File I/O — Text, Binary, Random Access

ফাইলসিস্টেমের সাথে কথোপকথন

~35 min Intermediate 12 practice problems Live code

1. Files as Streams

C-তে প্রতিটি ফাইল একটি FILE * stream-এর মাধ্যমে access করা হয়। fopen দিয়ে খুলে, fclose দিয়ে বন্ধ করতে হয়।

FILE *f = fopen("data.txt", "r");
if (!f) { perror("fopen"); return 1; }

// ... use f ...

fclose(f);

fopen ব্যর্থ হলে NULL ফেরত দেয়। Cause জানতে perror বা strerror(errno) ব্যবহার করুন।

2. Open Modes

Mode	Meaning
`"r"`	Read — file অবশ্যই থাকতে হবে
`"w"`	Write — না থাকলে তৈরি হয়, থাকলে truncate
`"a"`	Append — শেষে লেখা, না থাকলে তৈরি
`"r+"`	Read + write (file থাকতে হবে)
`"w+"`	Write + read (truncate)
`"rb"`, `"wb"`, …	Binary mode (Windows-এ গুরুত্বপূর্ণ)

3. Reading Line by Line

Online runner-এ সাধারণত ফাইল তৈরি করা যায় না, তাই এই demo stdin থেকে পড়ছে — কিন্তু pattern হুবহু একই, শুধু stdin-এর জায়গায় FILE * বসালেই হয়।

readlines.c — stdin acts as the file

#include <stdio.h>

int main(void) {
    FILE *f = stdin;          // in real code: fopen("data.txt", "r")
    char line[1024];
    int n = 0;
    while (fgets(line, sizeof line, f)) {
        printf("%3d | %s", ++n, line);
    }
    printf("(total %d lines)\n", n);
    return 0;
}

4. Writing and Reading — End-to-End Demo

নিচের প্রোগ্রামটি একটি temporary file-এ লিখে, বন্ধ করে, আবার খুলে পড়ে — পুরো cycle এক জায়গায়।

roundtrip.c

#include <stdio.h>
#include <stdlib.h>

int main(void) {
    const char *path = "notes.txt";

    // 1) write
    FILE *w = fopen(path, "w");
    if (!w) { perror("open for write"); return 1; }
    fprintf(w, "Hello, Bangladesh!\n");
    fprintf(w, "Line 2, with a number %d\n", 42);
    fclose(w);

    // 2) read back
    FILE *r = fopen(path, "r");
    if (!r) { perror("open for read"); return 1; }
    char line[256];
    while (fgets(line, sizeof line, r)) fputs(line, stdout);
    fclose(r);

    remove(path);              // clean up the temp file
    return 0;
}

5. Binary I/O — fread / fwrite

typedef struct { char name[32]; int id; double gpa; } Student;

Student arr[100];
fwrite(arr, sizeof arr[0], n, f);    // write n records
fread(arr, sizeof arr[0], n, f);     // read them back

Binary I/O-তে আপনি বাস্তবে যে bytes আছে সেগুলোই লিখেন/পড়েন — কোনো text formatting নেই। ফলে দ্রুত, কিন্তু file-এর content cross-platform পড়তে হলে endianness ও struct padding-এর যত্ন নিতে হয়।

6. Random Access — `fseek` & `ftell`

fseek(f, 0, SEEK_END);          // go to the end
long size = ftell(f);            // file size in bytes
rewind(f);                       // back to the start
fseek(f, 100, SEEK_SET);         // absolute offset 100
fseek(f, -10, SEEK_CUR);         // 10 bytes backwards

7. Error Handling

#include <errno.h>

FILE *f = fopen("nope.txt", "r");
if (!f) {
    fprintf(stderr, "open failed: %s\n", strerror(errno));
    perror("context");    // prints "context: No such file or directory"
    return 1;
}

প্রতিটি I/O function-এর return value যাচাই করুন। fgets NULL দিলে হয় EOF, না হয় read error — ferror(f) আর feof(f) দিয়ে পার্থক্য করা যায়।

8. Practice Problems

Copy one file to another byte-by-byte.

