Как открыть файл (a_file.text) с помощью функции в main?

Question

Глобальные переменные:

FILE* gfptrIn = NULL;
FILE* gfptrOut = NULL;
FILE* gfptrKey = NULL;
char gInFileName[256];
char gOutFileName[256];
char gKeyFileName[256];

Вот функции, которые открывают входной файл и выходной файл:

FILE* openInputFile(char* filename)
{
    FILE* fptr;
    long sz;

    fptr = fopen(filename, "rb");
    if (fptr == NULL)
    {
        fprintf(stderr, "\n\nError - Could not open input file %s!\n\n", filename);
        exit(-1);
    }

    fseek(fptr, 0, SEEK_END);
    sz = ftell(fptr);
    fseek(fptr, 0, SEEK_SET);

    if (sz == 0)
    {
        fprintf(stderr, "Error - File size is zero for \"%s\".\n\n", filename);
        fprintf(stderr, "Aborting operation - try again!\n\n");
        fclose(fptr);
        exit(0);
    }

    return fptr;
} // openInputFile

FILE* openOutputFile(char* filename)
{
    FILE* fptr;

    fptr = fopen(filename, "wb+");
    if (fptr == NULL)
    {
        fprintf(stderr, "\n\nError - Could not open output file %s!\n\n", filename);
        exit(-1);
    }
    return fptr;
} // openOutputFile

EDIT: анализ аргументов командной строки

void parseCommandLine(int argc, char* argv[])
{
    int cnt;
    char ch;
    bool i_flag, o_flag, k_flag, p_flag, err_flag;

    i_flag = k_flag = false;                // these must be true in order to exit this function
    err_flag = p_flag = o_flag = false;     // these will generate different actions

    cnt = 1;    // skip program name

    while (cnt < argc)
    {
        ch = *argv[cnt];
        if (ch != '-')
        {
            fprintf(stderr, "All options must be preceeded by a dash '-'\n\n");
            usage(argv);
        }

        ch = *(argv[cnt] + 1);
        if (0)
        {
        }

        else if (ch == 'e' || ch == 'E')
        {
            if (i_flag == true || gOp != 0)
            {
                fprintf(stderr, "Error! Already specifed an input file - can't encrypt/decrypt multiple files.\n\n");
                usage(argv);
            }

            i_flag = true;
            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must specify a filename after '-e'\n\n");
                usage(argv);
            }

            strncpy(gInFileName, argv[cnt], 256);
            gOp = 1;    // encrypt
        }

        else if (ch == 'd' || ch == 'D')
        {
            if (i_flag == true || gOp != 0)
            {
                fprintf(stderr, "Error! Already specifed an input file - can't decrypt/encrypt multiple files.\n\n");
                usage(argv);
            }

            i_flag = true;
            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must specify a filename after '-d'\n\n");
                usage(argv);
            }

            strncpy(gInFileName, argv[cnt], 256);
            gOp = 2;    // decrypt
        }

        else if (ch == 'o' || ch == 'O')
        {
            if (o_flag == true)
            {
                fprintf(stderr, "Error! Already specifed an output file.\n\n");
                usage(argv);
            }
            o_flag = true;
            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must specify a filename after '-o'\n\n");
                usage(argv);
            }
            strncpy(gOutFileName, argv[cnt], 256);
        }

        else if (ch == 'k' || ch == 'K')
        {
            if (k_flag == true)
            {
                fprintf(stderr, "Error! Already specifed a key file.\n\n");
                usage(argv);
            }
            k_flag = true;
            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must specify a filename after '-k'\n\n");
                usage(argv);
            }
            strncpy(gKeyFileName, argv[cnt], 256);
        }

        else if (ch == 'p' || ch == 'P')
        {
            if (p_flag == true)
            {
                fprintf(stderr, "Error! Already specifed a password.\n\n");
                usage(argv);
            }
            p_flag = true;
            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must enter a password after '-p'\n\n");
                usage(argv);
            }
            strncpy(gPassword, argv[cnt], 256);
        }

        else if (ch == 'r' || ch == 'R')
        {
            int x;

            cnt++;
            if (cnt >= argc)
            {
                fprintf(stderr, "Error! Must enter number between 1 and 3 after '-r'\n\n");
                usage(argv);
            }
            x = atoi(argv[cnt]);
            if (x < 1 || x > 3)
            {
                fprintf(stderr, "Warning! Entered bad value for number of rounds. Setting it to one.\n\n");
                x = 1;
            }
            gNumRounds = x;
        }

        else
        {
            fprintf(stderr, "Error! Illegal option in argument. %s\n\n", argv[cnt]);
            usage(argv);
        }

        cnt++;
    } // end while

    if (gOp == 0)
    {
        fprintf(stderr, "Error! Encrypt or Decrypt must be specified.\n\n)");
        err_flag = true;
    }

    if (i_flag == false)
    {
        fprintf(stderr, "Error! No input file specified.\n\n");
        err_flag = true;
    }

    if (k_flag == false)
    {
        fprintf(stderr, "Error! No key file specified.\n\n");
        err_flag = true;
    }

    if (p_flag == false)
    {
        fprintf(stderr, "Warning! Using default 'password'.\n\n");
    }

    if (o_flag == false && err_flag == false)   // no need to do this if we have errors
    {
        strcpy(gOutFileName, gInFileName);
        if (gOp == 1)   // encrypt
        {
            strcat(gOutFileName, ".enc");
        }
        if (gOp == 2)   // decrypt
        {
            strcat(gOutFileName, ".dec");
        }
    }

    if (err_flag)
    {
        usage(argv);
    }
    return;
} // parseCommandLine

и вот основная функция, над которой я работал:

int main(int argc, char* argv[])
{
    // open the input and output files
    gfptrIn = openInputFile(gInFileName);
    gfptrOut = openOutputFile(gOutFileName);

    fclose(gfptrIn);
    fclose(gfptrOut);

return 0;
}

Попытка выяснить, как открыть нужный файл, который является текстовым файлом, с помощью таких функций, как openInputFile(char* filename) и openOutputFile(char* filename) в основной функции, но я не мог понять как.