C Programming

1
C Programming

• File Processing
• Part II

2
Text processing

• A file can be opened and processed as either text
  file or binary file
• Text files
• contain printable characters and control
  characters
• organized into lines
• system may convert or remove some input
  characters
• system may insert or convert some output
  characters

3
Binary Files

• Contain a series of characters that require no
  translation
• Binary files are very similar to arrays of
  structures, except the structures are in a disk
  file rather than in an array in memory.
• Binary files have two features that distinguish
  them from text files
• You can jump instantly to any structure in the
  file, which provides random access as in an
  array.
• You can change the contents of a structure
  anywhere in the file at any time.
• Binary files also usually have faster read and
  write times than text files, because a binary
  image of the record is stored directly from
  memory to disk (or vice versa).
• In a text file, everything has to be converted
  back and forth to text, and this takes time.

4
fopen

• fopen mode determines if file is handled as text
  or binary
• add "b" to mode for binary processing
• fp2 fopen( fileb, "rb" )
• omit "b" for text processing
• fp1 fopen( filea, "r" )
• Same input file can be processed as text or
  binary, but characters read may be different
• Use binary when you want no translation to be
  done

5
Record processing, fread

• fread function reads up to a requested number of
  items of a specified size
• Format
• size_t fread( buffer, size, count, fp )
• Returns the number of items actually read, if
  less than requested must use feof or ferror
  function to determine if end-of-file or error
  occurred
• buffer
• character array to receive the input
• size
• size in bytes of each item
• count
• maximum number of items to be read
• fp
• file pointer to the input file

6
fread example

• char cal80, flo5100
• file filea
• filea fopen( "...", "rb" )
• ...
• / read one 80-byte record /
• i fread( cal, 80, 1, filea )
• ...
• / read five 100-byte records /
• i fread( flo, 5, 100, filea )

7
Record processing, fwrite

• fwrite function writes a requested number of
  items of a specified size
• Format
• size_t fwrite( buffer, size, count, fp )
• Returns the number of items actually written, if
  less than requested error occurred
• buffer
• character array containing data to be written
• size
• size in bytes of each item
• count
• number of items to write
• fp
• file pointer to the open output file

8
fwrite example

• char cal80, flo5100
• file filea
• fileb fopen( "...", "wb" )
• ...build data in buffers...
• / write one 80-byte record /
• i fwrite( cal, 80, 1, fileb )
• ...
• / write five 100-byte records /
• i fwrite( flo, 5, 100, fileb )

9
File position

• Bytes in file numbered from zero for first byte
• File position is where the next input or output
  operation will take place
• File position advances with each byte read or
  written
• ftell function returns current file position
• fseek function sets file position for next input
  or output
• Files opened in append mode always write at end
  of file regardless of previous file position

10

• fseek, set file position
• fseek function sets file position for next input
  or output operation
• Format int fseek( fileptr, offset, origin )
• Returns zero if successful, not zero otherwise
• fileptr
• open file pointer
• offset
• signed long integer added to determine new
  position
• origin
• constant specify how offset is to be used to
  determine new position, SEEK_SET
• offset is relative to beginning of file (should
  be positive)
• SEEK_CUR
• offset is relative to current position ( or -)
• SEEK_END
• offset is relative to end of file ( or -)
• Use to randomly read file
• Use to write to desired slot
• Use to reposition to remembered place (from index
  or checkpoint)
• Use to browse file forward and backward

11

• Set file pointer from beginning, SEEK_SET
• SEEK_SET sets file position from the beginning of
  the file
• Example
• long newp
• FILE filea
• char joe4
• filea fopen( ... , "rb" )
• ...
• newp 16175
• fseek( filea, newp, SEEK_SET )
• fscanf( filea, "3c", joe )

12
Set file pointer from current, SEEK_CUR

• SEEK_CUR sets the new position relative to the
  current file position. The offset can be positive
  to move forward, or negative to move backward.
• long newp
• FILE filea
• char sam5
• ...
• filea fopen( ... , "rb" )
• ...
• newp -8
• fseek( filea, newp, SEEK_CUR )
• fread( sam, 1, 4, filea )

13
Set file pointer from end, SEEK_END

• The following example sets the file position and
  reads the last 8 bytes of the file.
• long newp
• FILE filea
• char sam9
• ...
• filea fopen( ... , "rb" )
• ...
• newp 7
• fseek( filea, newp, SEEK_END )
• fread( sam, 1, 8, filea )

14

• include ltstdio.hgt
• struct rec
• int x,y,z
• int main()
• int i,j
• FILE f
• struct rec r
• ffopen("junk","w")
• if (!f)
• return 1
• for (i1ilt10 i)
• r.xi
• fwrite(r,sizeof(struct rec),1,f)

ffopen("junk","r") if (!f)
return 1 for (i1ilt10 i)
fread(r,sizeof(struct rec),1,f)
printf("d\n",r.x) fclose(f)
printf("\n")
15

• ffopen("junk","r")
• if (!f)
• return 1
• for (i9 igt0 i--)
• fseek(f,sizeof(struct rec)i,SEEK_SET)
• fread(r,sizeof(struct rec),1,f)
• printf("d\n",r.x)
• fclose(f)
• printf("\n")