Minimal ZIP File Format Specification

Goal:
Produce a valid ZIP archive containing a single file (e.g., "hello.txt"), with no compression ("Stored" method), using only the strict minimum of required features and fields, so that the archive is:

Extractable by all ZIP tools
Conforms with the ZIP spec

1. Structures Required

A valid ZIP file with one file requires the following, in this order:

Offset (relative to start of file)	Structure	Description
0	Local file header	Header preceding file data
+	File data	Raw file contents
+	Central directory header	Summary of one archive entry
+	End of central directory record	Points to the directory start

2. Structure Details

Below is each structure, with only required fields and offsets, as required by the PKWARE specification.

A. Local File Header (per-file)

Size	Field Offset	Field Name	Value
4	0	Signature	`0x04034b50` ("PK\x03\x04")
2	4	Version to extract	`20 00` (2.0)
2	6	Bit flags	`00 00` (no encryption, etc.)
2	8	Compression method	`00 00` (Store/no compression)
2	10	mod time	Usually `00 00`
2	12	mod date	Usually `00 00`
4	14	CRC32	CRC-32 of file data
4	18	Compressed size	Size of file
4	22	Uncompressed size	Size of file
2	26	Filename length	Length (N) of filename
2	28	Extra length	`00 00` (none)
N	30	Filename	e.g., `hello.txt`
M	...	Extra	omitted (length=0)

Directly after comes file data: ('Store' method: bytes are simply dumped in-place).

B. File Data

Size	Offset	Description
M	--	File bytes
---	--	(next structure)

C. Central Directory File Header

Size	Field Off	Field Name	Value
4	0	Signature	`0x02014b50` ("PK\x01\x02")
2	4	Version made by	`20 00` (2.0)
2	6	Version to extract	`20 00` (2.0)
2	8	Bit flags	`00 00`
2	10	Compression method	`00 00`
2	12	Mod time	`00 00`
2	14	Mod date	`00 00`
4	16	CRC32	CRC-32
4	20	Compressed size	Size of file
4	24	Uncompressed size	Size of file
2	28	Filename length	Length (N) of filename
2	30	Extra length	`00 00`
2	32	File comment length	`00 00`
2	34	Disk # start	`00 00`
2	36	Int file attrs	`00 00`
4	38	Ext file attrs	`00 00 00 00`
4	42	Local hdr offset	Offset to local file header start
N	46	Filename	e.g., `hello.txt`
M	...	Extra	(none: 0)
L	...	File comment	(none: 0)

D. End of Central Directory Record

Size	Field Offset	Field Name	Value
4	0	Signature	`0x06054b50` ("PK\x05\x06")
2	4	Disk #	`00 00`
2	6	Central dir start	`00 00` (single disk)
2	8	# entries disk	`01 00` (1 file)
2	10	# entries total	`01 00` (1 file)
4	12	Central dir size	Size of central directory (bytes)
4	16	Offset to directory	Offset to central directory
2	20	Comment length	`00 00` (no comment)
Q	...	Comment	(none)

3. Example File Layout

Suppose we are zipping "hello.txt" containing 12 bytes: Hello world\ .

Let:

N = filename length (9)
M = file size (12)
Local file header always ends at offset: 30+N

Layout

Offset	Content
0	Local file header
30+N	File data (12 bytes)
...	Central directory header
...	End of central directory

4. Fields Calculation

CRC32: Must be computed for the file data.
Sizes: All sizes must be correct and match actual data.
All offsets: Particularly, in the Central Directory header, the offset to the Local File Header (from the start of the file) must be set to 0.

5. C/C++ Implementation Tips

Use fixed values for all fields except the ones described (filename, file size, CRC, etc).
Write structures in order, respecting little-endian format for all integral fields.
Avoid all optional fields (extra, comment, etc).
Use only 1 entry in the archive.
No compression: Compression method = 0 ("store").
No encryption, no split, no volume zip, no extended attributes.

6. Visual Structure Example

[Local file header][hello.txt][Central directory][EOCD]

7. Smallest Possible ZIP (hex)

For a file with empty contents "empty.txt" (just for example):

50 4B 03 04 14 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 65 6D 70 74 79 2E 74 78 74
50 4B 01 02 14 00 14 00 00 00 00 00 00 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 65 6D 70 74 79 2E 74 78 74
50 4B 05 06 00 00 00 00 01 00 01 00 XX XX XX XX XX XX 00 00

(XX XX = correct directory data/offset sizes)

8. References

9. FAQ

Why not compression? Removes zlib/dependency and makes the smallest code.
Why not DOS file attributes? Most tools do not require them. Can be left default/zero.

10. Minimal Implementation Checklist

[x] Local file header
[x] File data
[x] Central directory header
[x] End of central directory record
[x] All sizes/offsets calculated and little-endian

11. Pseudocode / Skeleton

write_local_header();
write_file_data();
store_position_central_dir();
write_central_directory_header();
write_end_of_central_directory_record();

This spec provides everything needed to write the minimal possible .zip file compliant with all standard tools.

Minimal Zip Implementation

What is the most minimal implementation of C/C++ library to make a zip file that can be unzipped by all zip clients?**

Explanation

A zip file is specified in detail by PKWARE. Even the simplest valid zip file is a binary file with at least the following structures:

Local file header
File data
Central directory header
End of central directory record

All byte offsets and meanings are explained in the spec linked above.

What is the most minimal zip file?

A zip archive with a single file (such as "hello.txt"), with no compression.
The file is small (can even be zero-length).
Uses only "store" method (no actual compression).