This chapter describes systems calls used by the I/O library to perform asynchronous or synchronous I/O. This chapter also describes Fortran callable entry points to several C library routines.
The I/O library and programs use the system calls described in this chapter to perform synchronous and asynchronous I/O, to queue a list of distinct I/O requests, and to perform unbuffered I/O without system buffering.
With synchronous I/O, an executing program relinquishes control during the I/O operation until the operation is complete. An operation is not complete until all data is moved.
The read(2) and write(2) system calls perform synchronous reads and writes. The READ(3f) and WRITE(3f) functions provide a Fortran interface to the read and write system calls. The read system call reads a specified number of bytes from a file into a specified buffer. The write system call writes from a buffer to a file.
Asynchronous I/O lets the program use the time that an I/O operation is in progress to perform some other operations that do not involve the data in the I/O operation. In asynchronous I/O operations, control is returned to the calling program after the I/O is initiated. The program may perform calculations unrelated to the previous I/O request or it may issue another unrelated I/O request while waiting for the first I/O request to complete.
The asynchronous I/O routines provide functions that let a program wait for a particular I/O request to complete. The asynchronous form of BUFFER IN and BUFFER OUT statements used with UNIT and LENGTH routines provide this type of I/O.
The C library provides a set of routines that constitute a user-level I/O buffering scheme to be used by C programmers.
The getc(3c) and putc(3c) inline macros process characters. The getchar and putchar macros, and the higher-level routines fgetc, fgets, fprintf, fputc, fputs, fread, fscanf, fwrite, gets, getw, printf, puts, putw, and scanf all use or act as if they use getc and putc. They can be intermixed.
A file with this associated buffering is called a streams and is associated with a pointer to a defined type FILE. The fopen (3c) routine creates descriptive data for a stream and returns a pointer to designate the stream in all further transactions. Three open streams with constant pointers are usually declared in the <stdio.h> header file and are associated with stdin, stdout , and stderr.
Three types of buffering are available with functions that use the FILE type: unbuffered, fully buffered, and line buffered, as described in the following list:
If the stream is unbuffered, no library buffer is used.
For a fully buffered stream, data is written from the library buffer when it is filled, and read into the library buffer when it is empty.
If the stream is line buffered, the buffer is flushed when a new line character is written, the buffer is full, or when input is requested.
The setbuf and setvbuf functions let you change the type and size of the buffers. By default, output to a terminal is line buffered, output to stderr is unbuffered, and all other I/O is fully buffered. See the setbuf(3c) man page for details.
Mixing the use of C I/O functions with Fortran I/O on the same file may have unexpected results. If you want to do this, ensure that the Fortran file structure chosen does not introduce unexpected control words and that library buffers are flushed properly before switching between types of I/O.
The following example illustrates the use of some C routines. The assign environment does not affect these routines.
Example 5-1. C I/O from Fortran
PROGRAM STDIOEX
INTEGER FOPEN, FCLOSE, FWRITE, FSEEK
INTEGER FREAD, STRM
CHARACTER*25 BUFWR, BUFRD
PARAMETER(NCHAR=25)
C Open the file /tmp/mydir/myfile for update
STRM = FOPEN('/tmp/mydir/myfile','r+')
IF (STRM.EQ.0) THEN
STOP 'ERROR OPENING THE FILE'
ENDIF
C Write
I = FWRITE(BUFWR, 1, NCHAR, STRM)
IF (I.NE.NCHAR*1)THEN
STOP 'ERROR WRITING FILE'
ENDIF
C Rewind and read the data
I = FSEEK(STRM, 0, 0)
IF (I.NE.0)THEN
STOP 'ERROR REWINDING FILE'
ENDIF
I = FREAD(BUFRD, 1, NCHAR, STRM)
IF (I.NE.NCHAR*1)THEN
STOP 'ERROR READING FILE'
ENDIF
C Close the file
I = FCLOSE(STRM)
IF (I.NE.0) THEN
STOP 'ERROR CLOSING THE FILE'
ENDIF
END |