File Input/output in C


Input and output to and from files is identical to that at the command line, except the
fprintf and fscanf functions are used and they require another argument. This additional
argument is called a file pointer. In order to write two floating point numbers to a file, you
first need to declar the file pointer with the FILE type, and you need to open it, as in

float x=1, y=2;
FILE *file;
file = fopen(‘‘file.txt’’,’’w’’);
fprintf(file,’’%f %f\n’’,x,y);
fclose(file);

The function fprintf is identical to the printf function, except now we see it has another
argument file, which is a pointer to the file. Before you use the file variable, you need to
open the file with

file = fopen(‘‘file.txt’’,’’w’’);

This opens up the file ‘‘file.txt’’ and the ‘‘w’’ which is the mode and indicates how
the file will be used. The following three modes are allowed:
Mode String

Open for reading “r”
Open for writing “w”
Open and append “a”

When you are done with the file, you close it with

fclose(file);

The ‘‘r’’ mode is used when you would like to open a file for reading. To read two floating
point numbers from a file, you would use the fscanf function, which is identical to scanf,
except that it takes the file pointer as its first argument, as in

float x, y;
FILE *file;
file = fopen(‘‘file.txt’’,’’r’’);
fscanf(file,’’%f %f\n’’,&x,&y);
fclose(file);

You can check to make sure your files are opened correctly (that is, that they exist), by
checking to make sure file is not the predefined NULL pointer. We’ll discuss this pointer in
more detail later, but for now, to ensure that your file was opened correctly, you use

if(!file) printf(‘‘File did not open correctly!\n’’);
Handout 7 13/03/03 7

Note that in order to use the FILE type, we need to include the standard C header file with

#include<stdio.h>

The fscanf and fprintf functions can be used to print to and read from the terminal
using the stdin, stdout,and stderr file pointers defined in stdio.h. To write to the
standard output of the terminal using fprintf, then, you would use

fprintf(stdout,’’%f %f\n’’,x,y);

which is identical to using
printf(’’%f %f\n’’,x,y);

To write to the standard error, you would use
fprintf(stderr,’’%f %f\n’’,x,y);

To read from the terminal’s standard input with fscanf, you would use
fscanf(stdin,’’%f %f’’,&x,&y);

which is identical to using
scanf(’’%f %f’’,&x,&y);

Again, in order for the compiler to know what stdin, stdout, and stderr are, you need to
include stdio.h.