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K&R Chapter 7 covers the standard I/O library — fopen, fread, fwrite, fgets, fprintf, fclose. Reading and writing files is fundamental.

Opening and closing

#include <stdio.h>

FILE* fp = fopen("data.txt", "r");
if (fp == NULL) {
    perror("fopen failed");
    return 1;
}

/* ... use fp ... */

fclose(fp);

Always check the return value of fopen — it returns NULL on failure (file doesn't exist, permission denied, out of file descriptors). Use perror to print a system-derived error message.

Always close with fclose. Otherwise you leak file descriptors and may lose buffered writes.

File modes

The second arg to fopen is the mode string:

• "r" — read; file must exist
• "w" — write; TRUNCATES existing file or creates new
• "a" — append; creates if missing
• "r+" — read+write; file must exist
• "w+" — read+write; truncates
• "a+" — append+read

Add b for binary mode: "rb", "wb". On Unix this is the same as text mode, but on Windows binary mode is REQUIRED to avoid \r\n translation when reading binary files.

Reading a line at a time

char line[1024];
FILE* fp = fopen("input.txt", "r");
if (fp != NULL) {
    while (fgets(line, sizeof(line), fp) != NULL) {
        printf("got: %s", line);   // line includes trailing \n if present
    }
    fclose(fp);
}

fgets(buf, n, fp) reads up to n-1 chars OR until newline OR until EOF, then null-terminates. Returns NULL at EOF (or error).

Reading and writing binary

/* Write 10 ints to a binary file */
int nums[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};
FILE* fp = fopen("data.bin", "wb");
fwrite(nums, sizeof(int), 10, fp);
fclose(fp);

/* Read them back */
int loaded[10];
fp = fopen("data.bin", "rb");
size_t got = fread(loaded, sizeof(int), 10, fp);
fclose(fp);

fread(buf, size, count, fp) and fwrite(buf, size, count, fp) transfer count * size bytes. They return the number of ELEMENTS successfully transferred (which may be less than count on error / partial read).

Formatted I/O on files

Like printf/scanf but on a FILE*:

FILE* fp = fopen("out.txt", "w");
fprintf(fp, "name=%s age=%d\n", name, age);
fclose(fp);

fprintf is the file version of printf. fscanf is the file version of scanf.

stdin, stdout, stderr

The three standard streams are pre-opened FILE* values:

fprintf(stderr, "warning: %s\n", msg);    // write to stderr

These are the same as the unnamed streams that printf/scanf use by default.

Common mistakes

• Not checking fopen's return value — using a NULL FILE* is undefined behavior.
• Forgetting fclose — leaks file descriptors AND loses buffered output (writes can vanish without explicit flush).
• Mode mistakes — "w" truncates the file. Use "a" to add to an existing file.
• Forgetting binary mode on Windows — \r\n translation corrupts non-text data.
• Buffer overflow with fgets — the second arg must be the buffer's actual size (not its length-of-content).
• Not stripping the trailing \n from fgets output — common when comparing strings read from a file.