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1. Can stdout be forced to print somewhere other than the screen?

Although the stdout standard stream defaults to the screen, you can force it to print to another device using
something called redirection. For instance, consider the following program:

/* redir.c */
#include <stdio.h>
void main(void);
void main(void)
{
     printf("Let's get redirected!\n");
}

At the DOS prompt, instead of entering just the executable name, follow it with the redirection character
>, and thus redirect what normally would appear on-screen to some other device. The following example
would redirect the program's output to the prn device, usually the printer attached on LPT1:

C:>REDIR > PRN

Alternatively, you might want to redirect the program's output to a file, as the following example shows:

C:>REDIR > REDIR.OUT

In this example, all output that would have normally appeared on-screen will be written to the file REDIR.OUT.

2. How can I avoid the Abort, Retry, Fail messages?

When DOS encounters a critical error, it issues a call to interrupt 24, the critical error handler. Your C
compiler library contains a function named harderr() that takes over the handling of calls to interrupt 24.
The harderr() function takes one argument, a pointer to a function that is called if there is a hardware error.

Your user-defined hardware error-handling function is passed information regarding the specifics of the
hardware error that occurred. In your function, you can display a user-defined message to avoid the ugly
Abort, Retry, Fail message. This way, your program can elegantly handle such simple user errors as your
not inserting the disk when prompted to do so.

When a hardware error is encountered and your function is called, you can either call the C library function
hardretn() to return control to your application or call the C library function  hardresume() to return control
to DOS. Typically, disk errors can be trapped and your program can continue by using the hardresume()
function. Other device errors, such as a bat FAT (file allocation table) error, are somewhat fatal, and your
application should handle them by using the hardretn() function. Consider the following example, which
uses the harderr() function to trap for critical errors and notifies the user when such an error occurs:

#include <stdio.h>
#include <dos.h>
#include <fcntl.h>
#include <ctype.h>
void main(void);
void far error_handler(unsigned, unsigned, unsigned far*);
void main(void)
{
     int file_handle, ret_code;
     /* Install the custom error-handling routine. */
     _harderr(error_handler);
     printf("\nEnsure that the A: drive is empty, \n");
     printf("then press any key.\n\n");
     getch();
     printf("Trying to write to the A: drive...\n\n");
     /* Attempt to access an empty A: drive... */
     ret_code = _dos_open("A:FILE.TMP", O_RDONLY, &file_handle);
     /* If the A: drive was empty, the error_handler() function was
        called. Notify the user of the result of that function. */
     switch (ret_code)
     {
          case 100: printf("Unknown device error!\n");
                    break;
          case 2:   printf("FILE.TMP was not found on drive A!\n");
                    break;
          case 0:   printf("FILE.TMP was found on drive A!\n");
                    break;
          default:  printf("Unknown error occurred!\n");
                    break;
     }
}
void far error_handler(unsigned device_error, unsigned error_val, 
                        unsigned far* device_header)
{
     long x;
     /* This condition will be true only if a nondisk error occurred. */
     if (device_error & 0x8000)
          _hardretn(100);
     /* Pause one second. */
     for (x=0; x<2000000; x++);
         /* Retry to access the drive. */
         _hardresume(_HARDERR_RETRY);
}

In this example, a custom hardware error handler is installed named error_handler(). When the program
attempts to access the A: drive and no disk is found there, the error_handler() function is called. The
error_handler() function first checks to ensure that the problem is a disk error. If the problem is not a disk
error, it returns 100 by using the hardretn() function. Next, the program pauses for one second and issues
a hardresume() call to retry accessing the A: drive.

3. How can I make sure that my program is the only one accessing a file?

By using the sopen() function, you can open a file in shared mode and explicitly deny
reading and writing permissions to any other program but yours. This task is accomplished by using the
SH_DENYWR shared flag to denote that your program is going to deny any writing or reading attempts by other
programs. For example, the following snippet of code shows a file being opened in shared mode, denying
access to all other files:

/* Note that the sopen() function is not ANSI compliant... */
fileHandle = sopen("C:\\DATA\\SETUP.DAT", O_RDWR, SH_DENYWR);

By issuing this statement, all other programs are denied access to the SETUP.DAT file. If another program
were to try to open SETUP.DAT for reading or writing, it would receive an EACCES error code, denoting
that access is denied to the file.

