/**
 * Napisati fju:
 * void osCatFileFromPos(const char* srcPath, const char* destPath, const off_t pos);
 * koja od pozicije pos nadovezuje fajl sa putanje srcPath na fajl sa putanjom destPath.
 * Ispis nije potreban.
**/
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>

void osCatFileFromPos(const char* srcPath, const char* destPath, const off_t pos);

int main(int argc, char** argv) {
    if (argc != 4) {
        exit(EXIT_FAILURE); // usage incorrect
    }

    long pos = strtol(argv[3], 0, 10);
    osCatFileFromPos(argv[1], argv[2], (off_t)pos);

    exit(EXIT_SUCCESS);
}

void osCatFileFromPos(const char* srcPath, const char* destPath, const off_t pos) {
    char* sPath = realpath(srcPath, NULL);
    char* dPath = realpath(destPath, NULL);

    if (sPath == NULL) {
        exit(EXIT_FAILURE); // srcPath is NULL
    }

    if (dPath != NULL) {
        if (!strcmp(sPath, dPath)) {
            exit(EXIT_FAILURE); // srcPath and destPath are the same.
        }
    }

    free(sPath);
    if (dPath != NULL) {
        free(dPath);
    }

    int srcFd, destFd;
    if (srcFd = open(srcPath, O_RDONLY) == -1) {
        exit(EXIT_FAILURE); // open failed.
    }

    if (destFd = open(destPath, O_RDWR) == -1) {
        exit(EXIT_FAILURE); // open failed
    }

    static int bufSize = 1<<13U;
    char* buffer = malloc(bufSize);
    if (buffer == NULL) {
        exit(EXIT_FAILURE); // malloc failed
    }

    if (lseek(destFd, (off_t)pos, SEEK_SET) == -1) {
        exit(EXIT_FAILURE); // lseek failed
    }

    int readBytes = 0;
    while ( (readBytes = read(srcFd, buffer, bufSize)) > 0) {
        if (write(destFd, buffer, readBytes) != readBytes) {
            exit(EXIT_FAILURE); // write failed
        }
    }

    if (readBytes != 0) {
        exit(EXIT_FAILURE); // read failed.
    }

    free(buffer);

    close(srcFd);
    close(destFd);
}