C programming: Basic Operations on Text Files

The file I/O functions and types in the C language are straightforward and easy to understand. To make use of these functions and types you have to include the stdio library. (Like we already did in most of the tutorials).

The file I/O functions in the stdio library are:

fopen – opens a text file.
fclose – closes a text file.
feof – detects end-of-file marker in a file.
fscanf – reads formatted input from a file.
fprintf – prints formatted output to a file.
fgets – reads a string from a file.
fputs – prints a string to a file.
fgetc – reads a character from a file.
fputc – prints a character to a file.
File I/O: opening a text file

The fopen library function is used to open a text file. You also have to use a specified mode when you open a file. The three most common modes used are read (r), write (w), and append (a). Take a look at an example:

‪#‎include‬<stdio.h>

int main()
{
FILE *ptr_file;
int x;

ptr_file =fopen("output.txt", "w");

if (!ptr_file)
return 1;

for (x=1; x<=10; x++)
fprintf(ptr_file,"%d\n", x);

fclose(ptr_file);

return 0;
}

So let’s take a look at the example:

ptr_file =fopen(“output”, “w”);

The fopen statement opens a file “output.txt” in the write (w) mode. If the file does not exist it will be created. But you must be careful! If the file exists, it will be destroyed and a new file is created instead. The fopen command returns a pointer to the file, which is stored in the variable ptr_file. If the file cannot be opened (for some reason) the variable ptr_file will contain NULL.

if (!ptr_file)

The if statement after de fopen, will check if the fopen was successful. If the fopen was not successful, the program will return a one. (Indicating that something has gone wrong).

for (x=1; x<=10; x++)

This for loop will count to ten, starting from one.

fprintf(ptr_file,”%d\n”, x);

The fprintf statement should look very familiar to you. It can be almost used in the same way as printf. The only new thing is that it uses the file pointer as its first parameter.

fclose(ptr_file);

The fclose statement will close the file. This command must be given, especially when you are writing files. So don’t forget it. You have to be careful that you don’t type “close” instead of “fclose”, because the close function exists. But the close function does not close the files correctly. (If there are a lot of files open but not closed properly, the program will eventually run out of file handles and/or memory space and crash.)

File I/O: reading a text file

If you want to read a file you have to open it for reading in the read (r) mode. Then the fgets library functions can be used to read the contents of the file. (It is also possible to make use of the library function fscanf. But you have to be sure that the file is perfectly formatted or fscanf will not handle it correctly). Let’s take a look at an example:

#include<stdio.h>

int main()
{
FILE *ptr_file;
char buf[1000];

ptr_file =fopen("input.txt","r");
if (!ptr_file)
return 1;

while (fgets(buf,1000, ptr_file)!=NULL)
printf("%s",buf);

fclose(ptr_file);
return 0;
}

Note:The printf statement does not have the new-line (\n) in the format string. This is not necessary because the library function fgets adds the \n to the end of each line it reads.

A file “input.txt” is opened for reading using the function fopen en the mode read (r). The library function fgets will read each line (with a maximum of 1000 characters per line.) If the end-of-file (EOF) is reached the fgets function will return a NULL value. Each line will be printed on stdout (normally your screen) until the EOF is reached. The file is then closed and the program will end.