4. How can I open a file so that other programs can update it at the same time?

Your C compiler library contains a low-level file function called sopen() that can be used to open a file in
shared mode. Beginning with DOS 3.0, files could be opened in shared mode by loading a special program
named SHARE.EXE. Shared mode, as the name implies, allows a file to be shared with other programs as
well as your own. Using this function, you can allow other programs that are running to update the same
file you are updating.

The sopen() function takes four parameters: a pointer to the filename you want to open, the operational
mode you want to open the file in, the file sharing mode to use, and, if you are creating a file, the mode to
create the file in. The second parameter of the sopen() function, usually referred to as the "operation flag"
parameter, can have the following values assigned to it:

 Constant Description
 O_APPEND-Appends all writes to the end of the file
 O_BINARY-Opens the file in binary (untranslated) mode
 O_CREAT-If the file does not exist, it is created
 O_EXCL-If the O_CREAT flag is used and the file exists, returns an error
 O_RDONLY-Opens the file in read-only mode
 O_RDWR-Opens the file for reading and writing
 O_TEXT-Opens the file in text (translated) mode
 O_TRUNC-Opens an existing file and writes over its contents
 O_WRONLY-Opens the file in write-only mode

The third parameter of the sopen() function, usually referred to as the "sharing flag," can have the following values assigned to it:

 Constant Description
 SH_COMPAT-No other program can access the file
 SH_DENYRW-No other program can read from or write to the file
 SH_DENYWR-No other program can write to the file
 SH_DENYRD-No other program can read from the file
 SH_DENYNO-Any program can read from or write to the file

If the sopen() function is successful, it returns a non-negative number that is the file's handle. If an error occurs, -1 is returned, and the global variable errno is set to one of the following values:

 Constant Description
 ENOENT-File or path not found
 EMFILE-No more file handles are available
 EACCES-Permission denied to access file
 EINVACC-Invalid access code

The following example shows how to open a file in shared mode:

#include <stdio.h>
#include <fcntl.h>
#include <sys\stat.h>
#include <io.h>
#include <share.h>
void main(void);
void main(void)
{
     int file_handle;
     /* Note that sopen() is not ANSI compliant */
     file_handle = sopen("C:\\DATA\\TEST.DAT", O_RDWR, SH_DENYNO);
     close(file_handle);
}

Whenever you are sharing a file's contents with other programs, you should be sure to use the standard C
library function named locking() to lock a portion of your file when you are updating it.

5. How can I prevent another program from modifying part of a file that I am modifying?

 If your C compiler library comes with a function named  locking() that can be used to lock and unlock portions of shared files.

The locking function takes three arguments: a handle to the shared file you are going to lock or unlock, the
operation you want to perform on the file, and the number of bytes you want to lock. The file lock is placed
relative to the current position of the file pointer, so if you are going to lock bytes located anywhere but at
the beginning of the file, you need to reposition the file pointer by using the lseek() function.

The following example shows how a binary index file named SONGS.DAT can be locked and unlocked:

#include <stdio.h>
#include <io.h>
#include <fcntl.h>
#include <process.h>
#include <string.h>
#include <share.h>
#include <sys\locking.h>
void main(void);
void main(void)
{
     int file_handle, ret_code;
     char* song_name = "Six Months In A Leaky Boat";
     char rec_buffer[50];
     file_handle = sopen("C:\\DATA\\SONGS.DAT", O_RDWR, SH_DENYNO);
     /* Assuming a record size of 50 bytes, position the file
        pointer to the 10th record. */
     lseek(file_handle, 450, SEEK_SET);
     /* Lock the 50-byte record. */
     ret_code = locking(file_handle, LK_LOCK, 50);
     /* Write the data and close the file. */
     memset(rec_buffer, '\0', sizeof(rec_buffer));
     sprintf(rec_buffer, "%s", song_name);
     write(file_handle, rec_buffer, sizeof(rec_buffer));
     lseek(file_handle, 450, SEEK_SET);
     locking(file_handle, LK_UNLCK, 50);
     close(file_handle);
}

Notice that before the record is locked, the record pointer is positioned to the 10th record (450th byte) by
using the lseek() function. Also notice that to write the record to the file, the record pointer has to be
repositioned to the beginning of the record before unlocking the record.

