1. File Input/Output

2. Files A collection of related data treated as a unit. Two types Text
Binary
Stored in secondary storage devices.
Buffer
Temporary storage area that holds data while they are being transferred to or from memory.

3. Text Files Data are stored as human-readable characters
Each line of data ends with a newline character

4. Standard Files We have already discussed the standard files:
Standard input
Associated with the keyboard
Accessed using stdin
Standard output
Associated with the monitor
Accessed using stdout
Standard error
Accessed using stderr

5. User Files External files defined by the user
Must be explicitly opened in the program.
C assigns a logical name to each external file.
fopen() function
Prepares a file for processing
Makes the connection between the external file and the program
Creates a program file table or control structure to store the information needed to process the file

6. fopen FILE * fopen(char *filename, char *mode)
filename – string that supplies the name of the file as known to the external world. e.g. scores.dat
mode
Meaning r
Open file for reading
If file exists, the marker is positioned at beginning.
If file does not exist, error returned w
Open text file for writing
If file exists, it is emptied.
If file does not exist, it is created. a
Open text file for append.
If file exists, the marker is positioned at the end.

7. Examples
// Define and open the file scores.dat for input FILE *dataFile; dataFile = fopen("scores.dat", "r"); // Define and open the file prog.out for output FILE *outFile; outFile = fopen("prog.out", "w");

8. fopen() If succesful, will return a pointer to a FILE object
If not, a null pointer is returned
Always check to make sure the open was successful. If not, print an error message and exit
Example:
FILE *input = fopen("data.dat", "r");
if(input == NULL) {
printf("Failure to open file. Exiting\n");
exit(1); }

9. fopen() This semester, we will use the assert() function Example:
FILE *input = fopen("data.dat", "r");
assert(input != NULL);
We will need to have
#include <assert.h>
at the top of our program.

10. fclose int fclose(FILE *fp)
Note that the parameter is a FILE *, not the name of the file.
Used to close a file when no longer needed
Prevents associated file from being accessed again
Guarantees that data stored in the stream buffer is written to the file
Releases the FILE structure so that it can be used with another file
Frees system resources, such as buffer space
Returns zero on success, or EOF on failure

11. fclose()
Examples:
fclose(input);
fclose(dataFile);
fclose(outFile);

12. Field Specification
Consists of % character, a conversion code, and other formatting instructions
With one exception (*), each field specification must have a matching parameter in the parameter list that follows the string
The type in the field specification and the type of the parameter must match
Can have up to six elements for output
Can have up to five elements for input; precision is not allowed

13. Field Specification Elements of field specification scanf / fscanf:%
flag
Max width
size
conversion code
* do not store data
h short
l long int
l double (scan only)
L long double
left justify
+ sign (+ or -)
space if positive
0 zero padding
d, f, c, s, e, E, x, X, i, u, g, G %
flag
Max width
precision
size
conversion code
printf / fprintf

14. Field Specification
Examples %d
%7d
%+d
%lf
%7.2lf

15. End of File Controlled Loops
feof
int feof(FILE *fp)
Function to check if end of file has been reached.
For an end of file controlled loop
Read before the loop
Test for end of file: while (!feof(ptr))
Inside loop:
Process
Read at the bottom of the loop