**LISTING 5.1 SEARCH.C**

    /* Program to search the file specified by the first command-line
     * argument for the string specified by the second command-line
     * argument. Performs the search by reading and searching blocks
     * of size BLOCK_SIZE. */

    #include <stdio.h>
    #include <fcntl.h>
    #include <string.h>
    #include <alloc.h>   /* alloc.h for Borland compilers,
                            malloc.h for Microsoft compilers */

    #define BLOCK_SIZE  0x4000   /* we'll process the file in 16K blocks */

    /* Searches the specified number of sequences in the specified
       buffer for matches to SearchString of SearchStringLength. Note
       that the calling code should already have shortened SearchLength
       if necessary to compensate for the distance from the end of the
       buffer to the last possible start of a matching sequence in the
       buffer.
    */

    int SearchForString(unsigned char *Buffer, int SearchLength,
       unsigned char *SearchString, int SearchStringLength)
    {
       unsigned char *PotentialMatch;

       /* Search so long as there are potential-match locations
          remaining */
       while ( SearchLength ) {
         /* See if the first character of SearchString can be found */
         if ( (PotentialMatch =
               memchr(Buffer, *SearchString, SearchLength)) == NULL ) {
            break;   /* No matches in this buffer */
         }
          /* The first character matches; see if the rest of the string
             also matches */
          if ( SearchStringLength == 1 ) {
             return(1);  /* That one matching character was the whole
                            search string, so we've got a match */
          }
          else {
             /* Check whether the remaining characters match */
             if ( !memcmp(PotentialMatch + 1, SearchString + 1,
                   SearchStringLength - 1) ) {
                return(1);  /* We've got a match */
             }
          }
          /* The string doesn't match; keep going by pointing past the
             potential match location we just rejected */
          SearchLength -= PotentialMatch - Buffer + 1;
          Buffer = PotentialMatch + 1;
       }

       return(0);  /* No match found */
    }

    main(int argc, char *argv[]) {
       int Done;               /* Indicates whether search is done */
       int Handle;             /* Handle of file being searched */
       int WorkingLength;      /* Length of current block */
       int SearchStringLength; /* Length of string to search for */
       int BlockSearchLength;  /* Length to search in current block */
       int Found;              /* Indicates final search completion
                                  status */
       int NextLoadCount;      /* # of bytes to read into next block,
                                  accounting for bytes copied from the
                                  last block */
       unsigned char *WorkingBlock; /* Block storage buffer */
       unsigned char *SearchString; /* Pointer to the string to search for */
       unsigned char *NextLoadPtr;  /* Offset at which to start loading
                                       the next block, accounting for
                                       bytes copied from the last block */

       /* Check for the proper number of arguments */
       if ( argc != 3 ) {
          printf("usage: search filename search-string\n");
          exit(1);
       }

       /* Try to open the file to be searched */
       if ( (Handle = open(argv[1], O_RDONLY | O_BINARY)) == -1 ) {
          printf("Can't open file: %s\n", argv[1]);
          exit(1);
       }
       /* Calculate the length of text to search for */
       SearchString = argv[2];
       SearchStringLength = strlen(SearchString);
       /* Try to get memory in which to buffer the data */
       if ( (WorkingBlock = malloc(BLOCK_SIZE)) == NULL ) {
          printf("Can't get enough memory\n");
          exit(1);
       }

       /* Load the first block at the start of the buffer, and try to
          fill the entire buffer */
       NextLoadPtr = WorkingBlock;
       NextLoadCount = BLOCK_SIZE;
       Done = 0;      /* Not done with search yet */
       Found = 0;     /* Assume we won't find a match */
       /* Search the file in BLOCK_SIZE chunks */
       do {
          /* Read in however many bytes are needed to fill out the block
             (accounting for bytes copied over from the last block), or
             the rest of the bytes in the file, whichever is less */
          if ( (WorkingLength = read(Handle, NextLoadPtr,
                NextLoadCount)) == -1 ) {
             printf("Error reading file %s\n", argv[1]);
             exit(1);
          }
          /* If we didn't read all the bytes we requested, we're done
             after this block, whether we find a match or not */
          if ( WorkingLength != NextLoadCount ) {
             Done = 1;
          }

          /* Account for any bytes we copied from the end of the last
             block in the total length of this block */
          WorkingLength += NextLoadPtr - WorkingBlock;
          /* Calculate the number of bytes in this block that could
             possibly be the start of a matching sequence that lies
             entirely in this block (sequences that run off the end of
             the block will be transferred to the next block and found
             when that block is searched)
          */
          if ( (BlockSearchLength =
                   WorkingLength - SearchStringLength + 1) <= 0 ) {
                Done = 1;  /* Too few characters in this block for
                              there to be any possible matches, so this
                              is the final block and we're done without
                              finding a match
                           */
          }
          else {
             /* Search this block */
             if ( SearchForString(WorkingBlock, BlockSearchLength,
                   SearchString, SearchStringLength) ) {
                Found = 1;     /* We've found a match */
                Done = 1;
             }
             else {
                /* Copy any bytes from the end of the block that start
                   potentially-matching sequences that would run off
                   the end of the block over to the next block */
                if ( SearchStringLength > 1 ) {
                   memcpy(WorkingBlock,
                      WorkingBlock+BLOCK_SIZE - SearchStringLength + 1,
                      SearchStringLength - 1);
                }
                /* Set up to load the next bytes from the file after the
                   bytes copied from the end of the current block */
                NextLoadPtr = WorkingBlock + SearchStringLength - 1;
                NextLoadCount = BLOCK_SIZE - SearchStringLength + 1;
             }
          }
       } while ( !Done );

       /* Report the results */
       if ( Found ) {
          printf("String found\n");
       } else {
          printf("String not found\n");
       }
       exit(Found);   /* Return the found/not found status as the
                         DOS errorlevel */
    }