6. How can I read and write comma-delimited text?

Many of today's popular programs use comma-delimited text as a means of transferring data from one
program to another, such as the exported data from a spreadsheet program that is to be imported by a database
program. Comma-delimited means that all data (with the exception of numeric data) is surrounded by
double quotation marks ("") followed by a comma. Numeric data appears as-is, with no surrounding double
quotation marks. At the end of each line of text, the comma is omitted and a newline is used.

To read and write the text to a file, you would use the fprintf() and fscanf() standard C library functions.
The following example shows how a program can write out comma-delimited text and then read it back in.

#include <stdio.h>
#include <string.h>
typedef struct name_str
{
     char       first_name[15];
     char       nick_name[30];
     unsigned years_known;
} NICKNAME;
NICKNAME nick_names[5];
void main(void);
void set_name(unsigned, char*, char*, unsigned);
void main(void)
{
     FILE*     name_file;
     int       x;
     NICKNAME tmp_name;
     printf("\nWriting data to NICKNAME.DAT, one moment please...\n");
     /* Initialize the data with some values... */
     set_name(0,    "Sheryl",      "Basset",      26);
     set_name(1,    "Joel",        "Elkinator",    1);
     set_name(2,    "Cliff",       "Shayface",    12);
     set_name(3,    "Lloyd",       "Lloydage",    28);
     set_name(4,    "Scott",       "Pie",          9);
     /* Open the NICKNAME.DAT file for output in text mode. */
     name_file = fopen("NICKNAME.DAT", "wt");
     /* Iterate through all the data and use the fprintf() function
        to write the data to a file. */
     for (x=0; x<5; x++)
     {
          fprintf(name_file, "\"%s\", \"%s\", %u\n",
                      nick_names[x].first_name,
                      nick_names[x].nick_name,
                      nick_names[x].years_known);
     }
     /* Close the file and reopen it for input. */
     fclose(name_file);
     printf("\nClosed NICKNAME.DAT, reopening for input...\n");
     name_file = fopen("NICKNAME.DAT", "rt");
     printf("\nContents of the file NICKNAME.DAT:\n\n");
     /* Read each line in the file using the scanf() function
        and print the file's contents. */
     while (1)
     {
          fscanf(name_file, "%s %s %u",
                     tmp_name.first_name,
                     tmp_name.nick_name,
                     &tmp_name.years_known);
          if (feof(name_file))
               break;
          printf("%-15s %-30s %u\n",
                     tmp_name.first_name,
                     tmp_name.nick_name,
                     tmp_name.years_known);
     }
     fclose(name_file);
}
void set_name(unsigned name_num, char* f_name, char* n_name, unsigned years)
{
     strcpy(nick_names[name_num].first_name, f_name);
     strcpy(nick_names[name_num].nick_name,  n_name);
     nick_names[name_num].years_known = years;
}

7. How can you restore a redirected standard stream?

The preceding example showed how you can redirect a standard stream from within your program. But what
if later in your program you wanted to restore the standard stream to its original state? By using the standard
C library functions named dup() and fdopen(), you can restore a standard stream such as stdout to its
original state.

The  dup() function duplicates a file handle. You can use the  dup() function to save the file handle
corresponding to the  stdout standard stream. The  fdopen() function opens a stream that has been
duplicated with the dup() function. Thus, as shown in the following example, you can redirect standard
streams and restore them:

#include <stdio.h>
void main(void);

void main(void)
{
     int orig_stdout;
     /* Duplicate the stdout file handle and store it in orig_stdout. */
     orig_stdout = dup(fileno(stdout));
     /* This text appears on-screen. */
     printf("Writing to original stdout...\n");
     /* Reopen stdout and redirect it to the "redir.txt" file. */
     freopen("redir.txt", "w", stdout);
     /* This text appears in the "redir.txt" file. */
     printf("Writing to redirected stdout...\n");
     /* Close the redirected stdout. */
     fclose(stdout);
     /* Restore the original stdout and print to the screen again. */
     fdopen(orig_stdout, "w");
     printf("I'm back writing to the original stdout.\n");
}

8. How do you determine a file's attributes?

The file attributes are stored in the  find_t.attrib structure member. This structure member
is a single character, and each file attribute is represented by a single bit. Here is a list of the valid DOS file
attributes:

