer, MAXBUF, 1, fd);
printf("%s",buffer);
pclose(fd);
For more information read man page for popen.
Ok suppose I have a program ( in windows a .exe file ) and when I run it, it outputs some information... now I'm writing another program ( in c++ ) and I need it to automatically run that .exe file and read the output so that it can process that information for further actions...
what should I do ?
Use popen or on windows (per comment) _popen. Basically it functions as the thing behind the | in some program | thing.
Normally I'm against posting complete code but I literally wrote this today and have it on hand, so, here you go. From what I understand C++ doesn't have a great interface that replaces popen but if you're bringing in the boost libraries or something at that layer there are solutions.
Note I use char[10] because in my application I know the output will be that short.
PopenWrapper(const std::string& command) {
fd = popen(command.c_str(), "r");
if(fd == NULL) {
throw PopenException("Failed to open command: " + command);
}
}
std::string get() {
char line[10];
fgets(line, sizeof(line), fd);
return std::string(line);
}
~PopenWrapper() {
if(fd != NULL) {
pclose(fd);
}
}
This question already has answers here:
Can popen() make bidirectional pipes like pipe() + fork()?
(6 answers)
Closed 3 years ago.
Is it possible to read and write to a file descriptor returned by popen. I have an interactive process I'd like to control through C. If this isn't possible with popen, is there any way around it?
As already answered, popen works in one direction. If you need to read and write, You can create a pipe with pipe(), span a new process by fork() and exec functions and then redirect its input and outputs with dup2(). Anyway I prefer exec over popen, as it gives you better control over the process (e.g. you know its pid)
EDITED:
As comments suggested, a pipe can be used in one direction only. Therefore you have to create separate pipes for reading and writing. Since the example posted before was wrong, I deleted it and created a new, correct one:
#include<unistd.h>
#include<sys/wait.h>
#include<sys/prctl.h>
#include<signal.h>
#include<stdlib.h>
#include<string.h>
#include<stdio.h>
int main(int argc, char** argv)
{
pid_t pid = 0;
int inpipefd[2];
int outpipefd[2];
char buf[256];
char msg[256];
int status;
pipe(inpipefd);
pipe(outpipefd);
pid = fork();
if (pid == 0)
{
// Child
dup2(outpipefd[0], STDIN_FILENO);
dup2(inpipefd[1], STDOUT_FILENO);
dup2(inpipefd[1], STDERR_FILENO);
//ask kernel to deliver SIGTERM in case the parent dies
prctl(PR_SET_PDEATHSIG, SIGTERM);
//replace tee with your process
execl("/usr/bin/tee", "tee", (char*) NULL);
// Nothing below this line should be executed by child process. If so,
// it means that the execl function wasn't successfull, so lets exit:
exit(1);
}
// The code below will be executed only by parent. You can write and read
// from the child using pipefd descriptors, and you can send signals to
// the process using its pid by kill() function. If the child process will
// exit unexpectedly, the parent process will obtain SIGCHLD signal that
// can be handled (e.g. you can respawn the child process).
//close unused pipe ends
close(outpipefd[0]);
close(inpipefd[1]);
// Now, you can write to outpipefd[1] and read from inpipefd[0] :
while(1)
{
printf("Enter message to send\n");
scanf("%s", msg);
if(strcmp(msg, "exit") == 0) break;
write(outpipefd[1], msg, strlen(msg));
read(inpipefd[0], buf, 256);
printf("Received answer: %s\n", buf);
}
kill(pid, SIGKILL); //send SIGKILL signal to the child process
waitpid(pid, &status, 0);
}
The reason popen() and friends don't offer bidirectional communication is that it would be deadlock-prone, due to buffering in the subprocess. All the makeshift pipework and socketpair() solutions discussed in the answers suffer from the same problem.
