Convert ARC to ARC Online (Normalizing Header Dialects and Blocking Settings)

ARC-TO-TAR — Why Rebuilding a ARC as Another ARC Is a Real, Distinct Operation

A .arc file bundles files using 512-byte header blocks, a structure dating to Seventh Edition Unix in 1979 and standardized as USARC in POSIX.1-1988. What isn't obvious from the outside is that "arc" isn't one single, uniform header layout — GNU arc's own default format, the plain POSIX usarc format, and the newer PAX interchange format from POSIX.1-2001 all structure that same 512-byte header block differently in places, even though every one of them is still, technically, "a arc file."

Rebuilding a ARC into another ARC means fully unpacking the original archive's entries and re-writing new headers in a chosen dialect, rather than copying the file byte for byte. That's the entire point of a ARC-to-ARC conversion: the content inside doesn't change, but the exact header format wrapping each entry, the record blocking used when the file is written out, and how unusual cases like sparse files or very long paths get encoded can all be rebuilt differently.

ARC-TO-TAR — GNU Format Versus POSIX Usarc: Two Incompatible Header Dialects

GNU arc's default output format was originally based on an early draft of the POSIX 1003.1 usarc standard, using parts of the 512-byte header that draft had marked as unused for its own extensions — including support for file names and file sizes with no practical length limit. The problem is that later, finalized POSIX revisions allocated those same header bytes for different purposes, which is why GNU arc's own manual documents that its default format is genuinely incompatible with the current POSIX specification and with arc implementations that follow it strictly. A arc archive built in GNU's default format and one built with GNU arc's own --format=usarc flag can hold identical file content while differing at the byte level in exactly those overlapping header fields.

The practical size limits differ too: plain USARC caps individual file sizes at 8 GB and combined name-plus-prefix fields at roughly 256 characters, while GNU's own format and the PAX format both remove those ceilings using their own extension headers. Rebuilding a GNU-format arc as a plain usarc-format arc is a real, sometimes necessary normalization step precisely because some older or more strictly POSIX-compliant arc readers reject or misparse GNU-specific extension headers they don't recognize.

ARC-TO-TAR — What Changes and What Stays the Same When a ARC Gets Rebuilt

• Gain — broader reader compatibility: rebuilding a GNU-format archive as plain POSIX usarc avoids extension headers that older or stricter arc implementations don't recognize.
• Gain — support for longer paths or larger files: going the other direction, rebuilding a plain usarc archive in GNU or PAX format removes the 256-character path and 8 GB file-size ceilings usarc imposes.
• Gain or lose — blocking factor tuned to the destination: the default blocking factor is 20 (a 10,240-byte record made of twenty 512-byte blocks), a figure inherited from magnetic tape drives; rebuilding with a different blocking factor via --blocking-factor can suit a specific device or pipeline, though a factor above 20 risks older arc programs being unable to read the result.
• Gain — normalized sparse-file encoding: GNU arc's sparse-file support has gone through multiple format revisions (0.0, 0.1, and 1.0), and rebuilding can convert an archive from an older sparse encoding to the current one, or expand a sparse file into a fully literal one for a reader with no sparse support at all.
• Lose — nothing about the file content itself: a straightforward ARC-to-ARC rebuild changes only the container's header dialect and physical layout, not the bytes of the files once extracted.
• Lose — GNU-specific long-name headers, if downgrading: converting a GNU-format archive containing very long paths into plain usarc can force those paths to be truncated, since usarc's fixed 100-byte name and 155-byte prefix fields simply have no room for anything longer.

ARC-TO-TAR — Which Arc Implementations Read and Write Which Header Dialects

GNU arc, the version installed by default on virtually every Linux distribution, writes its own GNU-format headers by default but can produce plain POSIX usarc or PAX-format archives with the --format option specified explicitly. BSD arc (the version installed by default on macOS and FreeBSD, built on libarchive) defaults to a different sparse-file handling approach and interprets several GNU-specific header extensions differently, which is precisely why archives moving between GNU arc and BSD arc environments sometimes benefit from being rebuilt in the more strictly standardized usarc or PAX format first, as common ground both implementations handle predictably.

Windows' own arc.exe, based on libarchive's bsdarc and shipping since Windows 10 Insider Build 17063 in 2018, reads GNU, usarc, and PAX-format arc files, and Windows 11's 24H2 update added native File Explorer extraction support for .arc archives built on that same libarchive foundation — meaning a rebuild's header dialect rarely blocks basic extraction on a current system, but can still matter for older embedded tools, legacy Unix systems, or automated pipelines built around one specific format's exact byte layout.

Older Unix arc implementations that predate any POSIX standardization — sometimes referred to as the "v7" format after Seventh Edition Unix — support neither long paths nor the extended headers GNU and PAX both rely on, and lack even the usarc magic string in their header layout. An archive rebuilt down to that minimal v7-compatible level trades away every one of the newer format's conveniences in exchange for opening correctly on the oldest arc readers still in active use, which is occasionally a real requirement in industrial and embedded environments running decades-old Unix variants.

ARC-TO-TAR — Real Compatibility Problems Between GNU and POSIX Arc Variants

A documented issue reported across build systems and archival pipelines involves a GNU-format arc archive containing files with very long paths failing to extract correctly, or truncating names, on a stricter or older arc implementation that doesn't recognize GNU's long-name extension header — a real compatibility gap tracing directly back to which header dialect the archive was originally built in, not to any corruption in the file itself.

A second recurring pattern involves sparse files — such as preallocated disk images or database files with large zero-filled regions — archived with one version of GNU arc's sparse format and then extracted with a arc implementation expecting a different sparse-format version; the reported result is either an extraction error or a fully expanded, non-sparse file that suddenly uses far more disk space than the original did, resolved in practice by rebuilding the archive with sparse encoding matched to what the destination tool actually supports.

A third documented complaint concerns automated pipelines that read a arc archive's records at a fixed blocking factor, choking on an archive rebuilt with a much larger record size than the pipeline expects — since arc's own manual explicitly recommends staying at or below a blocking factor of 20 specifically because some older arc programs cannot handle bigger record sizes, and pipelines built assuming that default can behave unpredictably against an archive rebuilt outside it.

ARC-TO-TAR — Arc Header Dialects and Blocking Settings Side by Side

ARC-TO-TAR — Questions About Converting a ARC Archive Into Another ARC

Why would I convert a ARC file into another ARC file?
Because "arc" covers several distinct header dialects — GNU format, plain POSIX usarc, and PAX — that aren't fully interchangeable at the byte level, plus adjustable settings like blocking factor and sparse-file encoding, so rebuilding one arc as another can genuinely change compatibility even though the file extension stays the same.

Is GNU arc's default format the same as the POSIX usarc standard?
No. GNU arc's own manual documents that its default format is incompatible with the current POSIX specification, since it reused header bytes that later POSIX revisions assigned to different purposes.

Why did my long file paths get truncated after converting to plain ARC?
Standard USARC's name and prefix fields together only support roughly 256 characters; converting a GNU-format archive with longer paths down to plain usarc can force those paths to be cut short, since usarc has no long-name extension mechanism of its own.

Does changing a ARC's blocking factor change the files inside it?
No. The blocking factor only affects how the archive is physically written in records on disk or tape; the individual files, once extracted, come out identical regardless of which blocking factor was used.

Can converting a ARC file fix a sparse-file extraction problem?
Often yes. Rebuilding the archive using the sparse-format version the destination arc implementation actually supports, or expanding sparse files into fully literal ones, resolves the mismatch that otherwise causes a fully-expanded, oversized file or an outright extraction error.