This is the code for playing sound file in C++ linux code
string str1 = "aplay ";
str1 = str1 + " out.wav" + " & ";
const char *command = str1.c_str();
system(command);
** Entire code is available here : Playing sound C++ linux aplay : device or resource busy
I just want to know how to play this in a fork() as I read that system call is too taxing on cpu, which ofcourse is in my case.
Please help
fork will make a copy of your process, so you can easily write:
// fork the current process: beyond this point, you will have 2 process
int ret = fork();
if (ret == 0) {
// in child: execute the long command
system("aplay out.wav");
// exit the child process
exit(0);
}
// child process will not go here
if (ret < 0) {
perror("fork");
}
After, you should know that system will do for you fork + exec + wait. Since you don't want your parent process to wait the child, you can write:
// fork the current process: beyond this point, you will have 2 process
int ret = fork();
if (ret == 0) {
// in child: execute the long command
char program[] = "/usr/bin/aplay";
char *args[] = {"/usr/bin/aplay", "out.wav" };
ret = execv(program, args);
// this point will be reach only if `exec` fails
// so if we reach this point, we've got an error.
perror("execv");
exit(0);
}
// child process will not go here
if (ret < 0) {
perror("fork");
}
Related
When i write to a linux driver / device, in this case i want to put the embedded linux device to sleep:
echo "mem" > /sys/power/state
I get an error on the terminal if the above command fails
[ 2593.061030] dpm_run_callback(): elan_touch_suspend+0x0/0x114 returns 1
[ 2593.067578] PM: Device 0-0015 failed to suspend: error 1
[ 2593.072994] PM: Some devices failed to suspend, or early wake event detected
[ 2593.107845] ==== calc_soc_by_voltageMethod E60U22 ====
And i do this the same in c++:
int fd2 = open("/sys/power/state", O_RDWR);
write(fd2, "mem", 3);
close(fd2);
If the above command fails, i get the same error on the terminal. now i want to get this error as a string in c++, in shell i can do something like this:
echo "mem" > /sys/power/state 2>/tmp/sleep_error
But i cant figure this out in c++, I need to to try one more time if it fails
What I tryied:
Capturing cerr of the whole program, with freopen doesn't work. When I write to the device from another terminal, and do cat /dev/stderr from another, i get the output in the second one, I tryied to use it:
char byte[1000];
int stderrdevice = open("/dev/stderr", O_RDONLY | O_NOCTTY);
int fd2 = open("/sys/power/state", O_RDWR);
write(fd2, "mem", 3);
close(fd2);
ssize_t size = read(stderrdevice, &byte, 1000);
printf("Read byte %s\n", byte);
This doesn't work too. Any resources, documentation related to this are welcome
Thanks everyone for help and responding. user17732522 and Nate Eldredge were right. What i was trying to get was the kernel ring buffer, that was printing out to the serial connection. The same thing was in dmesg. I ended up using klogctl to get the errors. I couldn't get only the last line of dmesg with other klogctl options, and the code is a bit chaotic, but here is what I finally used:
bool continueSleeping = true;
int count = 0;
while (continueSleeping == true) {
// https://linux.die.net/man/3/klogctl
klogctl(5, NULL, 0);
log("Trying sleep");
std::this_thread::sleep_for(std::chrono::milliseconds(100));
int fd2 = open("/sys/power/state", O_RDWR);
int status = write(fd2, "mem", 3);
close(fd2);
std::this_thread::sleep_for(std::chrono::milliseconds(500));
log("After sleep");
// get dmesg, and then only lines containing <3>
char *logs_data;
ssize_t len = klogctl(10, NULL, 0);
logs_data = (char *)malloc(len);
klogctl(3, logs_data, len);
vector<string> dmesgErrorsVec;
boost::split(dmesgErrorsVec, logs_data, boost::is_any_of("\n"),
boost::token_compress_on);
// to show whole dmesg
//log("dmesg: " + (string)logs_data);
free(logs_data);
string dmesgErrors;
for (string line : dmesgErrorsVec) {
if (line.find("<3>") != std::string::npos) {
// tesdt
dmesgErrors.append(line);
dmesgErrors.append("\n");
}
}
dmesgErrorsVec.clear();
if (status == -1 or
dmesgErrors.find("PM: Some devices failed to suspend") != std::string::npos) {
log("Failed to suspend, dmesg errors:\n" + dmesgErrors);
log("status: " + to_string(status));
CEG();
count = count + 1;
if (count == 5) {
log("5 failed attemts at suspending, sleep a little longer...");
smartWait(10000);
} else if (count == 15) {
log("15 failed attempts at sleeping...");
// Write to fbink here a sad message
} else {
smartWait(3000);
}
} else {
// Exiting this sleeping hell
log("Tryied going to sleep " + to_string(count) + "times");
continueSleeping = false;
}
}
log("Sleep finished, Exiting going to sleep");
I've been tasked to create a program that takes a text file that contains a list of programs as input. It then needs to run valgrind on the programs (one at a time) until valgrind ends or until the program hits a max allotted time. I have the program doing everything I need it to do EXCEPT it isn't waiting for valgrind to finish. The code I'm using has this format:
//code up to this point is working properly
pid_t pid = fork();
if(pid == 0){
string s = "sudo valgrind --*options omitted*" + testPath + " &>" + outPath;
system(s.c_str());
exit(0);
}
//code after here seems to also be working properly
I'm running into an issue where the child just calls the system and moves on without waiting for valgrind to finish. As such I'm guessing that system isn't the right call to use, but I don't know what call I should be making. Can anyone tell me how to get the child to wait for valgrind to finish?
