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c++linuxpopendd

problem in reading output of dd command using pipe

I am developing an application, in which I want to redirect the output (progress information) of dd command to my C++ program, but it is not actually getting the output, here is the code

FILE * progressInfo = popen("gzip -dc backup/backup.img.gz | pv -ptrbe -i 2 -s 2339876653 | dd of=/dev/sdb","r");
if(!progressInfo)
{
return -1;
}
char buf[1024];
while(fgets(buff, sizeof(buff),progressInfo)!=NULL)
{
    std::cout << buff << endl;
}

but the problem is the progress information is not received in buff, and the output is continuously printed on terminal, and above program halts on while(fgets(buff, sizeof(buff),progressInfo)!=NULL), and as soon as the dd operation is completed, the very next line to loop block is executed.

if anyone has any idea why the output is not returned to buff, and its continuously retuned on terminal?

Solution

  • The output is probably being written to standard error rather than standard output. Just add " 2>&1" to the very end of your command string and you should see the output (note the leading space).