Beyond the Lifespan of a Program

Variables and arrays only exist in RAM. As soon as your program finishes or the power goes out, that data is gone forever. Files allow you to save data permanently on the hard drive.

The FILE Pointer

In C, you don't interact with the hard drive directly. You use a special structure called FILE provided by <stdio.h>. You always work with a pointer to this structure.

FILE *fp;

The Core Lifecycle of a File

1. Open: fp = fopen("data.txt", "w");
2. Check: if (fp == NULL) - Always check if the file actually opened!
3. Process: Use fprintf, fscanf, fputs, etc.
4. Close: fclose(fp);. Critical: If you forget this, your data might not actually be saved to the disk.

Opening Modes

Reading and Writing Functions

• fprintf(fp, ...): Just like printf, but sends the output to a file.
• fscanf(fp, ...): Just like scanf, but reads from a file.
• fputs(str, fp): Writes a string to the file.
• fgets(str, size, fp): Reads a string safely from the file.

Random Access (fseek and ftell)

Normally, files are read sequentially from start to finish. But C allows you to "jump" to any specific byte in a file using fseek. This is how database software finds a specific record in a 100GB file without reading the whole thing.

Always close your files! The operating system has a limit on how many files a program can keep open at once. If you keep opening files without closing them, your program will eventually fail.