File Handling Basics

Every variable, array, and struct you have used so far lives only in memory — the moment the program exits, it is gone. File I/O is how a C program persists data beyond its own execution: writing results to disk so they survive after the process ends, and reading them back later, possibly from an entirely different run of the program (or a different program altogether).

The FILE* Abstraction

The C standard library (<stdio.h>) represents an open file with an opaque type called FILE. You never touch a FILE directly — you always work through a pointer to it, FILE *, which the library hands you when you open a file and which you pass to every subsequent read, write, or close operation.

#include <stdio.h>

int main(void)
{
    FILE *fp;   /* a handle to an open file, not the file's data itself */

    fp = fopen("notes.txt", "w");
    if (fp == NULL) {
        printf("Could not open file.\n");
        return 1;
    }

    fprintf(fp, "Persisted to disk.\n");
    fclose(fp);

    return 0;
}

Think of FILE * as a handle or ticket, not the file's actual bytes — it tracks bookkeeping like the current read/write position, internal buffers, and error/EOF state on your behalf.

The General Workflow

Open the file with fopen(), specifying a path and a mode (read, write, append, ...)
Read or write through the returned FILE *, using functions like fprintf, fscanf, fgets, fread, fwrite, and friends
Close the file with fclose() once you are done, releasing the underlying resource and flushing any buffered writes to disk

Every file operation can fail — check it

Opening a file can fail because the path does not exist, a directory has no read permission, the disk is full, or the process has hit its limit of open file handles. Reading and writing can fail too, mid-stream. Production C code checks the return value of `fopen`, and the return values / error state of every subsequent read and write, rather than assuming success. This theme comes up again and again in the following file-handling pages — treat it as the golden rule of file I/O in C.

What's ahead

The next pages walk through opening and closing files properly, text-based reading and writing, binary I/O for raw data, seeking to arbitrary positions within a file, and handling the errors that inevitably come up along the way.