একটি file অপরটিতে byte-by-byte কপি করুন।

✨ Show Answer

fcopy.c

#include <stdio.h>

int main(void) {
    const char *src = "a.txt", *dst = "b.txt";
    FILE *in = fopen(src, "wb");
    fputs("ABCL TECH", in); fclose(in);

    in  = fopen(src, "rb");
    FILE *out = fopen(dst, "wb");
    int c;
    while ((c = fgetc(in)) != EOF) fputc(c, out);
    fclose(in); fclose(out);

    // print result
    out = fopen(dst, "r");
    char buf[64] = {0};
    fread(buf, 1, 63, out);
    puts(buf);
    fclose(out);
    remove(src); remove(dst);
    return 0;
}

Count lines in a file.

একটি file-এ লাইন সংখ্যা গুনুন।

✨ Show Answer

count_lines.c

#include <stdio.h>
int main(void) {
    const char *p = "sample.txt";
    FILE *w = fopen(p, "w");
    fputs("first\nsecond\nthird\n", w);
    fclose(w);

    FILE *r = fopen(p, "r");
    int n = 0, c;
    while ((c = fgetc(r)) != EOF) if (c == '\n') n++;
    fclose(r);
    printf("lines = %d\n", n);
    remove(p);
    return 0;
}

Count occurrences of a word in a file.

একটি file-এ একটি নির্দিষ্ট শব্দ কতবার আছে গুনুন।

✨ Show Answer

count_word.c

#include <stdio.h>
#include <string.h>

int main(void) {
    const char *p = "essay.txt", *needle = "C";
    FILE *w = fopen(p, "w");
    fputs("I love C. C is fun. Systems need C.\n", w);
    fclose(w);

    FILE *f = fopen(p, "r");
    char line[512]; int total = 0;
    while (fgets(line, sizeof line, f)) {
        char *q = line;
        while ((q = strstr(q, needle))) { total++; q += strlen(needle); }
    }
    fclose(f);
    printf(""%s" appears %d times\n", needle, total);
    remove(p);
    return 0;
}

Append a log entry to a file on each run.

প্রতিবার প্রোগ্রাম চালালে একটি log entry file-এ append হবে।

✨ Show Answer

append_log.c

#include <stdio.h>
#include <time.h>

int main(void) {
    const char *p = "app.log";

    FILE *f = fopen(p, "a");          // append
    fprintf(f, "run at time %ld\n", (long)time(NULL));
    fclose(f);

    // show contents
    f = fopen(p, "r");
    char line[256];
    while (fgets(line, sizeof line, f)) fputs(line, stdout);
    fclose(f);
    remove(p);
    return 0;
}

Read integers from a file into an array, then print sorted.

File থেকে integer পড়ে array-তে রাখুন, তারপর sort করে প্রিন্ট করুন।

✨ Show Answer

sort_file.c

#include <stdio.h>
#include <stdlib.h>
int cmp(const void *a, const void *b) { return *(const int*)a - *(const int*)b; }
int main(void) {
    const char *p = "nums.txt";
    FILE *w = fopen(p, "w");
    fputs("42 7 19 3 88 21 56 11", w);
    fclose(w);

    FILE *r = fopen(p, "r");
    int a[100], n = 0;
    while (n < 100 && fscanf(r, "%d", &a[n]) == 1) n++;
    fclose(r);
    qsort(a, n, sizeof *a, cmp);
    for (int i = 0; i < n; i++) printf("%d ", a[i]);
    putchar('\n');
    remove(p);
    return 0;
}

Save an array of Students to a binary file and read it back.

