Random File Access CHAPTER 7

Text and Binary Files While the concepts of “text files” given, we processed files sequentially, one character, integer at a time. Each program read a character, then read the next character, then the next character, and so on. It is also possible to process the characters in a file randomly. Thus, a program can begin processing a file at any desired location within the file, then go to another location in the file, and so on at random.

Text and Binary Files A text file is a sequence of characters that occasionally includes a new-line character. In some operating systems, most notable DOS (also this is true for versions of the MS Windows), when a program places the new-line character into a file, the new-line character is physically stored two characters: Carriage Return (CR) Line Feed (LF) So, the new-line character is also named as CRLF mark. Similarly, when a program reads a CRLF pair, the system converts them to single new-line character (that are seen as a single character in the program).

Text and Binary Files In Unix, Linux like operating systems, the new-line character is indicated only with LF (line feed) character, and there is no need to do any conversion. When randomly processing a file, a program must know its exact position in the file. If we open the file in text file format, it is not possible to process file randomly. In text files, the new-line to CRLF conversions make it impossible to control the positioning in the file. It is a need to open the file in binary mode, in order to process it randomly.

Text and Binary Files When a program opens a file as a binary file, none of the characters is ever changed when read or written by the program. In a binary file the CRLF pair is not accepted as a single new-line character, instead, these are accepted as a pair of characters. Opening a file in binary mode is just needing to place the letter “b” at the end of the opening mode string in the fopen() statement.

Opening a Binary File To open a file in binary mode, use one of the following as the opening mode in fopen(). Mode Meaning . rb Read wb Write (Create) ab Append r+b or rb+ Read and write w+b or wb+ Write (Create) and read a+b or ab+ Append and read Example: input_file = fopen(“TEST.TXT”, “rb”); It opens the “TEST.TXT” file for reading in binary mode.

Opening a Binary File Open a file in binary mode, when it must appear to the program exactly as it is stored. That includes files that are to be : Processed randomly Executable files Data files (image files (like gif, jpg), sound file (like wav, mp3), movie files (avi, mpeg). In order to process a binary file randomly, you need to use the Random Access Functions.

Random Access Functions Random access functions are used to manipulate the file’s position indicator, or FPI. We will use getc(), and putc() functions for input/output purposes (to access the characters in a file). In order to access the character on the desired location really we need a special function to manipulate the FPI. The most useful direct access function in the C library is fseek().

Random Access Functions The syntax of the fseek() function is: fseek(file_pointer, offset, location) The third argument is one of three integer values. The third argument determines the value that the function places initially in the file’s FPI. The possible values of the third argument can be: Value Symbolic Equiv. Meaning . 0 SEEK_SET Sets the FPI to the first character in the file 1 SEEK_CUR Does not alter the FPI, stays at its current position. 2 SEEK_END Sets the FPI to the end of the file

Random Access Functions The second argument in fseek() is the offset, which must be a long integer. After setting the FPI by using the third argument, fseek() then adds the value of the second argument to the FPI. It carries the FPI to a position that is far the value of the second argument away from the main position given in the first argument. Examples: fopen(“TEST.TXT”,”rb”); FPI a b c d e f g h eof

Examples fseek(input_file,3L,SEEK_CUR); fseek(input_file,0L,SEEK_END); FPI 3 a b c d e f g h eof fseek(input_file,0L,SEEK_END); FPI 8 a b c d e f g h eof fseek(input_file,-2L,SEEK_CUR); FPI 6 a b c d e f g h eof

Examples fseek(input_file,0L,SEEK_SET); FPI a b c d e f g h eof fseek(input_file,-3L,SEEK_END); FPI 5 a b c d e f g h eof fseek(input_file,2L,SEEK_SET); FPI 2 a b c d e f g h eof

Examples fseek(input_file,8L,SEEK_SET); FPI 8 a b c d e f g h eof fseek(input_file,-8L,SEEK_END); FPI a b c d e f g h eof fseek(input_file,6L,SEEK_CUR); FPI 6 a b c d e f g h eof

Examples To detect the End-of-File position on a file, the feof() function can be used. Syntax : int feof( FILE *file-pointer); The function returns a nonzero value if the end of file has been detected. Ex.: while (!feof(file_ptr) { … }

Examples The READ AHEAD RULE: Read sequentially a file until you reach the end of file. ex.: FILE *input_file; char ch; input_file = fopen(“TEST.TXT”,”rb”); ch=getc(input_file); while (!feof(input_file)) { printf(“%c”,ch); } fclose(input_file);

Examples #include <stdio.h> #include <stdlib.h> #include <ctype.h> void main() { FILE *input_file; char ch; input_file= fopen(“EXAMPLE.TXT”,”rb+”); ch = getc(input_file); while (!feof(input_file) fseek(input_file, -1L, SEEK_CUR); putc(toupper(ch),input_file); fseek(input_file,0L, SEEK_CUR); } flose(input_file);

Examples Recall that, executing a getc() obtains a character and moves the FPI to the next character. Also, putc() moves the FPI to the next character after the character is printed. So, on the previous example, it was needed to bring the FPI to back to the current position with “-1L” offset. On rewrite operations, after the read operation, you have to bring the FPI on to the same position to rewrite the modified version of the read character.