Can't invoke g++ with redirected stdout - c++

Question

I'm looking to invoke g++ and get the output. Here's my code:
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <iostream>
#include <boost/optional.hpp>
#include <vector>
#include <string>
namespace Util
{
template<typename T>
using optional = boost::optional<T>;
}
namespace Wide
{
namespace Driver
{
struct ProcessResult
{
std::string std_out;
int exitcode;
};
ProcessResult StartAndWaitForProcess(std::string name, std::vector<std::string> args, Util::optional<unsigned> timeout);
}
}
#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <iostream>
#include <fcntl.h>
Wide::Driver::ProcessResult Wide::Driver::StartAndWaitForProcess(std::string name, std::vector<std::string> args, Util::optional<unsigned> timeout) {
int filedes[2];
pipe(filedes);
pid_t pid = fork();
if (pid == 0) {
while ((dup2(filedes[1], STDOUT_FILENO) == -1) && (errno == EINTR)) {}
auto fd = open("/dev/null", O_RDWR);
while ((dup2(fd, STDIN_FILENO) == -1) && (errno == EINTR)) {}
//freopen("/dev/null", "rw", stdin);
//freopen("/dev/null", "rw", stderr);
//close(filedes[1]);
close(filedes[0]);
std::vector<const char*> cargs;
cargs.push_back(name.c_str());
for (auto&& arg : args)
cargs.push_back(arg.c_str());
cargs.push_back(nullptr);
execv(name.c_str(), const_cast<char* const*>(&cargs[0]));
}
std::string std_out;
close(filedes[1]);
char buffer[4096];
while (1) {
ssize_t count = read(filedes[0], buffer, sizeof(buffer));
if (count == -1) {
if (errno == EINTR) {
continue;
} else {
perror("read");
exit(1);
}
} else if (count == 0) {
break;
} else {
std_out += std::string(buffer, buffer + count);
}
}
close(filedes[0]);
int status;
ProcessResult result;
result.std_out = std_out;
waitpid(pid, &status, 0);
if (!WIFEXITED(status))
result.exitcode = 1;
else {
result.exitcode = WEXITSTATUS(status);
if (result.exitcode != 0) {
std::cout << name << " failed with code " << result.exitcode << "\n";
}
}
return result;
}
int main()
{
auto r = Wide::Driver::StartAndWaitForProcess("g++", { "-std=c++14", "main.cpp" }, 150);
std::cout << r.std_out << "!!!!\n!!!!\n" << r.exitcode << "\n";
}
The output:
read: Bad file descriptor
g++ failed with code 1
!!!!
!!!!
1
Just invoke g++ main.cpp -std=c++14 && ./a.out.
I've used strace but it doesn't really give any more interesting details- the process runs, then fork/exec, then the above error. I can invoke other processes with the above code so I don't know what's so different about g++. I can invoke GCC with popen without problems so I don't know what's so different here.
The error here is really not very helpful. How can I invoke g++ and get the output?

Answer 1

The problem here is that you call execv which requires a full path to the executable as its first argument.
What you need is execvp which uses the contents of the PATH environment variable to find the executable, and thus only requires a name like g++.