The following f90 command compiles the free-format DIGITAL Fortran 90 source files circle-calc.f90 and sub.f90 together using software pipelining optimizations ( -o5 ):

% f90 -O5 -unroll 3 circle-calc.f90 sub.f90

The loops within the program are unrolled 3 times ( -unroll 3 ). Loop unrolling occurs at optimization level -o3 or above.

2.1.9 Using Listing Files

If you expect your program to have compilation errors, you should request a separate listing file ( -v option).

For example, the following command compiles DIGITAL Fortran 90 source files with file names that end with .f , and ld creates an executable file named a.out :

% f90 -V *.f

The listing file assumes the name of the first file. If the first file was named aaa.f , the listing file is named aaa.l .

Using a listing file provides such information as the column pointer (1) that indicates the exact part of the line that caused the error (see Section 2.3.2). Especially for large files, consider obtaining a printed copy of the listing file you can reference while editing the source file.

For More Information:

• On calling programs written in other languages, see Chapter 11.
• On options for the f90 command, see Chapter 3.
• On f90 command options and their categories, see Table 3-1.

The f90 driver program passes options and files not intended for the DIGITAL Fortran 90 compiler to the cc driver program.

2.2.1 f90 Driver Program Interaction with cc and ld

The f90 driver program controls which software components operate on the files and options specified on the f90 command line and their order of use. After preprocessing and compilation by the DIGITAL Fortran 90 compiler, certain files or options are passed to the cc driver program. The cc driver:

1. Runs cpp
2. Runs the C compiler cc
3. Passes library-related information to and runs the ld linker

Figure 2-1 shows the sequence and use of related software components for an example f90 command.

Figure 2-1 f90 Driver Program and Software Components

The f90 driver does the following:

1. Examines file name suffix information and groups files and options specified on the f90 command line.
2. Runs the requested preprocessors (if any).
3. Runs the DIGITAL Fortran 90 compiler to process Fortran files.
4. Passes grouped input files, processed source files, and grouped options to the cc driver in the following order:
   1. All options, except for any -l string options, are parsed and passed to the DIGITAL Fortran 90 compiler, the cc compiler, or the cpp preprocessor (if you specified -cpp on the command line).
   2. All object files are grouped and passed to cc .
   3. All non-Fortran source files are grouped and passed to cc (such as a C program with a .c suffix).
   4. All archive libraries ( .a suffix) are grouped in the same order specified on the f90 command line and passed to cc .
   5. All shared libraries ( .so suffix) are grouped in the same order specified on the f90 command line and passed to cc .
   6. All user-specified -l string options are grouped in the same order specified on the f90 command line and passed to cc .
   7. All -l string options automatically added by the f90 command are grouped with other information and passed to cc .

Upon return to the command line, the f90 driver program returns one of the following status values:

• 0 (zero)--success
• 1--failure
• 2--subprocess failure (preprocessor, the decfort90 compiler, or ld )
• 3--signal

Because the f90 driver runs other software components, error messages may be returned by these other components. For instance, ld may return a message if it cannot resolve a global reference. Using the -v option on the f90 command line can help clarify which component is generating the error.

2.2.2 make Facility

The make facility is often used to automate building large programs. You can also use the integrated development environment and windowing interface provided by the DEC FUSE optional product, which provides a builder facility that uses make .

For More Information:

• On DEC FUSE, see the DEC Fuse Handbook.
• On make , see make(1).

Certain options are passed directly from the f90 command driver to the cc compiler driver. These options do not generally apply to compiling DIGITAL Fortran 90 source files, but might be used to:

• Process C language source files or intermediate files using the f90 command instead of the cc command
• Pass information to ld

With the -w c[c...],arg1[,arg2]... option, you can pass ld options not otherwise provided by the f90 command directly to ld .

When compiling a program that contains both DIGITAL Fortran 90 and C language source files, you can usually compile with a single f90 command. Any options that f90 does not recognize are passed to cc , such as the following:

• Compilation environment options, such as -systype name and -y environment
• Preprocessor ( cpp ) options, such as -m
• C language compatibility option, such as -migrate
• Memory location (performance) options, such as -d num, -t num, and -taso
• Other options listed in cc(1)

Certain options recognized and used by f90 also apply to cc , such as the -on option. If needed, you can compile the C files using the cc command (instead of the f90 command) with the -c option, and then compile the DIGITAL Fortran 90 files and the (C language) object files using the f90 command.

For more information on the options processed by cc , see cc(1) (for most options) or ld(1).

2.3 Compiler Limits, Compiler Messages, and Linker Messages

The following sections discuss the compiler limits and error messages from the compiler and linker. Other components can report messages, as described in Section 2.2.1.

2.3.1 Compiler Limits

Table 2-3 lists the limits to the size and complexity of a single DIGITAL Fortran 90 program unit and to individual statements contained within it.

Table 2-3 Compiler Limits

Language Element	Limit
Actual number of arguments per CALL or function reference	Limited only by memory constraints.
Arguments in a function reference in a specification expression	255
Array dimensions	7
Array elements per dimension	9,223,372,036,854,775,807 1
Constants; character and Hollerith	2000 characters
Constants; characters read in list-directed I/O	2048 characters
Continuation lines	511
DO and block IF statement nesting (combined)	128
DO loop index variable	9,223,372,036,854,775,807 1
Format group nesting	8
Fortran source line length	132 characters
INCLUDE file nesting	10 levels
Labels in computed or assigned GOTO list	500
Lexical tokens per statement	3000
Named common blocks	250
Parentheses nesting in expressions	40
Structure nesting	20
Symbolic-name length	31 characters

The following are usually limited by the amount of process virtual address space available, as determined by system parameters:

• The amount of data storage
• The size of arrays
• The total size of executable programs

For information on increasing your limits, see Section 1.1.

