cksum(1p) — Linux manual page


CKSUM(1P)               POSIX Programmer's Manual              CKSUM(1P)

PROLOG         top

       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.

NAME         top

       cksum — write file checksums and sizes

SYNOPSIS         top

       cksum [file...]

DESCRIPTION         top

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

       The encoding for the CRC checksum is defined by the generating


       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

        4. The bit sequence is complemented and the result is the CRC.

OPTIONS         top


OPERANDS         top

       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         top

       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         top

       The input files can be any file type.


       The following environment variables shall affect the execution of

       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.

       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).

                 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.



STDOUT         top

       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         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top




EXIT STATUS         top

       The following exit values shall be returned:

        0    All files were processed successfully.

       >0    An error occurred.



       The following sections are informative.


       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.

       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.

       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         top


RATIONALE         top

       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

           static unsigned long crctab[] = {
           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;

       The historical practice of writing the number of ``blocks'' has
       been changed to writing the number of octets, since the latter is
       not only more useful, but also since historical implementations
       have not been consistent in defining what a ``block'' meant.

       The algorithm used was selected to increase the operational
       robustness of cksum.  Neither the System V nor BSD sum algorithm
       was selected. Since each of these was different and each was the
       default behavior on those systems, no realistic compromise was
       available if either were selected—some set of historical
       applications would break. Therefore, the name was changed to
       cksum.  Although the historical sum commands will probably
       continue to be provided for many years, programs designed for
       portability across systems should use the new name.

       The algorithm selected is based on that used by the
       ISO/IEC 8802‐3:1996 standard (Ethernet) for the frame check
       sequence field. The algorithm used does not match the technical
       definition of a checksum; the term is used for historical
       reasons. The length of the file is included in the CRC
       calculation because this parallels inclusion of a length field by
       Ethernet in its CRC, but 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. It was
       decided that historical usage was the more important

       Early proposals contained modifications to the Ethernet algorithm
       that involved extracting table values whenever an intermediate
       result became zero. This was demonstrated to be less robust than
       the current method and mathematically difficult to describe or

       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
               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];

       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.

       The referenced Sarwate article also discusses generating the



SEE ALSO         top

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

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                         CKSUM(1P)

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