I think that you are looking for fork/execv. Here is an example:
http://www.cs.ecu.edu/karl/4630/spr01/example1.html
An other alternative could be popen.
You can fork and exec your program and then wait for it to finish. See the following example.
pid_t pid = vfork();
if(pid == -1)
{
perror("fork() failed");
return -1;
}
else if(pid == 0)
{
char *args[] = {"/bin/sleep", "5", (char *)0};
execv("/bin/sleep", args);
}
int child_status;
int child_pid = wait(&child_status);
printf("Child %u finished with status %d\n", child_pid, child_status);
When we use system() command, program wait until it complete but I am executing a process using system() and using load balance server due to which program comes to next line just after executing system command. Please note that that process may not be complete.
system("./my_script");
// after this I want to see whether it is complete or not using its pid.
// But how do i Know PID?
IsScriptExecutionComplete();
Simple answer: you can't.
The purpose of system() is to block when command is being executed.
But you can 'cheat' like this:
pid_t system2(const char * command, int * infp, int * outfp)
{
int p_stdin[2];
int p_stdout[2];
pid_t pid;
if (pipe(p_stdin) == -1)
return -1;
if (pipe(p_stdout) == -1) {
close(p_stdin[0]);
close(p_stdin[1]);
return -1;
}
pid = fork();
if (pid < 0) {
close(p_stdin[0]);
close(p_stdin[1]);
close(p_stdout[0]);
close(p_stdout[1]);
return pid;
} else if (pid == 0) {
close(p_stdin[1]);
dup2(p_stdin[0], 0);
close(p_stdout[0]);
dup2(p_stdout[1], 1);
dup2(::open("/dev/null", O_RDONLY), 2);
/// Close all other descriptors for the safety sake.
for (int i = 3; i < 4096; ++i)
::close(i);
setsid();
execl("/bin/sh", "sh", "-c", command, NULL);
_exit(1);
}
close(p_stdin[0]);
close(p_stdout[1]);
if (infp == NULL) {
close(p_stdin[1]);
} else {
*infp = p_stdin[1];
}
if (outfp == NULL) {
close(p_stdout[0]);
} else {
*outfp = p_stdout[0];
}
return pid;
}
Here you can have not only PID of the process, but also it's STDIN and STDOUT. Have fun!
Not an expert on this myself, but if you look at the man page for system:
system() executes a command specified in command by calling /bin/sh -c command, and returns after the command has been completed
You can go into the background within the command/script you're executing (and return immediately), but I don't think there's a specific provision in system for that case.
Ideas I can think of are:
Your command might return the pid through the return code.
Your code might want to look up the name of the command in the active processes (e.g. /proc APIs in unix-like environments).
You might want to launch the command yourself (instead of through a SHELL) using fork/exec
As the other answers said, std::system blocks until complete anyway. However, if you want to run the child process async and you are ok with boost you can use boost.process (ref):
#include <boost/process.hpp>
namespace bp = boost::process;
bp::child c(bp::search_path("echo"), "hello world");
std::cout << c.id() << std::endl;
// ... do something with ID ...
c.wait();
You can check exit status of your command by following code :
int ret = system("./my_script");
if (WIFEXITED(ret) && !WEXITSTATUS(ret))
{
printf("Completed successfully\n"); ///successful
}
else
{
printf("execution failed\n"); //error
}
Hi I am working on Linux and I am trying to create a GUI app to go with my executable I have made.