Student array-কে binary file-এ save করে আবার পড়ুন।

✨ Show Answer

students_bin.c

#include <stdio.h>
#include <string.h>

typedef struct { char name[32]; int id; double gpa; } Student;

int main(void) {
    const char *p = "students.dat";
    Student a[3] = { {"Arif",101,3.72}, {"Nusrat",102,3.92}, {"Zahid",103,3.55} };

    FILE *w = fopen(p, "wb");
    fwrite(a, sizeof a[0], 3, w);
    fclose(w);

    Student b[3] = {0};
    FILE *r = fopen(p, "rb");
    fread(b, sizeof b[0], 3, r);
    fclose(r);

    for (int i = 0; i < 3; i++)
        printf("%s  id=%d  gpa=%.2f\n", b[i].name, b[i].id, b[i].gpa);
    remove(p);
    return 0;
}

Implement tail -n — print the last N lines of a file.

tail -n-এর মতো file-এর শেষ N লাইন প্রিন্ট করুন।

✨ Show Answer

tail_n.c

#include <stdio.h>
#include <string.h>

int main(void) {
    const char *p = "sample.txt";
    FILE *w = fopen(p, "w");
    for (int i = 1; i <= 10; i++) fprintf(w, "line %d\n", i);
    fclose(w);

    const int N = 3;
    char buf[N][256] = {{0}};
    FILE *r = fopen(p, "r");
    int count = 0;
    char line[256];
    while (fgets(line, sizeof line, r)) {
        strncpy(buf[count % N], line, 255);
        count++;
    }
    fclose(r);

    int start = count >= N ? count % N : 0;
    int shown = count < N ? count : N;
    for (int i = 0; i < shown; i++)
        fputs(buf[(start + i) % N], stdout);
    remove(p);
    return 0;
}

Compute the size of a file without reading its contents.

Content না পড়েই একটি file-এর size বের করুন।

✨ Show Answer

filesize.c

#include <stdio.h>

int main(void) {
    const char *p = "big.bin";
    FILE *w = fopen(p, "wb");
    char pad[100] = {0};
    fwrite(pad, 1, 100, w);
    fclose(w);

    FILE *f = fopen(p, "rb");
    fseek(f, 0, SEEK_END);
    printf("size = %ld bytes\n", ftell(f));
    fclose(f);
    remove(p);
    return 0;
}

Find and replace all occurrences of one word in a text file.

একটি text file-এর সব জায়গায় একটি word-কে অন্যটি দিয়ে replace করুন।

✨ Show Answer

সহজ approach: পুরো ফাইল memory-তে পড়ুন, একটি নতুন buffer-এ find/replace করে write করুন। Large file হলে line-by-line process করুন।

replace.c

#include <stdio.h>
#include <string.h>

void process_line(const char *src, const char *find, const char *repl, FILE *out) {
    size_t fl = strlen(find);
    const char *p = src, *q;
    while ((q = strstr(p, find))) {
        fwrite(p, 1, q - p, out);
        fputs(repl, out);
        p = q + fl;
    }
    fputs(p, out);
}

int main(void) {
    const char *in_p = "in.txt", *out_p = "out.txt";
    FILE *w = fopen(in_p, "w");
    fputs("C is fun. C makes systems go.\n", w); fclose(w);

    FILE *r = fopen(in_p, "r");
    FILE *o = fopen(out_p, "w");
    char line[1024];
    while (fgets(line, sizeof line, r)) process_line(line, "C", "ABCL TECH", o);
    fclose(r); fclose(o);

    FILE *s = fopen(out_p, "r");
    while (fgets(line, sizeof line, s)) fputs(line, stdout);
    fclose(s);
    remove(in_p); remove(out_p);
    return 0;
}

Concatenate multiple files (Unix cat).