Under UNIX, most commands cannot be trusted to read one line and immediately process it and print it, except if their standard output is a tty. The reason is that stdio buffers output in userspace by default, and defers the write() system call until either the buffer is full or the stdio stream is closed (typically because the program or script is about to exit after having seen EOF on input). If you write to such a program's stdin through a pipe, and now wait for an answer from that program's stdout (without closing the ingress pipe), the answer is stuck in the stdio buffers and will never come out - This is a deadlock.
You can trick some line-oriented programs (eg grep) into not buffering by using a pseudo-tty to talk to them; take a look at libexpect(3). But in the general case, you would have to re-run a different subprocess for each message, allowing to use EOF to signal the end of each message and cause whatever buffers in the command (or pipeline of commands) to be flushed. Obviously not a good thing performance-wise.
See more info about this problem in the perlipc man page (it's for bi-directional pipes in Perl but the buffering considerations apply regardless of the language used for the main program).
You want something often called popen2. Here's a basic implementation without error checking (found by a web search, not my code):
// http://media.unpythonic.net/emergent-files/01108826729/popen2.c
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include "popen2.h"
int popen2(const char *cmdline, struct popen2 *childinfo) {
pid_t p;
int pipe_stdin[2], pipe_stdout[2];
if(pipe(pipe_stdin)) return -1;
if(pipe(pipe_stdout)) return -1;
//printf("pipe_stdin[0] = %d, pipe_stdin[1] = %d\n", pipe_stdin[0], pipe_stdin[1]);
//printf("pipe_stdout[0] = %d, pipe_stdout[1] = %d\n", pipe_stdout[0], pipe_stdout[1]);
p = fork();
if(p < 0) return p; /* Fork failed */
if(p == 0) { /* child */
close(pipe_stdin[1]);
dup2(pipe_stdin[0], 0);
close(pipe_stdout[0]);
dup2(pipe_stdout[1], 1);
execl("/bin/sh", "sh", "-c", cmdline, NULL);
perror("execl"); exit(99);
}
childinfo->child_pid = p;
childinfo->to_child = pipe_stdin[1];
childinfo->from_child = pipe_stdout[0];
close(pipe_stdin[0]);
close(pipe_stdout[1]);
return 0;
}
//#define TESTING
#ifdef TESTING
int main(void) {
char buf[1000];
struct popen2 kid;
popen2("tr a-z A-Z", &kid);
write(kid.to_child, "testing\n", 8);
close(kid.to_child);
memset(buf, 0, 1000);
read(kid.from_child, buf, 1000);
printf("kill(%d, 0) -> %d\n", kid.child_pid, kill(kid.child_pid, 0));
printf("from child: %s", buf);
printf("waitpid() -> %d\n", waitpid(kid.child_pid, NULL, 0));
printf("kill(%d, 0) -> %d\n", kid.child_pid, kill(kid.child_pid, 0));
return 0;
}
#endif
popen() can only open the pipe in read or write mode, not both. Take a look at this thread for a workaround.
In one of netresolve backends I'm talking to a script and therefore I need to write to its stdin and read from its stdout. The following function executes a command with stdin and stdout redirected to a pipe. You can use it and adapt it to your liking.
static bool
start_subprocess(char *const command[], int *pid, int *infd, int *outfd)
{
int p1[2], p2[2];
if (!pid || !infd || !outfd)
return false;
if (pipe(p1) == -1)
goto err_pipe1;
if (pipe(p2) == -1)
goto err_pipe2;
if ((*pid = fork()) == -1)
goto err_fork;
if (*pid) {
/* Parent process. */
*infd = p1[1];
*outfd = p2[0];
close(p1[0]);
close(p2[1]);
return true;
} else {
/* Child process. */
dup2(p1[0], 0);
dup2(p2[1], 1);
close(p1[0]);
close(p1[1]);
close(p2[0]);
close(p2[1]);
execvp(*command, command);
/* Error occured. */
fprintf(stderr, "error running %s: %s", *command, strerror(errno));
abort();
}
err_fork:
close(p2[1]);
close(p2[0]);
err_pipe2:
close(p1[1]);
close(p1[0]);
err_pipe1:
return false;
}
https://github.com/crossdistro/netresolve/blob/master/backends/exec.c#L46
(I used the same code in Can popen() make bidirectional pipes like pipe() + fork()?)
