4. COMPILING AND RUNNING FORTRAN PROGRAMS

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1 20 4. COMPILING AND RUNNING FORTRAN PROGRAMS 4.1. Introduction Fortran is the most commonly used high-level programming language in science and engineering. Developed in the 1950's, Fortran was essentially the first high level language. It has been updated many times since then to reflect current technology and programming philosophy, so today's industry standard Fortran-95 is a powerful and flexible language. This chapter will introduce you to Fortran programming on Unix by showing you how to create, compile, and run a simple Fortran program. It is assumed that you know the rudiments of Fortran. If you don't know the Fortran language, you should consult a textbook or manual. In this chapter we use the simplest (default) form for all commands and Fortran statements discussed. In reality, most commands allow many options, allowing you to customize your programming as you need to. You are referred to the Fortran Language Reference and User's Guide manuals for these details. 4.2 Creating the Source Code File After receiving the $ prompt, you are ready to write and edit your Fortran program. Assuming that you prefer to use the jove editor, type in: jove filename (The jove editor is described in section 3 of this manual). "Filename" will be the name of the file containing the source code for the program you are working on. "Filename" can be a full path name including directory and subdirectory information, or it can be a simple name plus file extension as in the following example. $ jove kwatt.f95 (return) The file extension, or qualifier,.f,.f90, and.f95 stand for Fortran and are required to distinguish it from other types of programs. The following example illustrates a program typed in with jove: This program totals the amount of energy used in 5 regions. (in millions of kwatt-hrs) program kwatt real kwh(5), totkwh integer region data kwh/1.5,0.75,2.0,1.25,1.5/ totkwh=0.0 do region=1,5 totkwh=totkwh + kwh(region) end do print*, totkwh, ' KWATT HOURS WERE USED IN THE FIVE REGIONS ' stop end After you have exited from the editor, you are ready to COMPILE, LINK and RUN your program.

2 The Compiler A compiler is a program which translates a program written in a high level language, Fortran for example, into machine language so the computer can execute it. To compile your program, type: f95 filename The computer will compile the program using the Absoft Fortran 95 compiler, link it together with the system run-time libraries, and produce an executable program. By default, the executable program will be named "a.out". If you wish, you may rename the executable file using the mv command, or by specifying to xlf95 the name you want using the "-o" option. $ f95 kwatt.f95 -o kwatt (return) If the computer can't convert an instruction into machine language, the computer will return an ERROR message describing the error and the line number. You can then go back and edit your program to make the necessary changes (you may need to consult the Fortran Language Reference to help you find and correct your errors) If no compiling errors occur, the compiler will produce a new file, "kwatt", for this example. This file is an executable binary image and cannot be read by humans (you can see it with the ls command). Once you have fixed any errors in your program, you may wish to compile it again, telling f95 to use additional steps to optimize the executable program it produces. This step adds time to the compilation process, but often produces a faster running result. Use this if you expect your program to run for a long time. $ f95 -O kwatt.f95 -o kwatt (return) 4.4 The Link Phase A linker is a program that links together all the different files you need to run your program, producing an executable image file. On Unix, the compiler invokes the linker automatically. If the compilation is successful, f95 will attempt to attach your program to a standard set of run-time routines and produce an executable program. Occasionally, though, you will want to tell f95 to also link your program to one or more additional subroutine libraries. For example, the LINPACK matrix manipulation subroutine library is not normally linked to all programs. If your program needs to access those routines, you must tell f95 to link your program to the system LINPACK library using the "-l" compiler option. $ f95 myprog.f95 -llinpack -o myprog (return) If no linking errors occur, the computer will produce a new file, myprog. Again, this program is an executable binary image stored on disk and is not readable by humans. 4.5 Running After the computer has finished linking, you are now in a position to run your program. To do this type the name of the executable program:./myprog This tells the computer to initiate the execution of your program.