একাধিক file concatenate করুন (cat-এর মতো)।

✨ Show Answer

cat_many.c

#include <stdio.h>

void cat_one(const char *p) {
    FILE *f = fopen(p, "r");
    if (!f) { perror(p); return; }
    int c; while ((c = fgetc(f)) != EOF) fputc(c, stdout);
    fclose(f);
}

int main(void) {
    // seed two sample files
    FILE *w = fopen("a.txt", "w"); fputs("first\n",  w); fclose(w);
    w = fopen("b.txt", "w");        fputs("second\n", w); fclose(w);

    const char *files[] = { "a.txt", "b.txt" };
    for (int i = 0; i < 2; i++) cat_one(files[i]);

    remove("a.txt"); remove("b.txt");
    return 0;
}

Build a tiny fixed-size-record database using fseek.

fseek দিয়ে ছোট fixed-record DB বানান।

✨ Show Answer

tiny_db.c

#include <stdio.h>
#include <string.h>

typedef struct { int id; char name[32]; } Rec;

int main(void) {
    const char *db = "mini.db";
    FILE *f = fopen(db, "wb+");
    Rec a = { 1, "Alice" }, b = { 2, "Bob" };
    fwrite(&a, sizeof a, 1, f);
    fwrite(&b, sizeof b, 1, f);

    // seek to record #1 (0-indexed) and read it
    fseek(f, 1 * sizeof(Rec), SEEK_SET);
    Rec r; fread(&r, sizeof r, 1, f);
    printf("rec[1] = id=%d name=%s\n", r.id, r.name);
    fclose(f);
    remove(db);
    return 0;
}

Why do you need "rb" on Windows but not on Linux?
Windows-এ "rb" কেন দরকার, Linux-এ কেন নয়?

✨ Show Answer
Windows-এর text mode read/write করার সময় \r\n-কে \n-এ রূপান্তর করে দেয় (এবং লেখার সময় উল্টোটা)। Binary ডেটার জন্য এটি বিপর্যয়কর — একটি 0x0D byte হারিয়ে যেতে পারে। "rb" / "wb" দিলে সেই auto-translation বন্ধ থাকে। Linux/macOS-এ text ও binary একই — তাই পার্থক্য নেই। তবু portable কোডে সবসময় "rb"/"wb" লিখুন।

Glossary (শব্দকোষ)

Term	Meaning	বাংলায়
`FILE *`	Opaque pointer representing an open file/stream.	খোলা file/stream-এর pointer।
`fopen`	Opens a file and returns a `FILE *`.	File খুলে `FILE *` ফেরত দেয়।
`fclose`	Flushes buffers and closes the file.	Buffer flush করে file বন্ধ করে।
Mode String	`"r"`, `"w"`, `"a"`, `"rb"`, `"wb"` — open semantics.	File খোলার পদ্ধতি বর্ণনাকারী string।
Text Mode	Newlines may be translated by the platform.	Platform অনুযায়ী newline রূপান্তর হয়।
Binary Mode	Reads/writes bytes verbatim.	Byte যেমন আছে তেমনই read/write হয়।
`fread` / `fwrite`	Block read/write of binary records.	Binary record read/write করা।
`fprintf` / `fscanf`	Formatted I/O on a file stream.	File stream-এ formatted I/O।
`fgets`	Reads a line of text up to a size limit.	Size সীমাসহ এক লাইন পড়া।
`fseek` / `ftell`	Move/get the file position for random access.	Random access-এর জন্য position সরানো/পড়া।
`rewind`	Resets file position to the beginning.	Position শুরুতে ফিরিয়ে আনা।
EOF	End-of-file indicator returned by read functions.	"ফাইল শেষ" নির্দেশক।
`ferror` / `perror`	Inspect / print stream errors.	Stream error যাচাই/print করা।

Summary — Module 20

Open, check, read/write, close। fgets text line-এর জন্য, fread/fwrite binary record-এর জন্য, fseek random access-এর জন্য। প্রতিটি কাজের error check করুন — perror বা ferror।

Next Module → Preprocessor & Macros।

Previous Next: Preprocessor

1. Files as Streams

2. Open Modes

3. Reading Line by Line

4. Writing and Reading — End-to-End Demo

5. Binary I/O — fread / fwrite

6. Random Access — fseek & ftell

7. Error Handling

8. Practice Problems

Glossary (শব্দকোষ)

Summary — Module 20

6. Random Access — `fseek` & `ftell`