How to capture output of strace in C++ program - c++

Question

I want to analyse the output of strace in my C++ program. While launching /bin/strace ps from my app I get an output from ps, but not from strace and strace output is printed to stdout (my terminal). I use standard technique of using pipes and redirecting streams.
Here is my source:
#include <stdlib.h>
#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>
int main(){
char *const parmList[] = {"/bin/strace", "ps", NULL};
int pipes[2];
pipe(pipes);
pid_t child = fork();
if(child == 0){
close(pipes[0]);
dup2(pipes[1],1);
execv(parmList[0], parmList);
}
else{
int status;
wait(&status);
fcntl(pipes[0], F_SETFL, O_NONBLOCK | O_ASYNC);
char buf[128] = {0};
ssize_t bytesRead;
std::string stdOutBuf;
while(1) {
bytesRead = read(pipes[0], buf, sizeof(buf)-1);
if (bytesRead <= 0)
break;
buf[bytesRead] = 0;
stdOutBuf += buf;
}
std::cout << "<stdout>\n" << stdOutBuf << "\n</stdout>" << std::endl;
}
close(pipes[0]);
close(pipes[1]);
return 0;
}
How can I get an output of strace in my program?

Answer 1

strace writes to stderr not to stdout, if you only want to capture the strace output just use stderr instead of stdout
change the dup2 line like this
dup2(pipes[1],2);
If you want combined strace and ps output do this:
dup2(pipes[1],1);
dup2(pipes[1],2);
if you want separated output you'll probably need to use non-blocking reads and select() or poll()
Also: after calling exec you should print an error message, if everything works exec won't return, but if something goes wrong with the exec, it's good to know.
std::cerr << "exec failed!";
I used this code and had success:
#include <stdlib.h>
#include <iostream>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <fcntl.h>
int main(){
char *const parmList[] = {"/usr/bin/strace", "ps", NULL};
int pipes[2];
pipe(pipes);
pid_t child = fork();
if(child == 0){
close(pipes[0]);
dup2(pipes[1],2);
execv(parmList[0], parmList);
std::cerr << "exec fail\n" ;
}
else{
int status;
wait(&status);
fcntl(pipes[0], F_SETFL, O_NONBLOCK | O_ASYNC);
char buf[128] = {0};
ssize_t bytesRead;
std::string stdOutBuf;
while(1) {
bytesRead = read(pipes[0], buf, sizeof(buf)-1);
if (bytesRead <= 0)
break;
buf[bytesRead] = 0;
stdOutBuf += buf;
}
std::cout << "<stdout>\n" << stdOutBuf << "\n</stdout>" << std::endl;
}
close(pipes[0]);
close(pipes[1]);
return 0;
}
HTH