Q: strange behaviour of procmail when piping content to c++ executable I have a working procmail config.
this is the rc.filters :  
:0 w :a.lock
* ^From:(.*\<)?(try@gmail\.com)\>
| $HOME/executable/a.out

this file compiles and works, procmail delivers the mail,
and the executable writes the content to the output file.
#include <stdlib.h>
#include <iostream>
#include <fstream>

using namespace std;

int main(void)
{
ofstream myfile;
myfile.open ("output.txt");

    string line;
    while (getline(cin, line)) 
    {
    myfile << line << endl;     
    }    
myfile.close();    
return EXIT_SUCCESS;
} 

the problem is I need a cin object with the content to pass
to a constructor of the Mimetic library.
I need this executable to work:   
#include <stdlib.h>
#include <iostream>
#include <fstream>
#include <mimetic/mimetic.h>

using namespace std;
using namespace mimetic;

int main(void)
{
ofstream myfile;
myfile.open ("output.txt");

MimeEntity me(cin);                         
const Header& h = me.header();  
string subjectString = h.subject();
myfile << subjectString;
myfile << "Check";      
myfile.close();
return EXIT_SUCCESS;
}

If I take a Mime message called message.txt and do the following with the second code :
cat message.txt | ./a.out

./a.out < message.txt

In both cases the executable works and I get the subject in an output.txt
but for some when it is invoked and the content piped by procmail it doesn't work,
and all I get in the output.txt is "Check" which means that the file
was at least invoked.
the procmail.log states that everything is fine. 

A: I don't know what's going on exactly but I would capture the input from std::cin into a string and then pass a std::istringstream constructed from that value to MimeEntity. This way you can inspect what the input from std::cin was while still having it processed by the library:
std::istreambuf_iterator<char> begin(std::cin), end;
std::string message(begin, end);
out << "received >>>" << message << "<<<\n";
std::istringstream in(message);
MimeEntity me(in);
// ...