3 22 Your program will now execute. If errors occur in the execution of your program you can go back to the editor and make the necessary corrections. After making the corrections, repeat the f95 compilation and run your program again. If any problems arise, there is an assortment of manuals available in room MLT 308. There is also a debugging program available to help you debug, or find the errors in, your program. Complete $ jove kwatt.f95 (return) THIS PROGRAM TOTALS THE AMOUNT OF ENERGY USED IN 5 REGIONS (IN MILLIONS OF KWATT-HRS) PROGRAM KWATT REAL KWH(5),TOTKWH INTEGER REGION DATA KWH/1.5,0.75,2.0,1.25,1.5/ TOTKWH=0.0 DO REGION=1,5 TOTKWH = TOTKWH + KWH(REGION) END DO PRINT*, TOTKWH, ' KWATT HOURS WERE USED IN THE FIVE REGIONS ' STOP END (control-x control-\) (control-x control-c) Now compile, link and run: $ f95 kwatt.f95 -o kwatt (return) $./kwatt (return) The output written to the terminal screen will be: KWATT HOURS WERE USED IN THE FIVE REGIONS 4.6 Input and Output In the above example, the data needed to run the program is given in a Fortran DATA statement. Often, it is useful to supply new data to a program for different runs, in which case we will need a way to input data each time the program runs. The most common way to input data into a Fortran program is with a READ statement. Data can be read in directly from the terminal or from a separate data file. To see how this is done on Unix, replace the DATA statement in the above example with the following READ statement:

4 23 READ*, (KWH(I),I=1,5) Now if you compile and run the program, nothing will happen. That is, the program starts to run, but pauses at the READ statement and waits for you to type in five numbers. When it gets five legal numbers, the program proceeds and writes the output to the terminal. It is often desirable to input data from a file or write results to a file. On Unix systems, this is done easily using the BASH shell's advanced feature of I/O redirection (see chapter 2). Any READ statements that would have taken input from your terminal may read from a file instead using the '<' symbol on the command line. Similarly, any output that would have printed on your terminal may be directed to a file using the '>' symbol on the command line. kwatt < file1 reads from file1, and writes to your terminal kwatt > file2 reads from your terminal, and writes to file2 kwatt < file1 > file2 reads from file1, and writes to file2 If one input stream is not enough, you may explicitly create more. To do this you first need to tell the computer that a data file exists using the Fortran OPEN statement. Then to read the data from that file: OPEN(UNIT=10, FILE='kwhdata') READ(10,*) (KWH(I),I=1,5) Then you must put the data into a file called kwhdata in your current directory, so when the program is run, it will find kwhdata and read the data from it. You can also use a full pathname if the data file is not in your current directory. In a similar way, you can cause output data to be written to a file, instead of the terminal. To do this you must OPEN the output file and replace the PRINT statement in the example with a WRITE statement: OPEN(UNIT=20, FILE='totkwh.dat') WRITE(20,*) TOTKWH, 'KWATT HOURS WERE USED IN THE FIVE REGIONS' The choice of I/O unit numbers is up to you, except that units 0, 5, and 6 are special. Unit 5 is automatically preopened to be used for input read from your keyboard, and is the default I/O unit used by the READ* statement. Similarly, unit 6 is pre-opened to be used for output written to your terminal. Units 5 and 6 are also the I/O units affected by BASH I/O redirection. Unit 0 is also pre-opened for output to your terminal, but is not redirected by BASH I/O redirection. The complete example using file input and output is given on the next page:

5 24 THIS PROGRAM TOTALS THE AMOUNT OF ENERGY USED IN 5 REGIONS (IN MILLIONS OF KWATT-HRS) PROGRAM KWATT REAL KWH(5),TOTKWH INTEGER REGION OPEN(UNIT=10, FILE='kwh.dat') READ(10,*) (KWH(I), I=1,5)) TOTKWH=0.0 DO REGION=1,5 TOTKWH=TOTKWH + KWH(REGION) END DO OPEN(UNIT=20, FILE='totkwh.dat') WRITE(20,*) TOTKWH, 'KWATT HOURS WERE USED IN THE FIVE REGIONS' STOP END Running the program now requires an existing input data file called kwh.dat, and produces a new output file called totkwh.dat.