C Read Files

To read from a file, you open it in read mode ("r"). Remember that read mode does not create a file. If the file does not exist, fopen() returns NULL, so checking for NULL is even more important when reading than when writing.

Reading Line by Line with fgets()

The fgets() function reads one line of text from a file and stores it in a character array (a string). It needs three things: the array to fill, the maximum number of characters to read, and the file handle. It is a safe choice because it will never read more characters than your array can hold.

#include <stdio.h>

int main() {
    FILE *fptr = fopen("greeting.txt", "r");
    if (fptr == NULL) {
        printf("The file does not exist.\n");
        return 1;
    }

    char line[100];
    // Read up to 99 characters into line
    fgets(line, 100, fptr);
    printf("First line: %s", line);

    fclose(fptr);
    return 0;
}

Most files have more than one line, so we usually read them inside a loop. The trick is knowing when to stop. When fgets() reaches the end of the file, it returns NULL, which is a perfect signal to end the loop.

#include <stdio.h>

int main() {
    FILE *fptr = fopen("greeting.txt", "r");
    if (fptr == NULL) {
        printf("Could not open the file.\n");
        return 1;
    }

    char line[100];
    // Keep reading until fgets returns NULL (end of file)
    while (fgets(line, 100, fptr) != NULL) {
        printf("%s", line);
    }

    fclose(fptr);
    return 0;
}

Other Ways to Read

• fgetc() reads a single character at a time, which is useful for scanning through a file character by character.
• fscanf() reads formatted data, such as pulling an int or a float out of a file, similar to how scanf() reads from the keyboard.
• fgets() is usually the safest and easiest choice for reading text one line at a time.