 Value Description Constant
 0x00-Normal-(none)
 0x01-Read Only-FA_RDONLY
 0x02-Hidden File-FA_HIDDEN
 0x04-System File-FA_SYSTEM
 0x08-Volume Label-FA_LABEL
 0x10-Subdirectory-FA_DIREC
 0x20-Archive File-FA_ARCHIVE

To determine the file's attributes, you check which bits are turned on and map them corresponding to the
preceding table. For example, a read-only hidden system file will have the first, second, and third bits turned
on. A "normal" file will have none of the bits turned on. To determine whether a particular bit is turned on,
you do a bit-wise AND with the bit's constant representation.

The following program uses this technique to print a file's attributes:

#include <stdio.h>
#include <direct.h>
#include <dos.h>
#include <malloc.h>
#include <memory.h>
#include <string.h>
typedef struct find_t FILE_BLOCK;
void main(void);
void main(void)
{
     FILE_BLOCK f_block;  /* Define the find_t structure variable */
     int ret_code;     /* Define a variable to store the return codes */
     printf("\nDirectory listing of all files in this directory:\n\n");
     /* Use the "*.*" file mask and the 0xFF attribute mask to list
        all files in the directory, including system files, hidden
        files, and subdirectory names. */
     ret_code = _dos_findfirst("*.*", 0xFF, &f_block);
     /* The _dos_findfirst() function returns a 0 when
        it is successful and has found a valid filename
        in the directory. */
     while (ret_code == 0)
     {
          /* Print the file's name */
          printf("%-12s  ", f_block.name);
          /* Print the read-only attribute */
          printf("%s ", (f_block.attrib & FA_RDONLY) ? "R" : ".");
          /* Print the hidden attribute */
          printf("%s ", (f_block.attrib & FA_HIDDEN) ? "H" : ".");
          /* Print the system attribute */
          printf("%s ", (f_block.attrib & FA_SYSTEM) ? "S" : ".");
          /* Print the directory attribute */
          printf("%s ", (f_block.attrib & FA_DIREC)  ? "D" : ".");
          /* Print the archive attribute */
          printf("%s\n", (f_block.attrib & FA_ARCH)  ? "A" : ".");
          /* Use the _dos_findnext() function to look
             for the next file in the directory. */
          ret_code = _dos_findnext(&f_block);
     }
     printf("\nEnd of directory listing.\n");
}

9. How do you determine whether to use a stream function or a low-level function?

Stream functions such as fread() and  fwrite() are buffered and are more efficient when reading and writing
text or binary data to files. You generally gain better performance by using stream functions rather than their
unbuffered low-level counterparts such as read() and write().

In multiuser environments, however, when files are typically shared and portions of files are continuously
being locked, read from, written to, and unlocked, the stream functions do not perform as well as the low-
level functions. This is because it is hard to buffer a shared file whose contents are constantly changing.

Generally, you should always use buffered stream functions when accessing nonshared files, and you should
always use the low-level functions when accessing shared files.

10. How do you list a file's date and time?

A file's date and time are stored in the  find_t structure returned from the  _dos_findfirst() and
_dos_findnext() functions.

The date and time stamp of the file is stored in the find_t.wr_date and find_t.wr_time structure members.
The file date is stored in a two-byte unsigned integer as shown here:

 Element Offset Range
 Seconds-5 bits-0-9 (multiply by 2 to get the seconds value)
 Minutes-6 bits-0-59
 Hours-5 bits-0-23

Similarly, the file time is stored in a two-byte unsigned integer, as shown here:

 Element Offset Range
 Day-5 bits-1-31
 Month-4 bits-1-12
 Year-7 bits-0-127 (add the value "1980" to get the year value)

Because DOS stores a file's seconds in two-second intervals, only the values 0 to 29 are needed. You simply
multiply the value by 2 to get the file's true seconds value. Also, because DOS came into existence in 1980,
no files can have a time stamp prior to that year. Therefore, you must add the value "1980" to get the file's
true year value.

