File Handling in C

File handling allows you to store and access data permanently on disk. Unlike variables (which lose their values when the program ends), a file retains its contents even after the program closes.

Steps in File Handling

Declare a file pointer
Open the file
Perform read/write operations
Close the file

FILE *fp;                         // Step 1: declare file pointer
fp = fopen("file.txt", "r");      // Step 2: open file
// Step 3: read/write operations
fclose(fp);                       // Step 4: close file

File Opening Modes

Mode	Description
`"r"`	Read — file must already exist
`"w"`	Write — creates new file, or overwrites existing
`"a"`	Append — adds data at the end of the file
`"r+"`	Read + Write — file must exist
`"w+"`	Read + Write — creates new or clears existing
`"a+"`	Read + Append — read anywhere, write at end only

Common File Functions

Function	Purpose
`fopen()`	Open a file
`fclose()`	Close a file
`fprintf()`	Write formatted data
`fscanf()`	Read formatted data
`fputc()`	Write a single character
`fgetc()`	Read a single character
`fputs()`	Write a string
`fgets()`	Read a string (line by line)
`feof()`	Check if end of file is reached

Writing to a File

FILE *fp = fopen("data.txt", "w");

if (fp == NULL) {
    printf("Error opening file!");
    return 1;
}

fprintf(fp, "Hello World!\n");
fputs("C Programming\n", fp);
fputc('A', fp);

fclose(fp);

Reading from a File

FILE *fp = fopen("data.txt", "r");

if (fp == NULL) {
    printf("File not found!");
    return 1;
}

char ch  = fgetc(fp);       // read one character
char str[50];
fgets(str, 50, fp);         // read one line

int num;
fscanf(fp, "%d", &num);     // read formatted value

fclose(fp);

Appending to a File

FILE *fp = fopen("data.txt", "a");

fprintf(fp, "New line added!\n");
fputs("More text\n", fp);

fclose(fp);

Checking for NULL

Always check if fopen() returned NULL — it means the file could not be opened (wrong path, no permissions, etc.):

FILE *fp = fopen("test.txt", "r");
if (fp == NULL) {
    printf("Cannot open file.\n");
    return 1;
}

Fundamentals

Functions

Array

String

Structure

Union

Pointer

File Handling

Preprocessor

File Handling in C

File Handling in C

Steps in File Handling

File Opening Modes

Common File Functions

Writing to a File

Reading from a File

Appending to a File

Checking for NULL

​File Handling in C

​Steps in File Handling

​File Opening Modes

​Common File Functions

​Writing to a File

​Reading from a File

​Appending to a File

​Checking for NULL

File Handling in C

Steps in File Handling

File Opening Modes

Common File Functions

Writing to a File

Reading from a File

Appending to a File

Checking for NULL