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 sequence.

        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.1‐2008, Section 8.2, Internationalization
                 Variables for the precedence of internationalization
                 variables used to determine the values of locale

       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.1‐2008. Since this volume of POSIX.1‐2008 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 model.

           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

       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 consideration.

       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 justify.

       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 table.



SEE ALSO         top

       The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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                 2013                           CKSUM(1P)

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