TEC 284

C Programming – Part 6File Input and Output

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

FILE

#include <stdio.h>main () {//create 3 file pointersFILE *pRead;FILE *pWrite;FILE *pAppend;

}

Each FILE pointer can essentially open and manage a separate file

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

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

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

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

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”);}

}

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

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);

Reading a file

For simplicity, the test file the example is referring to is just a text file containing names

Names.txtJohnMichaelFrankStacyRobert

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);fscanf(pRead, “%s”, name);

}fclose(pRead);

}}

Results of the program

Contents of file :JohnMichaelFrankStacyRobert

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

Other types of specifiers for file reads

Type 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

Reading multiple fields test file Hobbies.txt

Michael ProgrammingSheila ShoppingSpencer FootballOlivia Dancing

Code to read multiple fieldsmain () {

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);fscanf(pRead, “%s%s”, name, hobby);

}fclose(pRead);

}}

Results

Name Hobby

Michael ProgrammingSheila ShoppingSpencer FootballOlivia Dancing

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

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 variablesscanf( “%s%s%s%f”, fName, lName,id, &gpa );//write variable contents separated by tabsfprintf(pWrite, “%s\t%s\t%s\t%.2f\n”,fName, lName, id,

gpa);

fclose(pWrite);} //end if

}

Reading the same record backmain () {

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 variablesfscanf( “pRead”, “%s%s%s%f”, fName, lName,id, &gpa );//print variable data to standard outputprintf(“%s %s\t%s\t%.2f\n”,fName, lName, id, gpa);

fclose(pRead);} //end if

}

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

Appending code example//This program appends to our previous Hobbies.txt file examplemain () {

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 variablesscanf( “%s%s”, name, hobby);//write variable contents separated by tabsfprintf(pWrite, “%s\t%s\n”,name, hobby);

fclose(pWrite);} //end if

}