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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.

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Steps in File Handling

  1. Declare a file pointer
  2. Open the file
  3. Perform read/write operations
  4. 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

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File Opening Modes

ModeDescription
"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

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Common File Functions

FunctionPurpose
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

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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);

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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);

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Appending to a File

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

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

fclose(fp);

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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;
}