File: g77intro.txt Date: 1997 August 15 Version: 0.4 Author: Clive Page, cgp@star.le.ac.uk g77 vn: 0.5.18 This file contains a brief introduction to g77, the GNU Fortran compiler, here packaged for use on PCs with MS-DOS using the EMX DOS-extender. 1. What is g77? G77 is a Fortran compiler from the Free Software Foundation (GNU). The g77 compiler is designed to be portable, and there are implementations on a wide variety of systems including Linux, other Unixes, OS/2, and MS-DOS. The g77 compiler supports the whole of the (now obsolete) Fortran77 Standard plus many commonly-used extensions. This package, g77dos, packages the g77 compiler with the EMX DOS-extender of Eberhard Matthes and a few other useful utilities, so it can be used under MS-DOS. It requires a PC (386/486/Pentium) with around 4 MB of disc space. Typographical note: in this introduction Fortran keywords and code are shown in UPPER CASE merely make them stand out; g77 is equally happy with Fortran in lower-case. 2. How to download. The package is available by anonymous FTP from ftp.star.le.ac.uk in directory pub/fortran (and maybe other sites in due course). There are two zip files: g77dos.zip [insert size here] The main executables g77doc.zip [insert size here] Documentation and other stuff. FTP these in binary mode. This port is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with emx; see the file COPYING. If not, write to the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA. The GNU Public Licence requires the corresponding source code to be available. This can be obtained from a number of FTP sites including: [insert addresses here] 3. How to install. You will need at least 3 MB [check this] of free space on your hard disc. If you start from your root directory the files will unzip into subdirectories \EMX \EMX\BIN \EMX\LIB and \EMX\DOC as necessary. You are free to put them somewhere else but if so you will need to update the file SETUPG77.BAT to reflect the changes. The simplest commands are: CD \ UNZIP G77DOS.ZIP UNZIP G77DOC.ZIP To use the system you need to execute the very short batch file SETUPG77 which adds the \EMX\BIN directory to your path and defines an environment variable. 4. How to use the compiler. The command G77 will compile and link your program and produce an executable bound with the EMX DOS-extender (which is needed to get 32-bit memory access). There are various command options (see below) which need to be specified before the name of the program file(s). A simple demonstration program called MYTEST.F is included in the package. To compile this just use: G77 MYTEST.F This generates a file MYTEST.EXE, which can be executed simply using MYTEST It is good practice to get the compiler to warn you of unused and undeclared variables, the switch to do this is "-Wall" which can be used like this: G77 -Wall MYTEST.F Note that the switches are case-sensitive: -WALL will not work. The g77 compiler has a large number of other command switches - a few of the most useful are shown here: -c Compile-only: produces .OBJ files. -ffree-form Selects free-format source code -fpedantic Warns of non-portable/non-standard code. -fno-automatic Static storage for all variables, like universal SAVE -fno-backslash Interprets "\" as a normal character in strings -fvxt Use VAX Fortran interpretation of certain syntax -g Produces debugging information. -Idirectory Specifies directory to search for INCLUDE files -O Optimise code generation -Wimplicit Warns of any names with no explicit data type -Wuninitialised Warns of some cases of unset variables (if -O also set). -Wall Warns of both of above cases. Any number of source-code files can be used on the command-line, and wild-cards (*) are supported in file-names. The command-line can also contain compiled object modules (.OBJ) and library files (called "archives" in Unix-speak) which have .A as the extension. File-names are, of course, not case-sensitive in MS-DOS. The .EXE file produced by the compiler can only be run if a suitable DOS-extender (EMX.EXE or RSX.EXE) to be available on the search path. If you want to distribute your programs to other machines you may prefer to generate a stand-alone executable, although this will be larger. You can do this with the EMXBIND program by first extracting a version of the executable in "a.out" format, and then binding this with EMX. Assuming you have created a file MYPROG.EXE, the commands would be: EMXBIND -x MYPROG.EXE A.OUT EMXBIND -b -s EMX A.OUT MYPROG.EXE DEL A.OUT 5. Extensions to g77 compatible with Fortran90: The g77 compiler conforms fully only