1. EECE.2160 ECE Application Programming
Instructors:
Dr. Lin Li & Dr. Michael Geiger
Spring 2019
Lecture 32:
File I/O

2. ECE Application Programming: Lecture 32
Lecture outline
Announcements/reminders
Program 7 due today (4/24)
Program 8 due 5/3
Course evaluations to be posted online; returned at final exam
Today’s class
Program 8 intro
File I/O
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ECE Application Programming: Lecture 32

3. ECE Application Programming: Lecture 32
File information
Files commonly used for input/output
Interface uses FILE pointers
FILE type defined in <stdio.h>
Allows access to necessary file characteristics
File characteristics include:
Name—for example z:\Visual Studio 2010\Projects\fileio\fileio\myinput.txt
Read/write permission
Type (binary or ASCII text)
Access (security; single/multiple user)
Position in file
Programmer doesn’t have to account for this info
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ECE Application Programming: Lecture 32

4. Basic file I/O functions
Before reading/writing file, program must gain access to file
fopen() function used to open file
Returns FILE * if successful, NULL otherwise
When done with file, program should close it
fclose() function used to close file
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ECE Application Programming: Lecture 32

5. ECE Application Programming: Lecture 32
fopen()
FILE *fopen(char *fname, char *faccess)
fname: name of file (e.g., "f1.txt")
Name may require full path
faccess: string providing
First char: access mode
r/w/a (read/write/append)
Write starts at beginning of file, append starts at end
Either write or append creates new file if none exists
Additional (optional) char: file type
b/t (binary/text)
Text files are human readable (default--don’t need t)
Binary files are just raw bytes
Valid access strings: "r", "w", "a", "rb", "wb", "ab"
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ECE Application Programming: Lecture 32

6. ECE Application Programming: Lecture 32
fopen() (continued)
If successful, fopen() returns valid FILE * to be used with file read/write functions
If unsuccessful, fopen() returns NULL
Test this error condition to ensure file opened
Why might fopen() be unsuccessful?
Try to open input file that doesn’t exist
Try to open read-only file for writing
Try to open file that’s locked by another application
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ECE Application Programming: Lecture 32

7. ECE Application Programming: Lecture 32
fopen() example
FILE *fp; fp = fopen("in1.txt", "r"); if (fp == NULL) printf("Couldn’t open file\n"); else { … // FILE * is valid // Continue with program }
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ECE Application Programming: Lecture 32

8. File i/o function calls
fclose(FILE *file_handle)
Closes a file
Argument is address returned by fopen()
fclose(fp);
Recommended for input files
Required for output files
OS often doesn’t write last bit of file to disk until file is closed
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ECE Application Programming: Lecture 32

9. Example of basic file function usage
int main() { FILE *fp; // Open text file for reading fp = fopen("in.txt", "r"); if (fp == NULL) { printf("Error: could not open in.txt"); return 0; } ... // CODE TO EXECUTE IF FILE OPENS fclose(fp); // Close file when done
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ECE Application Programming: Lecture 32

10. ECE Application Programming: Lecture 33
Example: File I/O
Write a program to:
Read three integer values from the file myinput.txt
Determine sum and average
Write the original three values as well as the sum and average to the file myoutput.txt
Note that:
The program should exit if an error occurs in opening a file
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ECE Application Programming: Lecture 33

11. ECE Application Programming: Lecture 33
The program (part 1)
#include <stdio.h> int main() { int v1, v2, v3, sum; // Input values and sum double avg; // Average of x, y, and z FILE *fpIn, *fpOut; // File pointers // Open input file, exit if error fpIn = fopen("myinput.txt","r"); if (fpIn == NULL) { printf("Error opening myinput.txt\n"); return 0; } // Can actually open file as part of conditional statement if ((fpOut = fopen("myoutput.txt","w")) == NULL) { printf("Error opening myoutput.txt\n");
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ECE Application Programming: Lecture 33

12. ECE Application Programming: Lecture 33
The program (part 2)
// Read the three values
fscanf(fpIn, "%d %d %d", &v1, &v2, &v3);
// Compute sum and average
sum = v1 + v2 + v3;
avg = sum / 3.0;
// print out values
fprintf(fpOut, "Values: %d, %d, %d\n", v1, v2, v3);
fprintf(fpOut, "Sum: %d\n",sum);
fprintf(fpOut, "Avg: %lf\n",avg);
// close the files
fclose(fpIn);
fclose(fpOut);
return 0; }
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ECE Application Programming: Lecture 33

13. File i/o function calls: formatted I/O
fprintf(file_handle, format_specifier, 0+ variables)
file_handle: address returned by fopen()
Other arguments are same as printf()
Example: fprintf(fp, "x = %d", x);
fscanf(file_handle, format_specifier, 0 or more variables)
Other arguments are same as scanf()
Example: fscanf(fp, "%d%d", &a, &b);
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ECE Application Programming: Lecture 32

14. Unformatted I/O (cont.)
One benefit—ability to read/write entire array at once
For example:
Given int x[100];
Can read array from file pointed to by fp:
n = fread(x, sizeof(int), 100, fp);
n should equal 100
Can write array to file pointed to by fp:
fwrite(x, sizeof(int), 100, fp);
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ECE Application Programming: Lecture 33

15. ECE Application Programming: Lecture 33
End of file/error
Two ways to check for end of file:
Text files: Check if fscanf() == EOF
More common: do fscanf() as part of loop condition, and continue while EOF not reached
e.g. while (fscanf(fp, “%d”, &y) != EOF)
Binary files: feof(file_handle);
Note: both functions indicate EOF after failed read operation
Must try to read data and discover that there’s nothing to read before testing for EOF
Checking for error (binary only): ferror(file_handle);
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ECE Application Programming: Lecture 33

16. ECE Application Programming: Lecture 32
Next time
Character and line I/O
Reminders: remaining key dates
Program 7 due today (4/24)
Program 8 due 5/3
Course evaluations to be posted online; returned at final exam
7/24/2019
ECE Application Programming: Lecture 32