C Read Files
Read a File
In the previous chapter, we wrote to a file using w
and a modes inside the fopen() function.
To read from a file, you can use the
r mode:
Example
FILE *fptr;
// Open a file in read mode
fptr = fopen("filename.txt", "r");
This opens filename.txt for reading.
Reading a file in C takes a few steps. We will guide you through them, one by one.
Next, we need to create a string that should be big enough to store a line of text from the file.
For example, let's create a string that can store up to 100 characters:
Example
FILE *fptr;
// Open a file in read mode
fptr = fopen("filename.txt", "r");
// Store the content of the file
char myString[100];
In order to read the content of filename.txt,
we can use the
fgets() function.
The fgets() function takes three parameters:
Example
fgets(myString, 100,
fptr);
- The first parameter specifies
where to store the file content, which will be in the
myStringarray we just created. - The second parameter specifies the maximum size of data to read, which
should match the size of
myString(100). - The third parameter requires a file pointer that is used to read the file
(
fptrin our example).
Now, we can print the string, which will output the content of the file:
Example
FILE *fptr;
// Open a file in read mode
fptr = fopen("filename.txt", "r");
// Store the content of the file
char myString[100];
// Read
the content and store it inside myString
fgets(myString, 100, fptr);
// Print the file content
printf("%s", myString);
// Close the
file
fclose(fptr);
Hello World!
Note: The fgets function only reads the first line of the
file. If you remember, there were two lines of text in filename.txt.
To read every line of the file, you can use a while loop:
Example
FILE *fptr;
// Open a file in read mode
fptr =
fopen("filename.txt", "r");
// Store the content of the file
char
myString[100];
// Read the content and print it
while(fgets(myString, 100, fptr)) {
printf("%s", myString);
}
// Close the file
fclose(fptr);
Hello World!
Hi everybody!
The loop reads one line at a time. When there are no more lines, fgets() returns NULL (no value), and the loop stops.
Good Practice
If you try to open a file for reading that does not exist, the fopen() function will return NULL.
Tip: As a good practice, we can use an if statement to test for NULL,
and print some text instead (when the file does not exist):
Example
FILE *fptr;
// Open a file in read mode
fptr = fopen("loremipsum.txt", "r");
// Print some text if the file does not exist
if (fptr == NULL) {
printf("Not able to open the file.");
} else {
// Close the file (only if it was opened)
fclose(fptr);
}
If the file does not exist, the following text is printed:
Not able to open the file.
With this in mind, we can create more reliable code if we use our "read a file" example above again:
Example
If the file exists, read the content and print it. If the file does not exist, print a message:
FILE *fptr;
// Open a file in read mode
fptr = fopen("filename.txt", "r");
// Store the content of the file
char myString[100];
// If the file exists
if (fptr != NULL) {
// Read the content and print it
while (fgets(myString, 100, fptr)) {
printf("%s", myString);
}
// Close the file
fclose(fptr);
} else {
// If the file does not exist
printf("Not able to open the file.");
}
Hello World!
Hi everybody!
Note: Only close a file with fclose() if it was opened. When fopen() returns NULL, there is no file to close, and fclose(NULL) can make the program crash. You will learn more about NULL in our NULL chapter.