The following example program shows how you can get a directory listing along with each file's date and time stamp:

#include <stdio.h>
#include <direct.h>
#include <dos.h>
#include <malloc.h>
#include <memory.h>
#include <string.h>
typedef struct find_t FILE_BLOCK;
void main(void);
void main(void)
{
     FILE_BLOCK f_block;   /* Define the find_t structure variable */
     int ret_code;         /* Define a variable to store return codes */
     int hour;             /* We're going to use a 12-hour clock! */
     char* am_pm;          /* Used to print "am" or "pm" */
     printf("\nDirectory listing of all files in this directory:\n\n");
     /* Use the "*.*" file mask and the 0xFF attribute mask to list
        all files in the directory, including system files, hidden
        files, and subdirectory names. */
     ret_code = _dos_findfirst("*.*", 0xFF, &f_block);
     /* The _dos_findfirst() function returns a 0 when it is successful
        and has found a valid filename in the directory. */
     while (ret_code == 0)
     {
          /* Convert from a 24-hour format to a 12-hour format. */
          hour = (f_block.wr_time >> 11);
          if (hour > 12)
          {
               hour  = hour - 12;
               am_pm = "pm";
          }
          else
               am_pm = "am";
          /* Print the file's name, date stamp, and time stamp. */
          printf("%-12s  %02d/%02d/%4d  %02d:%02d:%02d %s\n",
                    f_block.name,                      /* name  */
                    (f_block.wr_date >> 5) & 0x0F,     /* month */
                    (f_block.wr_date) & 0x1F,          /* day   */
                    (f_block.wr_date >> 9) + 1980,     /* year  */
                    hour,                              /* hour  */
                    (f_block.wr_time >> 5) & 0x3F,     /* minute  */
                    (f_block.wr_time & 0x1F) * 2,      /* seconds */
                    am_pm);
          /* Use the _dos_findnext() function to look
             for the next file in the directory. */
          ret_code = _dos_findnext(&f_block);
     }
     printf("\nEnd of directory listing.\n");
}

Notice that a lot of bit-shifting and bit-manipulating had to be done to get the elements of the time variable
and the elements of the date variable. If you happen to suffer from bitshiftophobia (fear of shifting bits), you
can optionally code the preceding example by forming a union between the find_t structure and your own
user-defined structure, such as this:

/* This is the find_t structure as defined by ANSI C. */
struct find_t
{
     char reserved[21];
     char attrib;
     unsigned wr_time;
     unsigned wr_date;
     long size;
     char name[13];
}
/* This is a custom find_t structure where we
   separate out the bits used for date and time. */
struct my_find_t
{
     char reserved[21];
     char attrib;
     unsigned seconds:5;
     unsigned minutes:6;
     unsigned hours:5;
     unsigned day:5;
     unsigned month:4;
     unsigned year:7;
     long size;
     char name[13];
}
/* Now, create a union between these two structures
   so that we can more easily access the elements of
   wr_date and wr_time. */
union file_info
{
     struct find_t ft;
     struct my_find_t mft;
}

Using the preceding technique, instead of using bit-shifting and bit-manipulating, you can now extract date and time elements like this:

...
file_info my_file;
...
printf("%-12s  %02d/%02d/%4d  %02d:%02d:%02d %s\n",
          my_file.mft.name,             /* name    */
          my_file.mft.month,            /* month   */
          my_file.mft.day,              /* day     */
          (my_file.mft.year + 1980),    /* year    */
          my_file.mft.hours,            /* hour    */
          my_file.mft.minutes,          /* minute  */
          (my_file.mft.seconds * 2),    /* seconds */
          am_pm);

11. How do you list files in a directory?

Unfortunately, there is no built-in function provided in the C language such as dir_list() that would easily
provide you with a list of all files in a particular directory. By utilizing some of C's built-in directory functions,
however, you can write your own dir_list() function.

First of all, the include file dos.h defines a structure named  find_t, which represents the structure of the DOS
file entry block. This structure holds the name, time, date, size, and attributes of a file. Second, your C
compiler library contains the functions _dos_findfirst() and _dos_findnext(), which can be used to find
the first or next file in a directory.

The  _dos_findfirst() function requires three arguments. The first argument is the file mask for the
directory list. A mask of *.* would be used to list all files in the directory. The second argument is an attribute
mask, defining which file attributes to search for. For instance, you might want to list only files with the
Hidden or Directory attributes. The last argument of the  _dos_findfirst() function is a pointer to the variable that is to hold the directory information (the find_t structure variable).

