CKSUM(1P)

📦 version 2017a, in alpine 3.22, alpine 3.23, alpine 3.24, arch rolling, nixpkgs unstable, opensuse-leap 16.0

👑 CKSUM(1P) — write file checksums and sizes 💖

okay bestie, welcome to the POSIX Programmer's Manual ✨ this page is literally the POSIX spec for cksum, so buckle up.

🌸 PROLOG 🌸

Real talk first: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. 😭 so like, don't come for me if Linux does its own thing, okay? check the Linux page, iykyk.

💅 NAME 💅

cksum — write file checksums and sizes 🧮

✨ SYNOPSIS ✨

cksum [file...]

🦋 DESCRIPTION 🦋

okay so here's the vibe. The cksum utility shall calculate and write to standard output a cyclic redundancy check (CRC) for each input file, and also write to standard output the number of octets in each file. that's two things per file, bestie — the checksum AND the size. main character energy fr fr.

The CRC used is based on the polynomial used for CRC error checking in the ISO/IEC 8802‐3:1996 standard (Ethernet). iconic.

The encoding for the CRC checksum is defined by the generating polynomial:

G(x)=x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1

Mathematically, the CRC value corresponding to a given file shall be defined by the following procedure:

  1. The n bits to be evaluated are considered to be the coefficients of a mod 2 polynomial M(x) of degree n-1. These n bits are the bits from the file, with the most significant bit being the most significant bit of the first octet of the file and the last bit being the least significant bit of the last octet, padded with zero bits (if necessary) to achieve an integral number of octets, followed by one or more octets representing the length of the file as a binary value, least significant octet first. The smallest number of octets capable of representing this integer shall be used.
  2. M(x) is multiplied by x32 (that is, shifted left 32 bits) and divided by G(x) using mod 2 division, producing a remainder R(x) of degree ≤ 31.
  3. The coefficients of R(x) are considered to be a 32-bit sequence.
  4. The bit sequence is complemented and the result is the CRC.

like… it's giving math class but make it slay 💜

🎀 OPTIONS 🎀

None. 😌 yes, zero. the options list is empty, we love a low-maintenance utility.

🌈 OPERANDS 🌈

The following operand shall be supported:

  • file — A pathname of a file to be checked. If no file operands are specified, the standard input shall be used. 🫶

⭐ STDIN ⭐

The standard input shall be used if no file operands are specified, and shall be used if a file operand is '-' and the implementation treats the '-' as meaning standard input. Otherwise, the standard input shall not be used. See the INPUT FILES section. 👀

💖 INPUT FILES 💖

The input files can be any file type. literally any. no gatekeeping here. 🌈

🧚 ENVIRONMENT VARIABLES 🧚

The following environment variables shall affect the execution of cksum:

  • LANG — Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1‐2017, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
  • LC_ALL — If set to a non-empty string value, override the values of all the other internationalization variables. she said OVERRIDE 💅
  • LC_CTYPE — Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).
  • LC_MESSAGES — Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
  • NLSPATH — Determine the location of message catalogs for the processing of LC_MESSAGES.

🔥 ASYNCHRONOUS EVENTS 🔥

Default. (yes that's it, no drama 🌸)

👑 STDOUT 👑

For each file processed successfully, the cksum utility shall write in the following format:

"%u %d %s\n", <checksum>, <# of octets>, <pathname>

If no file operand was specified, the pathname and its leading <space> shall be omitted. ✂️

💀 STDERR 💀

The standard error shall be used only for diagnostic messages. no small talk, only diagnostics.

🙈 OUTPUT FILES 🙈

None.

🍓 EXTENDED DESCRIPTION 🍓

None.

✨ EXIT STATUS ✨

The following exit values shall be returned:

  • 0 — All files were processed successfully. 🎉
  • >0 — An error occurred. 😭

💜 CONSEQUENCES OF ERRORS 💜

Default.

The following sections are informative.

🎀 APPLICATION USAGE 🎀

The cksum utility is typically used to quickly compare a suspect file against a trusted version of the same, such as to ensure that files transmitted over noisy media arrive intact. However, this comparison cannot be considered cryptographically secure. The chances of a damaged file producing the same CRC as the original are small; deliberate deception is difficult, but probably not impossible.

bestie listen: NOT cryptographically secure. it's a vibe check, not a security guard. do not use this to protect secrets, fr fr. 🚨

Although input files to cksum can be any type, the results need not be what would be expected on character special device files or on file types not described by the System Interfaces volume of POSIX.1‐2017. Since this volume of POSIX.1‐2017 does not specify the block size used when doing input, checksums of character special files need not process all of the data in those files. 😬 so like, character special files? results may be unhinged.

