How does Objective C stdin/stdout piping work when called from C/C++?

Question

This is a problem I've been working on all afternoon, I think I have reduced it to its central problem, which appears to be unexpected behavior when piping data to/from an Objective C command line application that is called from a C++ application.

When executed alone, the Objective C program works as expected. When the C++ Pipe (which is the "master" in this case, the C++ is calling the Objective C executable) is calling a C/C++ executable similar to the below Objective C code, everything also works as expected.

Furthermore, if the input code is removed from the Objective C, or if the C++ program orders Objective C to be piped to a file (so the command would be "./HelloWorld > dump.txt" instead of "./HelloWorld") everything performs as expected.

However, when the code as presented bellow is executed, the C++ hangs when attempting to read the Objective C's stdout, on the first try before any attempts to read stdin have been made by Objective C.

Objective C

#import <Foundation/Foundation.h>

void c_print(NSString* prnt)
{
    printf("%s", [prnt cStringUsingEncoding:NSUTF8StringEncoding]);
}
void c_print_ln(NSString* prnt)
{
    printf("%s\n", [prnt cStringUsingEncoding:NSUTF8StringEncoding]);
}
NSString* read_till(char c)
{
    NSMutableString* ret = [[NSMutableString alloc] initWithString:@""];

    char r = getchar();
    while(r!=c && r!= '\0')
    {
        [ret appendFormat:@"%c",r];
        r = getchar();
    }
    return ret;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        c_print_ln(@"Hello, World!");
        NSString* exmp = read_till('\n');
        c_print_ln([[NSString alloc] initWithFormat:@"String I read: \"%@\"",exmp]);
    }
    return 0;
}

C++ (.h file)

#ifndef PIPE_H
#define PIPE_H

#include <sys/types.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <string>
#include <iostream>

#define PIPE_READ 0
#define PIPE_WRITE 1

class outsideExecutable
{
private:
    char buf[1024];
    bool is_good;

    int infp, outfp;

public:
    outsideExecutable(char* command);
    ~outsideExecutable();

    bool isGood();
    std::string readline();
    void writeline(std::string source);

};
#endif

C++ (.cpp file)

#include "Pipe.h"

using namespace std;

int main()
{
    cout<<"Testing Pipe"<<endl;
    outsideExecutable* exe = new outsideExecutable((char*)"./HelloWorld");
    exe->readline();
    exe->writeline("reading example");
    exe->readline();
    delete exe;
}
static pid_t popen2(const char *command, int *infp, int *outfp)
{
    int p_stdin[2], p_stdout[2];
    pid_t pid;

    if (pipe(p_stdin) != 0 || pipe(p_stdout) != 0)
        return -1;

    pid = fork();

    if (pid < 0)
        return pid;
    else if (pid == 0)
    {
        close(p_stdin[PIPE_WRITE]);
        dup2(p_stdin[PIPE_READ], PIPE_READ);
        close(p_stdout[PIPE_READ]);
        dup2(p_stdout[PIPE_WRITE], PIPE_WRITE);

        execl("/bin/sh", "sh", "-c", command, NULL);
        perror("execl");
        exit(1);
    }

    if (infp == NULL)
        close(p_stdin[PIPE_WRITE]);
    else
        *infp = p_stdin[PIPE_WRITE];

    if (outfp == NULL)
        close(p_stdout[PIPE_READ]);
    else
        *outfp = p_stdout[PIPE_READ];

    return pid;
}

outsideExecutable::outsideExecutable(char* command)
{
    is_good = false;

    if (popen2(command, &infp, &outfp) <= 0)
        return;

    is_good = true;
}
outsideExecutable::~outsideExecutable()
{

}

bool outsideExecutable::isGood()
{
    return is_good;
}
std::string outsideExecutable::readline()
{
    if(!is_good)
        return "";
    string ret = "";
    char hld;
    read(outfp, &hld, 1);
    while(hld!='\n' && hld!='\0')
    {
        ret = ret + hld;
        read(outfp, &hld, 1);
    }
    cout<<"We read:"<<ret<<endl;
    return ret;
}
void outsideExecutable::writeline(std::string source)
{
    if(!is_good)
        return;
    //Do nothing
    cout<<"Sending command: "<<source<<endl;
    source = source+"\n";
    write(infp, source.c_str(), source.length());
}

#endif

Anyone have any ideas what could be wrong with this? I've got quite a bit of experience with C/C++, and it appears that the piping code from that side of things is working well. It really seems like this is an example of Objective C just not playing nice, I've never seen an example of piping failing like this.

Answer 1

rich (https://stackoverflow.com/users/1566221/rici) just provided the answer to this question in the comment above. Here is the updated Objective C code to fix it:

void c_print(NSString* prnt)
{
    printf("%s", [prnt cStringUsingEncoding:NSUTF8StringEncoding]);
    fflush(stdout);
}
void c_print_ln(NSString* prnt)
{
    printf("%s\n", [prnt cStringUsingEncoding:NSUTF8StringEncoding]);
    fflush(stdout);
}

Apparently stdout needs to be flushed in Objective C for piping to work properly.