Differences

This shows you the differences between two versions of the page.

--- haas:fall2017:cprog:projects:cbf0 [2017/09/17 18:56] – [Experiencing xxd] wedge
+++ haas:fall2017:cprog:projects:cbf0 [2017/10/15 20:49] (current) – wedge
@@ Line 3: / Line 3: @@
 <WRAP><fs 150%>CSCS1320 C/C++ Programming</fs></WRAP>
 </WRAP>
-~~TOC~~
 ======Project: C Binary Fun (cbf0)======
@@ Line 35: / Line 33: @@
 The computer works in units of **bytes**, which these days means groups of 8 bits. C has the ability to arbitrarily read and write individual bytes of data, and we will want to make use of that to aid us in our current task.
+=====Opening and reading from files=====
+The nice thing about C is that it tends to embody the "everything is a file" mantra from UNIX.
+What this means, basically, is that interacting with data in a file is really no different that interacting with data from the keyboard or data to the screen. We merely need a FILE pointer and appropriate resources allocated.
+To interact with a file, we must first declare a pointer to type FILE that will be our point of transaction.
+Common names for our file pointer variable are **fp**, **fPtr**, **input**, **inp**, but in reality can be anything you want.
+The intention is that, of course, you name variables so they are meaningful in the context of the overall implementation.
+<code c>
+    FILE *input  = NULL;
+</code>
+====Opening a file with fopen()====
+To attach a file stream to a FILE pointer, we utilize a file opening function such as **fopen()**.
+It takes two arguments:
+  - the path and name of file we wish to open (provided as a string)
+  - the mode we wish to open the file as (provided as a string)
+There are 3 common file opening modes (and combinations thereof, among others, sometimes dependent on the particular operating system being run). For now, I highly recommend just sticking to ONE mode of operation per FILE pointer. This can avoid messy things like data corruption and indirect logic/runtime errors.
+The 3 file modes:
+  * r - open file for reading (start at beginning)
+  * w - open file for writing (start at beginning)
+  * a - open file for appending (add to end)
+If we wanted to open the file "**sample0.txt**" in the current directory for **reading**, using the file pointer **input**, we would do the following:
+<code c>
+    input  = fopen ("sample0.txt", "r");
+</code>
+Note the double quotes around each argument. They both need to be strings (ie array of char terminated with NULL terminator characters), and the double quotes enables this.
+====Reading from the file====
+If the file is filled with a set format of data you'd like to retrieve, such as one short integer per line (basically, a text file filled with numbers), we can just use our trusty and familiar **fscanf()** function. We merely have have to indicate the correct file pointer:
+<code c>
+    short int value  = 0;
+    ...
+    fscanf (input, "%hd", &value);
+</code>
+If there is no simple universal "format" to the file, or if the raw information in the file is the information we are interested in, we need to instead look at it as a consecutive collection of bytes, and we can grab a char's worth of data (I would recommend starting out by looking at a file like this as a byte-by-byte or char-by-char endeavor... ignore trying to transact with groups of them until you get the process down with individual chars).
+The **fscanf()** function is still viable here, but if all we're after is a char value, there's a special purpose input function we can use instead: **fgetc()**
+To read a byte of data from a file and store it in our variable (called byte), we would do the following:
+<code c>
+    char byte  = 0;
+    ...
+    byte  = fgetc (input);
+</code>
+The **fgetc()** function takes the intended FILE pointer it is to read from as its argument, so **input** should be a FILE pointer AND should have previously been **fopen()**'ed (and for **reading**!) prior to calling **fgetc()**.
+To make things easier, placing logic to read from a file in a loop can be a very powerful combination.
 =====Task=====
@@ Line 112: / Line 176: @@
   * The hex values and rendered ASCII displayed will be sourced from the file specified on the command-line. While the target files for this project are less than 512 bytes, your program should be able to handle larger and smaller files, and update its display accordingly.
   * If a line throttle is given, your program is to stop output of data and ASCII rendering at that line, once it completes.
-  * Once the data in the file has been exhausted, you need to wrap up as appropriate; finish the current line (even if you have to pad spaces), display the corresponding ascii field (padding spaces as appropriate), and display the closing footer.
+  * Once the data in the file has been exhausted, you need to wrap up as appropriate; finish the current line (even if you have to pad spaces), and display the corresponding ascii field (padding spaces as appropriate).
   * Don't forget to **fclose()** any open file pointers! And **free()** any **malloc()**'ed or **calloc()**'ed memory.

Lab46 Wiki

User Tools

Site Tools

Differences

Page Tools