File Handling in C
File access in C is achieved by associating a stream with a file. C communicates with files using a new data type called a file pointer. This type is defined within stdio.h, and written as FILE *. A file pointer called output_file is declared in a statement like
FILE *output_file;
The File Modes of fopen Function
Your program must open a file before it can access it. This is done using the fopen function, which returns the required file pointer. If the file cannot be opened for any reason then the value NULL will be returned. You will usually use fopen as follows
if ((output_file = fopen("output_file", "w")) == NULL)
fprintf(stderr, "Cannot open %s\n",
"output_file");
fopen takes two arguments, both are strings, the first is the name of the file to be opened, the second is an access character, which is usually one of r, a or w etc. Files may be opened in a number of modes, as shown in the following table.
File Modes |
r |
Open a text file for reading. |
w |
Create a text file for writing. If the file exists, it is overwritten. |
a |
Open a text file in append mode. Text is added to the end of the file. |
rb |
Open a binary file for reading. |
wb |
Create a binary file for writing. If the file exists, it is overwritten. |
ab |
Open a binary file in append mode. Data is added to the end of the file. |
r+ |
Open a text file for reading and writing. |
w+ |
Create a text file for reading and writing. If the file exists, it is overwritten. |
a+ |
Open a text file for reading and writing at the end. |
r+b or rb+ |
Open binary file for reading and writing. |
w+b or wb+ |
Create a binary file for reading and writing. If the file exists, it is overwritten. |
a+b or ab+ |
Open a text file for reading and writing at the end. |
The update modes are used with fseek, fsetpos and rewind functions. The fopen function returns a file pointer, or NULL if an error occurs.
The following example opens a file, tarun.txt in read-only mode. It is good programming practice to test the file exists.
if ((in = fopen("tarun.txt", "r")) == NULL)
{
puts("Unable to open the file");
return 0;
}
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