2.3.2 Compiler Diagnostic Messages and Error Conditions

The DIGITAL Fortran 90 compiler identifies syntax errors and violations of language rules in the source program. If the compiler finds any such errors, it writes messages to the stderr output file and any listing file. If you enter the f90 command interactively, the messages are displayed on your terminal.

Compiler messages have the following format:

f90: severity: filename, line n, message-text [text-in-error] --------^

The pointer ( ---^ ) indicates the exact place on the source program line where the error was found. For example, the following error message shows the format and message text in a listing file when an END DO statement was omitted:

f90: Severe: echar.f, line 7: Unclosed DO loop or IF block DO I=1,5 --------^

Diagnostic messages usually provide enough information for you to determine the cause of an error and correct it.

2.3.3 Linker Diagnostic Messages and Error Conditions

If the linker detects any errors while linking object files, it displays messages about their cause and severity. If any errors occur, the linker does not produce an executable program file.

Linker messages are descriptive, and you do not normally need additional information to determine the specific error. The general format for ld messages follows:

ld: message-text

The message-text may be on multiple lines and is sometimes accompanied by an f90 error.

Some common errors that occur during linking resemble the following:

• An object file has compilation errors. This error occurs when you attempt to link a file that had warnings or errors during compilation. Although you can usually link compiled files for which the compiler generated messages, you should verify that the files will actually produce the output you expect.
• The files being linked define more than one transfer address. The linker generates a warning if more than one main program has been defined. For example, this can occur when an extra END statement exists in the program. The executable program file created by the linker in this case can be run; the entry point to which control is transferred is the first one that the linker found.
• A reference to a symbol name remains unresolved. This error occurs when you omit required file or library names from the f90 or ld command and the linker cannot locate the definition for a specified global symbol reference.

If an error occurs when you link files, you may be able to correct it by retyping the command string and specifying the correct routines or libraries ( -l string option, -l dir option), or by specifying the object library or object files on the command line.

For More Information:

• On specifying object library or object files on the command line and related information on linking, see Section 2.5.
• On linker messages, see your operating system documentation.
• On run-time messages, see Chapter 8.

In addition to option arguments on the f90 command, several statements used in the body of a Fortran program and cpp directives also influence the compilation process.

• The INCLUDE statement incorporates external source code into your programs during the compilation process.
  If you omit the directory path from the file specification in the INCLUDE statement, the directory searched depends on whether the -vms option was specified or omitted. For more information, see Section 2.1.5.
• The USE statement incorporates a post-compiled module file into your programs during the compilation process. For more information, see Section 2.1.4.
• The OPTIONS statement sets compiler options that would otherwise be specified on the f90 command. The options specified on an OPTIONS statement take precedence over options specified on the f90 command line if a conflict occurs. Each OPTIONS statement applies only to the program unit in which it appears.
• You can use the general cDEC$ directives (such as !DEC$) to perform tasks during compilation. You can specify the following, for example:
  • The identification field in the object file
  • An alternate name (alias) for external objects, such as subprograms
  • Alternate ways of passing arguments and naming for external objects
  • Common block attributes
  • The -align attributes for record structures and common blocks
  • The title and subtitle for listings
  • Properties for data objects and procedures
  
  cDEC$ directives take precedence over the f90 command options.
• A # character in column 1 indicates a cpp directive when processing if the -cpp option is specified. You can specify that only preprocessing (no compilation) occurs by specifying the -p option, perhaps for debugging purposes.

For More Information:

• On DIGITAL Fortran 90 statements, including INCLUDE, OPTIONS, and USE, see the DIGITAL Fortran Language Reference Manual.
• cDEC$ directives related to passing arguments and external names, see Section 11.2.
• On cpp and its directives, see cpp(1).

Within a Fortran 90 program, references to procedures defined outside your program unit need to be declared as external symbols by using the EXTERNAL statement. (In order for BLOCK DATA statement symbols to be resolved by the linker, the BLOCK DATA symbol must be declared EXTERNAL and not have its data type declared in the source program unit.)

During compilation of multiple source files that will be placed into a single object or executable file, the DIGITAL Fortran 90 compiler resolves those symbols referenced in one compilation unit and defined in another before linking occurs.

Upon successful compilation of a DIGITAL Fortran 90 program, the f90 command specifies certain libraries for ld to search for unresolved external symbols in the object file (or files), such as calls to routines in libraries.

You can also specify additional object libraries on the f90 command line by using certain options or by providing the file name of the library. These object libraries are also searched by ld for unresolved external references.

When f90 specifies certain libraries to ld , it provides a standard list of f90 library file names to ld . The ld linker tries to locate each of these library file names in a standard list of library directories; ld attempts to locate each object library file name first in one directory, then in the second, and then in the third directory on its search list of directories.

To display a list of the compilers invoked, files processed, and libraries accessed during linking, specify the -v option on the f90 command line.

In addition to an object file created by the compiler, any linker options and object files specified on the f90 command are also passed to the ld linker. The linker loads object files according to the order in which they are specified on the command line. Because of this, you must specify object libraries after all source and object files on the f90 command line.

For more details on the interaction of the f90 command with other components, see Section 2.2.1.

To help you identify undefined references to routines or other symbols in an object file, consider using the nm command. For instance, the following nm command filtered by the grep command lists all undefined (U) symbols:

% nm -o ex.o | grep U

If the symbol is undefined, a "U" appears in the column before the symbol name. Any symbols with a U in their names are also displayed by this use of grep .