1. C Programming – Part 6 File Input and Output
TEC 284

2. Streams
In C we use pointers to manage streams that read and write data
Streams are just file or hardware devices such as a monitor or pointer which can be controlled by a C program
We use pointers to the stream in order to control them
To point to and manage a file stream in C, an internal data structure called FILE is used

3. FILE Each FILE pointer can essentially open and manage a separate file
#include <stdio.h>
main () {
//create 3 file pointers
FILE *pRead;
FILE *pWrite;
FILE *pAppend; }
Each FILE pointer can essentially open and manage a separate file

4. Opening and Closing Files
When working with files, the basic steps involve opening the file, processing the file and then closing the file
After opening the file, there should always be some error checking/handling to ensure the file was successfully opened before working on the file
The fopen() function is used
fopen() returns a FILE pointer when the file is successfully opened

5. Opening a file in C main () { FILE *pRead;
pRead = fopen(“file1.txt”,“r”); }
fopen() takes two arguments
The name of the file to be opened
A flag to indicate how the file should be opened
In this example, the file file1.txt is opened in read-only mode

6. Text file open modes r - read text mode
w - write text mode (truncates file to zero length or creates new file)
a - append text mode for writing (opens or creates file and sets file pointer to the end-of-file)
r+ - read and write text mode
w+ - read and write text mode (truncates file to zero length or creates new file)
a+ - read and write text mode (opens or creates file and sets file pointer to the end-of-file

7. fopen() results
If the file does not exist and it is opened with read mode (r), then the open fails.
If the file is opened with append mode (a), then all write operations occur at the end of the file regardless of the current file position.
On success a pointer to the file stream is returned. On failure a null pointer is returned

8. Opening a file in C with error handling
main () { FILE *pRead; pRead = fopen(“file1.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nFile was opened for reading”); }

9. Check for NULL if(pRead == NULL) Can be shortened to if(pRead)
If pRead returns a non-NULL, the if condition is true.
If pRead returns NULL, the condition is false

10. Closing a file
After opening and processing a file, the file should be closed using fclose()
fclose() uses the FILE pointer to flush the stream and close the file
This frees up memory resources for the CPU and is good programming practice
fclose() takes a pointer to a file as its argument
fclose(pRead);

11. Reading a file
For simplicity, the test file the example is referring to is just a text file containing names
Names.txt
John
Michael
Frank
Stacy
Robert

12. Reading a file
main () { FILE *pRead; char name[20]; pRead = fopen(“Names.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nContents of file:\n”); fscanf(pRead, “%s”, name); while(!feof(pRead)) { //feof() checks for end-of-file //feof() returns a non-zero value when the end-of-file //marker is reached printf(“%s\n”,name); } fclose(pRead);

13. Results of the program
Contents of file : John Michael Frank Stacy Robert

14. Reading multiple fields
fscanf() can read multiple fields by supplying a series of type specifiers for the second argument
E.g. fscanf(pRead, “%s%s”, name, hobby);
The %s specifier will read a series of characters until a white space is found, including a blank, new line or tab

15. Other types of specifiers for file reads
Description c
Single character d
Decimal integer
e, E, f, g, G
Floating point o
Octal integer s
String of characters u
Unsigned decimal integer
x, X
Hexadecimal integer

16. Reading multiple fields test file Hobbies.txt
Michael Programming Sheila Shopping Spencer Football Olivia Dancing

17. Code to read multiple fields
main () { FILE *pRead; char name[20], char hobby[15]; pRead = fopen(“Hobbies.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nName\tHobby\n\n”); fscanf(pRead, “%s%s”, name, hobby); while(!feof(pRead)) { //feof() checks for end-of-file //feof() returns a non-zero value when the end-of-file //marker is reached printf(“%s\t%s\n”,name, hobby); } fclose(pRead);

18. Results
Name Hobby Michael Programming Sheila Shopping Spencer Football Olivia Dancing

19. Writing Data
Similarly to how printf() is used to send information to the screen fprintf() can be used to send information to a file
fprintf() uses a FILE pointer to write data to a file
The fprintf() function takes a FILE pointer, a list of data types and a list of values (or variables) to write the information to a file

20. Writing to a file
main () { FILE *pWrite; char fName[20], char lName[20], char id[15], float gpa; pWrite = fopen(“Students.txt”,“w”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nEnter first name, last name, id and GPA\n”); printf(“Enter data separated by spaces: ”); //store data entered by the user into variables scanf( “%s%s%s%f”, fName, lName,id, &gpa ); //write variable contents separated by tabs fprintf(pWrite, “%s\t%s\t%s\t%.2f\n”,fName, lName, id, gpa); fclose(pWrite); } //end if }

21. Reading the same record back
main () { FILE *pRead; char fName[20], char lName[20], char id[15], float gpa; pRead = fopen(“Students.txt”,“r”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { //print heading printf(“\nName\t\tID\t GPA\n\n”); //read field info from file and store in variables fscanf( “pRead”, “%s%s%s%f”, fName, lName,id, &gpa ); //print variable data to standard output printf(“%s %s\t%s\t%.2f\n”,fName, lName, id, gpa); fclose(pRead); } //end if }

22. Appending to a file
When opening a file using fopen() with a w argument value will erase any previous data stored in the file
Use the a attribute to append data at the end of the file
If the file does not exist, a new one will be created

23. Appending code example
//This program appends to our previous Hobbies.txt file example main () { FILE *pWrite; char name[20], char hobby[20]; pWrite = fopen(“Hobbies.txt”,“a”); if(pRead == NULL) { printf(“\nFile could not be opened”); } else { printf(“\nEnter a new name and hobby: ”); //store data entered by the user into variables scanf( “%s%s”, name, hobby); //write variable contents separated by tabs fprintf(pWrite, “%s\t%s\n”,name, hobby); fclose(pWrite); } //end if }