File I/O

After this lesson, you will be able to: Use fopen, fread, fwrite, fclose, fprintf, fscanf for text + binary file I/O.

File I/O is identical across Unix + Windows in standard C.

Prerequisites:Structs

Text file reading + writing

Open → read/write → close.

tsx

#include <stdio.h>

int main(void) {
    // Write
    FILE *f = fopen("out.txt", "w");
    if (f == NULL) { perror("fopen"); return 1; }
    fprintf(f, "Hello, %s\n", "world");
    fclose(f);

    // Read line by line
    f = fopen("out.txt", "r");
    if (f == NULL) { perror("fopen"); return 1; }
    char buf[256];
    while (fgets(buf, sizeof(buf), f) != NULL) {
        printf("%s", buf);
    }
    fclose(f);
    return 0;
}

Binary I/O

For structured / non-text data.

tsx

typedef struct { int id; double balance; } Record;

int main(void) {
    Record records[] = { {1, 100.0}, {2, 250.5}, {3, 99.99} };

    FILE *f = fopen("records.bin", "wb");
    fwrite(records, sizeof(Record), 3, f);
    fclose(f);

    Record loaded[3];
    f = fopen("records.bin", "rb");
    size_t read = fread(loaded, sizeof(Record), 3, f);
    fclose(f);
    printf("Read %zu records\n", read);
    return 0;
}

// Caveat: binary formats are NOT portable across architectures (endianness, struct padding).
// For cross-platform: use a defined format (JSON, protobuf, flatbuffers) or write byte-by-byte.

Error handling pattern

Always check + close.

tsx

FILE *f = fopen(path, "r");
if (f == NULL) {
    fprintf(stderr, "Cannot open %s: %s\n", path, strerror(errno));
    return -1;
}

// ... use f ...

if (ferror(f)) {
    fprintf(stderr, "Read error\n");
}
if (fclose(f) != 0) {
    perror("fclose");
}

Common mistakes only experienced C devs avoid

Forgetting fclose. Leaks file descriptors. Not checking fopen's return value. Crash on next fread. Using fgets without checking the buffer for the newline. May truncate. Binary I/O without thinking about endianness + struct padding. Cross-platform pain.

Quick Check

Why might binary I/O of a struct break when moving the file between machines?

Pick the cleanest answer.

←Structs and Unions

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