to the Fortran77 Standard. Although this is technically obsolete, the vast majority of Fortran programs in the world were written (more or less) to this Standard. The new standard, Fortran90, has many more advanced features. The g77 compiler already supports some features of Fortran90, the most notable of these being: IMPLICIT NONE (to flag non-explicit data types in a program unit). INCLUDE 'filename' Automatic arrays (a form of dynamic storage within subprograms). Free-format input (if the switch -ffree-form is used) Multiple statements on a line (with ";" as the separator). Symbolic names can be up to 31-characters long, can contain lower-case letters (equivalent to upper-case) and underscores. End-of-line comments may be used starting with ! (but ! must not be in column 6 with fixed-format source-code). Relational operators > >= < <= == /= can be used instead of .GT. etc. Character constants can use "double" or 'single' quotes. Program unit names permitted on END, e.g. END SUBROUTINE MYSUB. DO without labels and END DO are permitted, also indefinite DO (but a conditional EXIT or STOP is obviously needed in such loops). DO WHILE(logical expression) with END DO is permitted. EXIT and CYCLE are allowed in DO loops. SELECT CASE structure is supported but only with integer/logical selectors. Construct names are allowed with IF/DO/CYCLE/EXIT/SELECT CASE. Zero-length strings are valid. Substrings of character constants are permitted. Character intrinsic functions ACHAR, IACHAR, and LEN_TRIM are provided. Bit-wise integer functions BTEST, IAND, IBCLR, IBITS, IBSET, IEOR, IOR, ISHFT, ISHFT, MVBITS, NOT (the MIL-STD 1753 intrinsics) are provided. OPEN with STATUS='REPLACE' is supported. NAMELIST input/output is also supported. Type declarations may use KIND values (but this is of limited use because kind-selection functions are not yet provided). 6. Other g77 extensions (NOT compatible with Fortran90) Many extensions to the official Fortran77 Standard were introduced by companies which produced Fortran compilers for sale, but not all of these were incorporated into Fortran90. You may find that existing "Fortran77" code makes use of some of these non-standard features. Fortunately g77 supports some of the more common extensions, especially those of VAX Fortran. The most important ones are listed below. They make it possible to use "legacy" code with a minimum of alteration, but are NOT recommended if you are writing new code. Data types BYTE, INTEGER*1, INTEGER*2, INTEGER*4 (default), INTEGER*8, REAL*4 (default), REAL*8, DOUBLE COMPLEX, COMPLEX*8 (default), COMPLEX*16, LOGICAL*1, LOGICAL*2, LOGICAL*4 (default) are supported. DATA statements may be intermixed with specifications statements. Arguments of procedure calls may use %VAL, %REF, %LOC, %DESCR functions. Additional intrinsic functions: LOC, AND, OR, LSHIFT, RSHIFT, XOR, CDABS, CDCOS, CDEXP, CDLOG, CDSIN, CDSQRT, DCMPLX, DCONJG, DFLOAT, DIMAG, DREAL, IMAG, ZABS, ZCOS, ZEXP, ZLOG, ZSIN, and ZSQRT supported. Any number of continuations lines may be used. Symbolic names may include "$" if -fdollar-ok switch is specified.. Integer constants may be specified in other number bases: e.g. B'01', O'01234567', X'01EF', Z'FFFF' etc.; in addition "typeless" constants may be given in a similar form but with the letter following the string of digits. FORMAT specifications may include $ to suppress the final carriage-return. Debug lines starting "D" or "d" are treated as comments. 7. Notes on Input/Output I/O unit numbers must be in the range 0 to 99, with 0 and 6 pre-connected to the screen, 5 to the keyboard (but best to use UNIT=* for both). Unformatted direct-access files have bytes as units of record length. Output to the screen does not count as "printing" in Fortran terms, so the first character of each record is never removed for carriage-control. Output to unit N is sent to file "FORT.N" if no file was opened for it. 8. The Fortran Library G77 supports, even on MS-DOS, most of the routines commonly used to access system services on Unix. These include: ABORT, GETARG, IARGC, EXIT, CTIME, DATE, DTIME, ETIME, FDATE, GMTIME, LTIME, IDATE, ITIME, MCLOCK, SECNDS, SECOND, SYSTEM, TIME, ERF, DERF, ERFC, DERFC, IRAND, RAND, SRAND, ACCESS, CHDIR, CHMOD, GETCWD, FSTAT, SSTAT, LSTAT, RENAME, UMASK, UNLINK, GERROR, IERRNO, PERROR, GETENV, FGETC, GFET, FPUTC, FPUT, FLUSH, FNUM, FSEEK, FTELL, ISATTY, TTYNAM, SLEEP. Note that I have not checked all of these. HOSTNM and GETLOG are not supported, and SYSTEM (which executes an operating ...
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