Use forkpty (it's non-standard, but the API is very nice, and you can always drop in your own implementation if you don't have it) and exec the program you want to communicate with in the child process.
Alternatively, if tty semantics aren't to your liking, you could write something like forkpty but using two pipes, one for each direction of communication, or using socketpair to communicate with the external program over a unix socket.
You can't use popen to use two-way pipes.
In fact, some OSs don't support two-way pipes, in which case a socket-pair (socketpair) is the only way to do it.
popen works for me in both directions (read and write)
I have been using a popen() pipe in both directions..
Reading and writing a child process stdin and stdout with the file descriptor returned by popen(command,"w")
It seems to work fine..
I assumed it would work before I knew better, and it does.
According posts above this shouldn't work.. which worries me a little bit.
gcc on raspbian (raspbery pi debian)
What is fork and what is pipe?
Any scenarios explaining why their use is necessary will be appreciated.
What are the differences between fork and pipe in C?
Can we use them in C++?
I need to know this is because I want to implement a program in C++ which can access live video input, convert its format and write it to a file.
What would be the best approach for this?
I have used x264 for this. So far I have implemented the part of conversion on a file format.
Now I have to implement it on a live stream.
Is it a good idea to use pipes? Capture video in another process and feed it to the other?
A pipe is a mechanism for interprocess communication. Data written to the pipe by one process can be read by another process. The primitive for creating a pipe is the pipe function. This creates both the reading and writing ends of the pipe. It is not very useful for a single process to use a pipe to talk to itself. In typical use, a process creates a pipe just before it forks one or more child processes. The pipe is then used for communication either between the parent or child processes, or between two sibling processes. A familiar example of this kind of communication can be seen in all operating system shells. When you type a command at the shell, it will spawn the executable represented by that command with a call to fork. A pipe is opened to the new child process and its output is read and printed by the shell. This page has a full example of the fork and pipe functions. For your convenience, the code is reproduced below:
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
/* Read characters from the pipe and echo them to stdout. */
void
read_from_pipe (int file)
{
FILE *stream;
int c;
stream = fdopen (file, "r");
while ((c = fgetc (stream)) != EOF)
putchar (c);
fclose (stream);
}
/* Write some random text to the pipe. */
void
write_to_pipe (int file)
{
FILE *stream;
stream = fdopen (file, "w");
fprintf (stream, "hello, world!\n");
fprintf (stream, "goodbye, world!\n");
fclose (stream);
}
int
main (void)
{
pid_t pid;
int mypipe[2];
/* Create the pipe. */
if (pipe (mypipe))
{
fprintf (stderr, "Pipe failed.\n");
return EXIT_FAILURE;
}
/* Create the child process. */
pid = fork ();
if (pid == (pid_t) 0)
{
/* This is the child process.
Close other end first. */
close (mypipe[1]);
read_from_pipe (mypipe[0]);
return EXIT_SUCCESS;
}
else if (pid < (pid_t) 0)
{
/* The fork failed. */
fprintf (stderr, "Fork failed.\n");
return EXIT_FAILURE;
}
else
{
/* This is the parent process.
Close other end first. */
close (mypipe[0]);
write_to_pipe (mypipe[1]);
return EXIT_SUCCESS;
}
}
Just like other C functions you can use both fork and pipe in C++.
there are stdin and stdout for common input and output.
A common style is like this:
input->process->output
But with pipe, it becomes:
input->process1->(tmp_output)->(tmp-input)->process2->output
pipe is the function that returns the two temporary tmp-input and tmp-output, i.e. fd[0] and fd[1].