How do I execute a command and get the output of the command within C++ using POSIX?
#include <cstdio>
#include <iostream>
#include <memory>
#include <stdexcept>
#include <string>
#include <array>
std::string exec(const char* cmd) {
std::array<char, 128> buffer;
std::string result;
std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(cmd, "r"), pclose);
if (!pipe) {
throw std::runtime_error("popen() failed!");
}
while (fgets(buffer.data(), buffer.size(), pipe.get()) != nullptr) {
result += buffer.data();
}
return result;
}
Pre-C++11 version:
#include <iostream>
#include <stdexcept>
#include <stdio.h>
#include <string>
std::string exec(const char* cmd) {
char buffer[128];
std::string result = "";
FILE* pipe = popen(cmd, "r");
if (!pipe) throw std::runtime_error("popen() failed!");
try {
while (fgets(buffer, sizeof buffer, pipe) != NULL) {
result += buffer;
}
} catch (...) {
pclose(pipe);
throw;
}
pclose(pipe);
return result;
}
Replace popen
and pclose
with _popen
and _pclose
for Windows.
Getting both stdout and stderr (and also writing to stdin, not shown here) is easy peasy with my pstreams header, which defines iostream classes that work like popen
:
#include <pstream.h>
#include <string>
#include <iostream>
int main()
{
// run a process and create a streambuf that reads its stdout and stderr
redi::ipstream proc("./some_command", redi::pstreams::pstdout | redi::pstreams::pstderr);
std::string line;
// read child's stdout
while (std::getline(proc.out(), line))
std::cout << "stdout: " << line << '\n';
# if reading stdout stopped at EOF then reset the state:
if (proc.eof() && proc.fail())
proc.clear();
// read child's stderr
while (std::getline(proc.err(), line))
std::cout << "stderr: " << line << '\n';
}
I'd use popen() (++waqas).
But sometimes you need reading and writing...
It seems like nobody does things the hard way any more.
(Assuming a Unix/Linux/Mac environment, or perhaps Windows with a POSIX compatibility layer...)
enum PIPE_FILE_DESCRIPTERS
{
READ_FD = 0,
WRITE_FD = 1
};
enum CONSTANTS
{
BUFFER_SIZE = 100
};
int
main()
{
int parentToChild[2];
int childToParent[2];
pid_t pid;
string dataReadFromChild;
char buffer[BUFFER_SIZE + 1];
ssize_t readResult;
int status;
ASSERT_IS(0, pipe(parentToChild));
ASSERT_IS(0, pipe(childToParent));
switch (pid = fork())
{
case -1:
FAIL("Fork failed");
exit(-1);
case 0: /* Child */
ASSERT_NOT(-1, dup2(parentToChild[READ_FD], STDIN_FILENO));
ASSERT_NOT(-1, dup2(childToParent[WRITE_FD], STDOUT_FILENO));
ASSERT_NOT(-1, dup2(childToParent[WRITE_FD], STDERR_FILENO));
ASSERT_IS(0, close(parentToChild [WRITE_FD]));
ASSERT_IS(0, close(childToParent [READ_FD]));
/* file, arg0, arg1, arg2 */
execlp("ls", "ls", "-al", "--color");
FAIL("This line should never be reached!!!");
exit(-1);
default: /* Parent */
cout << "Child " << pid << " process running..." << endl;
ASSERT_IS(0, close(parentToChild [READ_FD]));
ASSERT_IS(0, close(childToParent [WRITE_FD]));
while (true)
{
switch (readResult = read(childToParent[READ_FD],
buffer, BUFFER_SIZE))
{
case 0: /* End-of-File, or non-blocking read. */
cout << "End of file reached..." << endl
<< "Data received was ("
<< dataReadFromChild.size() << "): " << endl
<< dataReadFromChild << endl;
ASSERT_IS(pid, waitpid(pid, & status, 0));
cout << endl
<< "Child exit staus is: " << WEXITSTATUS(status) << endl
<< endl;
exit(0);
case -1:
if ((errno == EINTR) || (errno == EAGAIN))
{
errno = 0;
break;
}
else
{
FAIL("read() failed");
exit(-1);
}
default:
dataReadFromChild . append(buffer, readResult);
break;
}
} /* while (true) */
} /* switch (pid = fork())*/
}
You also might want to play around with select() and non-blocking reads.
fd_set readfds;
struct timeval timeout;
timeout.tv_sec = 0; /* Seconds */
timeout.tv_usec = 1000; /* Microseconds */
FD_ZERO(&readfds);
FD_SET(childToParent[READ_FD], &readfds);
switch (select (1 + childToParent[READ_FD], &readfds, (fd_set*)NULL, (fd_set*)NULL, & timeout))
{
case 0: /* Timeout expired */
break;
case -1:
if ((errno == EINTR) || (errno == EAGAIN))
{
errno = 0;
break;
}
else
{
FAIL("Select() Failed");
exit(-1);
}
case 1: /* We have input */
readResult = read(childToParent[READ_FD], buffer, BUFFER_SIZE);
// However you want to handle it...
break;
default:
FAIL("How did we see input on more than one file descriptor?");
exit(-1);
}