Advanced · C PROGRAMMING

File input and output

What you will learn

Write, rewind and read a temporary file.

Understand the concept

FILE represents a stream managed by the C library. tmpfile opens a temporary binary update stream, removed when closed. Check whether opening, writing and reading succeed. rewind moves the position to the beginning so that the program can read back its data. Some hosted compilers restrict filesystem access.

Read the example

The complete program appears below. Read it first, predict the output, then run it.

Expected output

Repair ready

Practise

Write two lines, rewind, and read them using a loop around fgets.

Common mistake

Always check the file pointer before using it. Reading a file changes its current position; it does not automatically start over.

Lesson discussion

#include <stdio.h>

int main(void) {
    FILE *file = tmpfile();
    if (file == NULL) { perror("tmpfile"); return 1; }
    if (fputs("Repair ready\n", file) == EOF) { fclose(file); return 1; }
    rewind(file);
    char line[64];
    if (fgets(line, sizeof line, file) != NULL) printf("%s", line);
    else { fclose(file); return 1; }
    if (fclose(file) == EOF) return 1;
    return 0;
}

PRACTICE LAB

Read. Edit. Run.

C source · .c

Experiment with the program below. Your code and input are sent to the selected compiler only when you run it.

Compiler

C source

Tab indents · Shift+Tab unindents · Esc then Tab leaves the editor · Ctrl/Cmd+Enter runs · Ctrl/Cmd+S saves a draft

Changes save automatically to your account.

Standard input

Input / output example

Example input

No input required.

Expected output for the original program

Repair ready

Output will appear here.

File input and output

What you will learn

Understand the concept

Read the example

Expected output

Practise

Common mistake

Lesson discussion

Read. Edit. Run.

Input / output example

Debugging Lab

Interactive Memory Visualizer

Stack frames

Memory and pointers

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