C Files

1. What is File Handling in C?

C Files handling allows a program to store data permanently in a file on disk.
Types of files in C:
1. Text files → store readable characters (.txt)
2. Binary files → store data in binary format (.bin)
File handling in C uses the FILE structure provided in stdio.h.

2. Opening a File

filename → name of the file
mode → how the file is to be opened

Mode	Description
`"r"`	Read (file must exist)
`"w"`	Write (creates file or overwrites)
`"a"`	Append (write at end, creates file)
`"r+"`	Read & write (file must exist)
`"w+"`	Read & write (creates/overwrites)
`"a+"`	Read & append (creates file)

3. Closing a File

Always close a file after operations to save data and free resources.

4. Reading from a File

fgetc() → read a single character
fgets() → read a line/string
fscanf() → formatted input

Example: Reading a file character by character

5. Writing to a File

fputc() → write a single character
fputs() → write a string
fprintf() → formatted output

Example: Writing to a file

#include <stdio.h>

int main() {
FILE *fp = fopen("output.txt", "w");
if (fp == NULL) {
printf("Unable to create file!\n");
return 1;
}

fputs("Hello, C File Handling!\n", fp);
fprintf(fp, "Number: %d\n", 100);

fclose(fp);
return 0;
}

#include <stdio.h>

int main() {

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

if (fp == NULL) {

printf("Unable to create file!\n");

return 1;

}

fputs("Hello, C File Handling!\n", fp);

fprintf(fp, "Number: %d\n", 100);

fclose(fp);

return 0;

}

6. Reading & Writing (r+) Example

#include <stdio.h>

int main() {
FILE *fp = fopen("data.txt", "r+");
if (fp == NULL) {
printf("File not found!\n");
return 1;
}

// Write at the beginning
fprintf(fp, "Updated Line\n");

// Move pointer to beginning
fseek(fp, 0, SEEK_SET);

char line[100];
while (fgets(line, sizeof(line), fp) != NULL) {
printf("%s", line);
}

fclose(fp);
return 0;
}

#include <stdio.h>

int main() {

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

if (fp == NULL) {

printf("File not found!\n");

return 1;

}

// Write at the beginning

fprintf(fp, "Updated Line\n");

// Move pointer to beginning

fseek(fp, 0, SEEK_SET);

char line[100];

while (fgets(line, sizeof(line), fp) != NULL) {

printf("%s", line);

}

fclose(fp);

return 0;

}

7. Checking File Open

Always check if fopen was successful before performing operations.

8. Key Points About C File Handling

Use FILE * for file operations.
Always open a file with proper mode (r, w, a, etc.).
Always close files using fclose() after operations.
Use fgetc(), fgets(), fputc(), fputs(), fprintf(), fscanf() for reading/writing.
Use fseek(), ftell(), and rewind() for file pointer control.
File handling allows persistent storage beyond program execution.