#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

#define osErrorFatal(userMsg) osErrorFatalImpl((userMsg), __FILE__, __func__, __LINE__)
#define osAssert(expr, userMsg) \
    do { \
        if (!(expr)) \
            osErrorFatal(userMsg); \
    } while(0)


void osErrorFatalImpl(const char *userMsg, const char *fileName,
                      const char *functionName, const int lineNum);

static const char *os_Usage = "Usage: ./stdio_buffering path/to/reg/file";

int main(int argc, char **argv) {
    osAssert(2 == argc, os_Usage);

    /**
     * Magic mesage can be controlled with setvbuf calls:
     * 1) setvbuf(stdout, NULL, _IONBF, 0); -> magic message goes to stdout [no buffering is default for stderr]
     * 2) setvbuf(stdout, NULL, _IOLBF, 0); -> magic message goes to file [line buffering is default for stdout]
     * 3) setvbuf(stdout, NULL, _IOFBF, 0); -> magic message goes to file [full buffering is default for reg. files]
     * Also, if you append new line to prinf call magic message will go to file only in case 3).
     * */

    /**
     * This message is not printed on stdout cause of line buffering,
     * it is stored in printf's private buffer
     * */
    const char *magicMessage = "Magic happens to this message!";
    printf("%s", magicMessage);

    /**
     * Open file and redirect stdout to file
     * */
    int fd = open(argv[1], O_CREAT | O_TRUNC | O_WRONLY, 0644);
    osAssert(-1 != fd, "Opening file failed");

    /**
     * Save a stdout fd
     * */
    int fdCpy = dup(STDOUT_FILENO);
    osAssert(-1 != fdCpy, "Copying stdout fd failed");

    /**
     * Redirect stdout to file
     * */
    osAssert(dup2(fd, STDOUT_FILENO), "Redirecting stdout to file failed");
    printf("\nThis message will go to file along with magic message!\n");
    fflush(NULL); // assure that all streams are flushed

    /**
     * Restore stdout fd
     * */
    osAssert(-1 != dup2(fdCpy, STDOUT_FILENO), "Restoring stdout failed");

    /**
     * This goes to stdout
     * */
    printf("Finally stdout!\n");

    return 0;
}

void osErrorFatalImpl(const char *userMsg, const char *fileName,
                      const char *functionName, const int lineNum) {
    perror(userMsg);
    fprintf(stderr, "File: '%s'\nFunction: '%s'\nLine: '%d'\n", fileName, functionName, lineNum);
    exit(EXIT_FAILURE);
}