For some reason it unexpectedly ends. There is no error message, it just says in the Qt console window it unexpectedly ended with exit code 0.
Can someone please have a look at it for me. I am working on Linux.
I will also paste the code here.
void MainWindow::on_pushButton_clicked()
{
QString stringURL = ui->lineEdit->text();
ui->labelError->clear();
if(stringURL.isEmpty() || stringURL.isNull()) {
ui->labelError->setText("You have not entered a URL.");
stringURL.clear();
return;
}
std::string cppString = stringURL.toStdString();
const char* cString = cppString.c_str();
char* output;
//These arrays will hold the file id of each end of two pipes
int fidOut[2];
int fidIn[2];
//Create two uni-directional pipes
int p1 = pipe(fidOut); //populates the array fidOut with read/write fid
int p2 = pipe(fidIn); //populates the array fidIn with read/write fid
if ((p1 == -1) || (p2 == -1)) {
printf("Error\n");
return;
}
//To make this more readable - I'm going to copy each fileid
//into a semantically more meaningful name
int parentRead = fidIn[0];
int parentWrite = fidOut[1];
int childRead = fidOut[0];
int childWrite = fidIn[1];
//////////////////////////
//Fork into two processes/
//////////////////////////
pid_t processId = fork();
//Which process am I?
if (processId == 0) {
/////////////////////////////////////////////////
//CHILD PROCESS - inherits file id's from parent/
/////////////////////////////////////////////////
::close(parentRead); //Don't need these
::close(parentWrite); //
//Map stdin and stdout to pipes
dup2(childRead, STDIN_FILENO);
dup2(childWrite, STDOUT_FILENO);
//Exec - turn child into sort (and inherit file id's)
execlp("htmlstrip", "htmlstrip", "-n", NULL);
} else {
/////////////////
//PARENT PROCESS/
/////////////////
::close(childRead); //Don't need this
::close(childWrite); //
//Write data to child process
//char strMessage[] = cString;
write(parentWrite, cString, strlen(cString));
::close(parentWrite); //this will send an EOF and prompt sort to run
//Read data back from child
char charIn;
while ( read(parentRead, &charIn, 1) > 0 ) {
output = output + (charIn);
printf("%s", output);
}
::close(parentRead); //This will prompt the child process to quit
}
return;
}
EDIT:: DEBUGGING RESULTS
I ran the debugger and this is the error I received:
The inferior stopped because it received a signal from the Operating System.
Signal name : SIGSEGV
Signal meaning : Segmentation fault
You haven't initialized the "output" variable. On the last lines of your code, you do this:
while ( read(parentRead, &charIn, 1) > 0 ) {
output = output + (charIn);
printf("%s", output);
}
Which will do nasty things, since you are adding a byte read from your child process, to the output variable, which is a pointer that contains garbage, and then printing the contents of the "output" variable's address as a string. You probably want "output" to be a std::string, that way your code could make sense:
std::string output;
/* ... */
while ( read(parentRead, &charIn, 1) > 0 ) {
output += (charIn);
}
std::cout << output;
Once you have read all the data your child process has generated, you can write it to stdout.
EDIT: since you want to set the contents of "output" to a QPlainTextEdit, you can use QPlainTextEdit::setPlainText:
while ( read(parentRead, &charIn, 1) > 0 ) {
output += (charIn);
}
plainTextEdit.setPlainText(output.c_str());
Using Linux and C++, I would like a function that does the following:
string f(string s)
{
string r = system("foo < s");
return r;
}
Obviously the above doesn't work, but you get the idea. I have a string s that I would like to pass as the standard input of a child process execution of application "foo", and then I would like to record its standard output to string r and then return it.
What combination of Linux syscalls or POSIX functions should I use?
I'm using Linux 3.0 and do not need the solution to work with older systems.
The code provided by eerpini does not work as written. Note, for example, that the pipe ends that are closed in the parent are used afterwards. Look at
close(wpipefd[1]);
and the subsequent write to that closed descriptor. This is just transposition, but it shows this code has never been used. Below is a version that I have tested. Unfortunately, I changed the code style, so this was not accepted as an edit of eerpini's code.
