fprintf() / std::cout doesn't print part of the string to stdout/stderr

Question

I have this strange problem with stdout/stderr.

I want to apologize for not being able to put here the original code, it's too long / too many libraries dependent etc...
So please let me know if you ever encountered anything like it, or what may cause this issue without getting the original code, only the idea and examples of the simple things I tried to do:

• I'm using g++ (GCC) 4.4.6 20120305 (Red Hat 4.4.6-4) on RHEL 6.3
• I couldn't isolate the problem for putting it here, I'll give code examples of what I did.
• fprintf() / printf() / std::cout stops working after a while. I'm using boost::asio::io_service with deadline_timer in order to call a my_print() function.
  This my_print() function prints to screen every 1 second some information.

In order to print, I use alignments, like the following:

fprintf(stdout, "%*s\n", -printWidth, someEnumToStr[i]);
fprintf(stdout, "%s\n", aString);
fprintf(stdout, "%u\n", num);

While aString is a std::string. Sometimes I construct aString from std::ostringstream.
Sometimes I construct it with snprintf().

• I have an std::map with information, exactly 16 elements inside the map. I iterate over it, and for each element I try to print data with the example of fprintf() above.
• For an unknown reason, the line of element 16 isn't printed.
• If I call the executable, and redirect stdout to a file (./a.out > aaa.txt) the line of element 16 is getting printed.
• If I open a new FILE* and fprintf() to this file, again, everything is getting printed (all lines, including line of element 16)
• Before using fprintf() I tried to use std::cout (and alignments with std::cout.width(printWidth) << std::left...), The same behavior happened, but when line 16 wasn't drawn, stdout got stuck (I mean, the program still worked, but nothing was printed to stdout never again. I had to call std::cout.clear() for it to work again). Since a point in the code, which I couldn't lay my hands on, std::cout.failbit and badbit were 1.
• If I run the code with valgrind this behavior doesn't happen. valgrind doesn't say anything wrong.
• If I run it with gdb it happens, but gdb doesn't say anything wrong.
• If I run it in an IDE (clion) in debug mode, it doesn't happen.
• If I run it in IDE, without debug, it happens.
• I figure it depends on the printWidth I give for the alignment in fprintf() - When printWidth is bigger, it happens sooner (when it's smaller, line 16 is randomly getting printed).
• Another important thing: it happens more frequently when there is more to print.
• I tried to give std::cout a bigger buffer (not his default) and it didn't work.
• I tried to buffer all of the output into a buffer (instead of printing each line), then to only fprintf() once. Same behavior happens.
• I didn't find anywhere in the code I try to print a NULL pointer.
• I print with \n every couple of fprintf()s, and do fflush() in the end of my_print()

Please let me know if you know anything.

---

Illustration:

deadline_timer..... every 1 sec... my_print()
boost::asio::io_service.run

my_print() {

    for(std::map<>::iterator... begin, end, ++it....) {
        fprintf()s....
    }
}

Answer 1

Non printable characters may be breaking terminal.

fprintf(stdout,"%s", astdstring.cstr() );

Is how to print std::string

Answer 2

I use boost::asio, I have a callback to read from stdin. this read is nonblocking - happens with async_read_some().
The problem was stdin was turned to be nonblocking, and it also caused stdout to be nonblocking as well because they point to the same file description (explanation).
It caused the fprintf() calls to fail (returned -1 with errno 11) and not all of the output got printed out on the screen.

It has no relation to boost.
I succeeded isolating the problem, the following code creates this problem:

#include <iostream>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <stdlib.h>

using namespace std;

int main(int argc, char *argv[]) {

    const int problem = 8000;
    const int myBuffSize = 32000;
    char *myBuff = new char[myBuffSize];
    int myoffset = 0;
    memset(myBuff, '-', myBuffSize);
    int flags;

    bool toogle = true;
    bool running = true;

    // Comment from here
    if ((flags = fcntl(STDIN_FILENO, F_GETFL, 0)) < 0) {

        printf("error fcntl()\n");
        return 0;
    }
    if (fcntl(STDIN_FILENO, F_SETFL, flags | O_NONBLOCK) < 0) {
        printf("error fcntl()\n");
        return 0;
    }
    // Comment until here

    while(running) {

        toogle = toogle ? false : true;
        if (toogle) {

            snprintf(myBuff + problem, myBuffSize - problem, "fin\n\n");
        } else {
            snprintf(myBuff + problem, myBuffSize - problem, "end\n\n");
        }
        fprintf(stdout, "%s", myBuff);
        sleep(1);
    }

    delete[] myBuff;
    return 0;
}

If you'll comment the // Comment from here to // Comment untill here, it will print all of the output (fin and end will be printed).

One solution to this problem is to open another fd to the current tty using fopen(ttyname(STDOUT_FILENO), "w") and to print into it.
I believe another solution is to async_write() into screen.

Answer 3

The output might be stuck in a buffer, and not flushed before program termination.

Try adding exit(0) at the end of the program, and see if it helps.

http://www.cplusplus.com/reference/cstdlib/exit/

All C streams (open with functions in <cstdio>) are closed (and flushed, if buffered), and all files created with tmpfile are removed.