The second function you will use is the _dos_findnext() function. Its only argument is a pointer to the
find_t structure variable that you used in the _dos_findfirst() function. Using these two functions and
the find_t structure, you can iterate through the directory on a disk and list each file in the directory. Here
is the code to perform this task:

#include <stdio.h>
#include <direct.h>
#include <dos.h>
#include <malloc.h>
#include <memory.h>
#include <string.h>
typedef struct find_t FILE_BLOCK;
void main(void);
void main(void)
{
     FILE_BLOCK f_block;      /* Define the find_t structure variable */
     int ret_code;     /* Define a variable to store the return codes */
     printf("\nDirectory listing of all files in this directory:\n\n");
     /* Use the "*.*" file mask and the 0xFF attribute mask to list
        all files in the directory, including system files, hidden
        files, and subdirectory names. */
     ret_code = _dos_findfirst("*.*", 0xFF, &f_block);
     /* The _dos_findfirst() function returns a 0 when it is successful
        and has found a valid filename in the directory. */
     while (ret_code == 0)
     {
          /* Print the file's name */
          printf("%-12s\n", f_block.name);
          /* Use the _dos_findnext() function to look
             for the next file in the directory. */
          ret_code = _dos_findnext(&f_block);
     }
     printf("\nEnd of directory listing.\n");
}

12. How do you redirect a standard stream?

Most operating systems, including DOS, provide a means to redirect program input and output to and from
different devices. This means that rather than your program output (stdout) going to the screen, it can be
redirected to a file or printer port. Similarly, your program's input (stdin) can come from a file rather than
the keyboard. In DOS, this task is accomplished using the redirection characters, < and >. For example, if
you wanted a program named PRINTIT.EXE to receive its input (stdin) from a file named STRINGS.TXT, you would enter the following command at the DOS prompt:

C:>PRINTIT < STRINGS.TXT

Notice that the name of the executable file always comes first. The less-than sign (<) tells DOS to take the
strings contained in STRINGS.TXT and use them as input for the PRINTIT program.

Redirection of standard streams does not always have to occur at the operating system. You can redirect a
standard stream from within your program by using the standard C library function named freopen(). For
example, if you wanted to redirect the  stdout standard stream within your program to a file named
OUTPUT.TXT, you would implement the freopen() function as shown here:

...
freopen("output.txt", "w", stdout);
...

Now, every output statement (printf(), puts(), putch(), and so on) in your program will appear in the file OUTPUT.TXT.

13. How do you sort filenames in a directory?

When you are sorting the filenames in a directory, the one-at-a-time approach does not work. You need some
way to store the filenames and then sort them when all filenames have been obtained. This task can be
accomplished by creating an array of pointers to find_t structures for each filename that is found. As each
filename is found in the directory, memory is allocated to hold the find_t entry for that file. When all
filenames have been found, the qsort() function is used to sort the array of find_t structures by filename.

The  qsort() function can be found in your compiler's library. This function takes four parameters: a pointer
to the array you are sorting, the number of elements to sort, the size of each element, and a pointer to a
function that compares two elements of the array you are sorting. The comparison function is a user-defined
function that you supply. It returns a value less than zero if the first element is less than the second element,
greater than zero if the first element is greater than the second element, or zero if the two elements are equal.
Consider the following example:

#include <stdio.h>
#include <direct.h>
#include <dos.h>
#include <malloc.h>
#include <memory.h>
#include <string.h>
typedef struct find_t FILE_BLOCK;
int  sort_files(FILE_BLOCK**, FILE_BLOCK**);
void main(void);
void main(void)
{
     FILE_BLOCK f_block;       /* Define the find_t structure variable */
     int ret_code;             /* Define a variable to store the return
                                  codes */
     FILE_BLOCK** file_list;   /* Used to sort the files */
     int file_count;           /* Used to count the files */
     int x;                    /* Counter variable */
     file_count = -1;
     /* Allocate room to hold up to 512 directory entries. */
     file_list = (FILE_BLOCK**) malloc(sizeof(FILE_BLOCK*) * 512);
     printf("\nDirectory listing of all files in this directory:\n\n");
     /* Use the "*.*" file mask and the 0xFF attribute mask to list
        all files in the directory, including system files, hidden
        files, and subdirectory names. */
     ret_code = _dos_findfirst("*.*", 0xFF, &f_block);
     /* The _dos_findfirst() function returns a 0 when it is successful
        and has found a valid filename in the directory. */
     while (ret_code == 0 && file_count < 512)
     {
          /* Add this filename to the file list */
          file_list[++file_count] =
              (FILE_BLOCK*) malloc(sizeof(FILE_BLOCK));
          *file_list[file_count] = f_block;
          /* Use the _dos_findnext() function to look
             for the next file in the directory. */
          ret_code = _dos_findnext(&f_block);
     }
     /* Sort the files */
     qsort(file_list, file_count, sizeof(FILE_BLOCK*), sort_files);
     /* Now, iterate through the sorted array of filenames and
        print each entry. */
     for (x=0; x<file_count; x++)
     {
          printf("%-12s\n", file_list[x]->name);
     }
     printf("\nEnd of directory listing.\n");
}
int sort_files(FILE_BLOCK** a, FILE_BLOCK** b)
{
     return (strcmp((*a)->name, (*b)->name));
}