The only structural change is that I only redirect the I/O in the child (note the dup2 calls are only in the child path.) This is very important, because otherwise the parent's I/O gets messed up. Thanks to eerpini for the initial answer, which I used in developing this one.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#define PIPE_READ 0
#define PIPE_WRITE 1
int createChild(const char* szCommand, char* const aArguments[], char* const aEnvironment[], const char* szMessage) {
int aStdinPipe[2];
int aStdoutPipe[2];
int nChild;
char nChar;
int nResult;
if (pipe(aStdinPipe) < 0) {
perror("allocating pipe for child input redirect");
return -1;
}
if (pipe(aStdoutPipe) < 0) {
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
perror("allocating pipe for child output redirect");
return -1;
}
nChild = fork();
if (0 == nChild) {
// child continues here
// redirect stdin
if (dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1) {
exit(errno);
}
// redirect stdout
if (dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1) {
exit(errno);
}
// redirect stderr
if (dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1) {
exit(errno);
}
// all these are for use by parent only
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
// run child process image
// replace this with any exec* function find easier to use ("man exec")
nResult = execve(szCommand, aArguments, aEnvironment);
// if we get here at all, an error occurred, but we are in the child
// process, so just exit
exit(nResult);
} else if (nChild > 0) {
// parent continues here
// close unused file descriptors, these are for child only
close(aStdinPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
// Include error check here
if (NULL != szMessage) {
write(aStdinPipe[PIPE_WRITE], szMessage, strlen(szMessage));
}
// Just a char by char read here, you can change it accordingly
while (read(aStdoutPipe[PIPE_READ], &nChar, 1) == 1) {
write(STDOUT_FILENO, &nChar, 1);
}
// done with these in this example program, you would normally keep these
// open of course as long as you want to talk to the child
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
} else {
// failed to create child
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);
}
return nChild;
}
Since you want bidirectional access to the process, you would have to do what popen does behind the scenes explicitly with pipes. I am not sure if any of this will change in C++, but here is a pure C example :
void piped(char *str){
int wpipefd[2];
int rpipefd[2];
int defout, defin;
defout = dup(stdout);
defin = dup (stdin);
if(pipe(wpipefd) < 0){
perror("Pipe");
exit(EXIT_FAILURE);
}
if(pipe(rpipefd) < 0){
perror("Pipe");
exit(EXIT_FAILURE);
}
if(dup2(wpipefd[0], 0) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(dup2(rpipefd[1], 1) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(fork() == 0){
close(defout);
close(defin);
close(wpipefd[0]);
close(wpipefd[1]);
close(rpipefd[0]);
close(rpipefd[1]);
//Call exec here. Use the exec* family of functions according to your need
}
else{
if(dup2(defin, 0) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
if(dup2(defout, 1) == -1){
perror("dup2");
exit(EXIT_FAILURE);
}
close(defout);
close(defin);
close(wpipefd[1]);
close(rpipefd[0]);
//Include error check here
write(wpipefd[1], str, strlen(str));
//Just a char by char read here, you can change it accordingly
while(read(rpipefd[0], &ch, 1) != -1){
write(stdout, &ch, 1);
}
}
}
Effectively you do this :
Create pipes and redirect the stdout and stdin to the ends of the two pipes (note that in linux, pipe() creates unidirectional pipes, so you need to use two pipes for your purpose).
Exec will now start a new process which has the ends of the pipes for stdin and stdout.
Close the unused descriptors, write the string to the pipe and then start reading whatever the process might dump to the other pipe.
dup() is used to create a duplicate entry in the file descriptor table. While dup2() changes what the descriptor points to.
Note : As mentioned by Ammo# in his solution, what I provided above is more or less a template, it will not run if you just tried to execute the code since clearly there is a exec* (family of functions) missing, so the child will terminate almost immediately after the fork().
Ammo's code has some error handling bugs. The child process is returning after dup failure instead of exiting. Perhaps the child dups can be replaced with:
if (dup2(aStdinPipe[PIPE_READ], STDIN_FILENO) == -1 ||
dup2(aStdoutPipe[PIPE_WRITE], STDOUT_FILENO) == -1 ||
dup2(aStdoutPipe[PIPE_WRITE], STDERR_FILENO) == -1
)
{
exit(errno);
}
// all these are for use by parent only
close(aStdinPipe[PIPE_READ]);
close(aStdinPipe[PIPE_WRITE]);
close(aStdoutPipe[PIPE_READ]);
close(aStdoutPipe[PIPE_WRITE]);