The algorithm is expressed in terms of a bitstream divided into octets. If a file is transmitted between two systems and undergoes any data transformation (such as changing little-endian byte ordering to big-endian), identical CRC values cannot be expected. Implementations performing such transformations may extend cksum to handle such situations.

🌸 EXAMPLES 🌸

None.

✨ RATIONALE ✨

okay so bestie, here's the whole lore on why cksum does what it does. The following C-language program can be used as a model to describe the algorithm. It assumes that a char is one octet. It also assumes that the entire file is available for one pass through the function. This was done for simplicity in demonstrating the algorithm, rather than as an implementation model.

static unsigned long crctab[] = {
0x00000000,
0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};

unsigned long memcrc(const unsigned char *b, size_t n)
{
/*  Input arguments:
 *  const unsigned char*   b == byte sequence to checksum
 *  size_t                 n == length of sequence
 */

    register size_t i;
    register unsigned c, s = 0;

    for (i = n; i > 0; --i) {
        c = *b++;
        s = (s << 8) ^ crctab[(s >> 24) ^ c];
    }

    /* Extend with the length of the string. */
    while (n != 0) {
        c = n & 0377;
        n >>= 8;
        s = (s << 8) ^ crctab[(s >> 24) ^ c];
    }

    return ~s;
}

okay so real talk: back in the day people used to write the number of blocks, but that's been changed to writing the number of octets. Why? Because octets are way more useful, and also because historical implementations were literally not consistent about what a block even meant. 💀

The algorithm they picked was chosen to increase the operational robustness of cksum. They did not pick the System V sum algorithm, and they did not pick the BSD sum algorithm. Why? Because those two were different from each other, and each one was the default behavior on its own system. So there was no realistic compromise—if they picked either one, some set of historical applications would break. So they changed the name to cksum. The historical sum commands will probably keep being provided for many years, but if you're writing programs designed for portability across systems, bestie, you should use the new name. Periodt.

The algorithm they chose is based on the one used by the ISO/IEC 8802‐3:1996 standard (Ethernet) for the frame check sequence field. But heads up: the algorithm used does not match the technical definition of a checksum. The term is just used for historical reasons. The length of the file is included in the CRC calculation because that parallels how Ethernet includes a length field in its CRC, and also because it guards against inadvertent collisions between files that begin with different series of zero octets. The chance that two different files produce identical CRCs is much greater when their lengths are not considered. Keeping the length and the checksum of the file itself separate would yield a slightly more robust algorithm, but historical usage has always been that a single number (the checksum as printed) represents the signature of the file. They decided historical usage was the more important consideration. slay.

Early proposals had modifications to the Ethernet algorithm that involved extracting table values whenever an intermediate result became zero. But that was demonstrated to be less robust than the current method, and it was mathematically difficult to describe or justify. So they said no. 🙅‍♀️

The calculation used is identical to that given in pseudo-code in the referenced Sarwate article. The pseudo-code rendition is:

X <- 0; Y <- 0;
for i <- m -1 step -1 until 0 do
    begin
    T <- X(1) ^ A[i];
    X(1) <- X(0); X(0) <- Y(1); Y(1) <- Y(0); Y(0) <- 0;
    comment: f[T] and f'[T] denote the T-th words in the
        table f and f' ;
    X <- X ^ f[T]; Y <- Y ^ f'[T];
    end

The pseudo-code is reproduced exactly as given; however, note that in the case of cksum, A[i] represents a byte of the file, the words X and Y are treated as a single 32-bit value, and the tables f and f' are a single table containing 32-bit values. iykyk.

The referenced Sarwate article also discusses generating the table. cute.

🔮 FUTURE DIRECTIONS 🔮

None.

literally none. the future is now. 💅

👀 SEE ALSO 👀

  • The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables

go read that if you want the full lore. 🫶

okay so legal bestie moment, but make it cute:

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

This page is from IEEE/The Open Group, 2017, and it's the CKSUM(1P) man page. bye bestie, go checksum something. 💖✨

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