This example uses the user-defined function named sort_files() to compare two filenames and return the
appropriate value based on the return value from the standard C library function strcmp(). Using this same
technique, you can easily modify the program to sort by date, time, extension, or size by changing the element
on which the sort_files() function operates.

14. How do you view the PATH?

Your C compiler library contains a function called getenv() that can retrieve any specified environment
variable. It has one argument, which is a pointer to a string containing the environment variable you want
to retrieve. It returns a pointer to the desired environment string on successful completion. If the function
cannot find your environment variable, it returns NULL.

The following example program shows how to obtain the PATH environment variable and print it on-screen:

#include <stdio.h>
#include <stdlib.h>
void main(void);
void main(void)
{
     char* env_string;
     env_string = getenv("PATH");
     if (env_string == (char*) NULL)
          printf("\nYou have no PATH!\n");
     else
          printf("\nYour PATH is: %s\n", env_string);
}

15. If errno contains a nonzero number, is there an error?

The global variable errno is used by many standard C library functions to pass back to
your program an error code that denotes specifically which error occurred. However, your
program should not check the value of errno to determine whether an error occurred.

Usually, the standard C library function you are calling returns with a return code which denotes that an error has occurred and that the value of errno has been set to a specific error number. If no error has occurred or if you are using a library function that does not reference errno, there is a good chance that errno will contain an erroneous value. For performance enhancement, the errno variable is sometimes not cleared by the functions that use it.

You should never rely on the value of errno alone; always check the return code from the function you are
calling to see whether errno should be referenced. Refer to your compiler's library documentation for
references to functions that utilize the errno global variable and for a list of valid values for errno.

16. What is a stream?

A stream is a continuous series of bytes that flow into or out of your program. Input and output from devices
such as the mouse, keyboard, disk, screen, modem, and printer are all handled with streams. In C, all streams
appear as files - not physical disk files necessarily, but rather  logical files that refer to an input/output source.
The C language provides five "standard" streams that are always available to your program. These streams
do not have to be opened or closed. These are the five standard streams:

 Name Description Example
 stdin-Standard Input-Keyboard
 stdout-Standard Output-Screen
 stderr-Standard Error-Screen
 stdprn-Standard Printer-LPT1: port
 stdaux-Standard Auxiliary-COM1: port

Note that the stdprn and stdaux streams are not always defined. This is because LPT1: and COM1: have
no meaning under certain operating systems. However, stdin, stdout, and stderr are always defined. Also,
note that the stdin stream does not have to come from the keyboard; it can come from a disk file or some
other device through what is called redirection. In the same manner, the stdout stream does not have to
appear on-screen; it too can be redirected to a disk file or some other device. See the next FAQ for an
explanation of redirection.

17. What is the difference between text and binary modes?

Streams can be classified into two types: text streams and binary streams. Text streams are interpreted, with
a maximum length of 255 characters. With text streams, carriage return/line feed combinations are translated
to the newline  \n character and vice versa. Binary streams are uninterpreted and are treated one byte at a time
with no translation of characters. Typically, a text stream would be used for reading and writing standard
text files, printing output to the screen or printer, or receiving input from the keyboard.

A binary text stream would typically be used for reading and writing binary files such as graphics or word processing documents,
reading mouse input, or reading and writing to the modem.