Q. 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.
Q. 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");
}
Q. 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);
}
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