If you’ve ever seen an error like:

EMFILE: too many open files

you’ve already met one of the most fundamental pieces of an operating system: the file descriptor (FD).

This blog explains:

• what an FD is

• how its lifecycle works

• why your system ran out of them during image export

• how memory differs from FD (second problem you hit)

• and how Excel image export actually behaves internally (third problem)

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🧠 What is a File Descriptor (FD)?

A file descriptor is a small integer that the operating system gives your process when it opens a file or resource.

Think of it as a ticket number:

• You ask the OS: “Open this file”

• OS says: “Here’s FD = 7”

• You use 7 to read/write

• When done → you close it

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🧾 Example

const fs = require('fs');

const fd = fs.openSync('data.txt', 'r'); // fd = 7 (example)
const buffer = Buffer.alloc(100);

fs.readSync(fd, buffer, 0, 100, 0);
fs.closeSync(fd);

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📦 What counts as a file?

In Unix systems:

Everything is treated like a file

Resource	Uses FD?
Disk file	✅
HTTP request	✅
Socket	✅
Stream	✅

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🔄 FD Lifecycle (Step-by-Step)

[1] Request resource
        ↓
[2] OS opens it
        ↓
[3] OS assigns FD (integer)
        ↓
[4] Your app uses FD
        ↓
[5] You close FD
        ↓
[6] OS frees FD

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📊 Diagram

Application                  Operating System
-----------                 -----------------
   open()   ─────────────▶   Allocate FD (e.g., 5)
                            Open file/socket

   read(fd=5) ◀──────────▶   Read data

   write(fd=5) ◀─────────▶   Write data

   close(5) ─────────────▶   Release FD
                            Free slot

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🔢 Why FD Limits Exist

Each process has a limit:

ulimit -n

Typical values:

• 1024

• 4096

• 65535 (tuned systems)

---

💥 What is EMFILE?

EMFILE = Too Many Open Files

It means:

❗ You opened more resources than allowed at the same time

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🧨 Real Example (Your Case)

• 50 concurrent downloads

• Each opens:

  • 1 HTTP socket (FD)

  • 1 file write (FD)

👉

50 × (socket + file) = ~100 FDs

With multiple jobs:

5 jobs → ~500 FDs → EMFILE

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❌ Bad Pattern

await Promise.all(urls.map(download));

👉 Opens everything at once

---

✅ Good Pattern

for (const url of urls) {
  await download(url);
}

👉 Controlled FD usage

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🧠 NEW TOPIC 1: FD vs Memory (Critical Difference)

This is where many people get confused.

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📊 Comparison

Concept	FD	Memory
What it is	Handle	Data
Managed by	OS	Your app
Limit	OS limit (ulimit)	RAM
Error	EMFILE	OOM (Out of Memory)

---

🔍 Example

const buffer = await fs.readFile('image.jpg');

What happens:

open file → FD used
read file → buffer created in RAM
close file → FD released

👉 After this:

• FD = ❌ gone

• Buffer = ✅ still in memory

---

💥 Key Insight

❗ Closing a file does NOT free memory

---

🧨 Your second problem (memory)

Even after fixing EMFILE:

workbook.addImage({ buffer })

👉 ExcelJS stores ALL buffers internally

So:

6000 images × 150KB ≈ 900MB RAM

👉 This leads to memory pressure, not FD issue

---

🧠 Simple analogy

• FD = number of doors open 🚪

• Memory = stuff inside the room 📦

Closing doors doesn’t remove the stuff.

---

🧠 NEW TOPIC 2: Excel Image Rendering Limitation (Your 3rd Issue)

You noticed:

After ~3000 rows → images overlap at top

---

🔍 Root cause

Excel calculates image positions using:

Row height → converted to EMU
1pt = 12,700 EMU

Each image Y position = sum of all previous row heights

---

⚠️ Limitation

Excel internally uses:

32-bit signed integer
Max = 2,147,483,647

---

📊 Calculation

If:

row height = 50pt
→ 50 × 12700 = 635,000 EMU per row

Then:

2,147,483,647 / 635,000 ≈ 3,382 rows

---

💥 What happens after that?

Value overflows → becomes negative
→ Excel places image at wrong position (top)
→ all images overlap

---

📊 Visual

Correct:
Row 1 → Y = 0
Row 2 → Y = 635k
Row 3000 → Y = valid

After overflow:
Row 3500 → Y = negative ❗
→ jumps to top
→ overlap

---

❌ Why your “row height fix” felt bad

You reduced:

row height = 16pt

👉 avoids overflow BUT:

• image still 50px

• row smaller than image

• UI breaks

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🏁 Proper Solutions (Summary)

✅ For FD issue

• limit concurrency

• avoid massive Promise.all

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✅ For Memory issue

• compress images

• reduce unique images

• limit concurrent exports

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✅ For Excel overlap issue

Best:

👉 Split sheets

Sheet 1 → 0–3000 rows
Sheet 2 → 3001–6000

Alternative:

• reduce image size

• or fallback to URLs

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🧠 Final Mental Model

Problem 1 → FD (too many things open at once)
Problem 2 → Memory (too much data kept)
Problem 3 → Excel limit (internal integer overflow)

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💬 One-line takeaway

FD issues are about “how many things are open at once”, memory issues are about “how much data you keep”, and Excel issues are about “format limitations you cannot bypass.”