You can control the libraries to be searched with these methods:

• To specify additional object library file names for ld to search, use the -l string option at the end of the f90 command line. Each occurrence of the -l string option specifies an additional file name that is added to the list of object libraries for ld to locate.
  Table 2-4 shows the standard f90 library file names that are searched by ld .

  Table 2-4 Libraries Automatically Searched When Using the f90 Command

  File Name	-lstring Option Form
  libufor	-lufor
  libfor	-lfor
  libfutil	-lfutil
  libshpf , libphpf , or libphpfp (see text)	-lshpf , -lphpf , or -lphpfp (see text)
  libm	-lm
  libots	-lots
  libc	-lc

  
  If you specify -omp or -mp (requests directed parallel processing), the libots3 library is searched.
  Which High Performance Fortran (HPF) library is searched depends on which f90 options are specified:

  • If you specify -pprof and specify -wsf (or -wsf 2 or greater), libphpfp is searched.
  • If you omit -pprof and specify -wsf (or -wsf 2 or greater), libphpf is searched.
  • If you omit -wsf or specify -wsf 1 , libshpf is searched.
  
  The DIGITAL Fortran 90 kit provides both shared and archive libraries. For a complete list of DIGITAL Fortran 90 Run-Time Library files, see f90(1).
• In addition to the standard directories in which ld tries to locate the library file names, you can use the -l dir option to specify another directory.
  Unlike the -l string option that adds an object library file name for ld to search, the -l dir option adds an additional directory in which ld will look for library file names.
  The standard ld directories are searched before directories specified by the -l dir option.
  The following example specifies the additional object library path /usr/lib/mytest :

  % f90 simtest.f -L/usr/lib/mytest

  
  For a list of the standard ld directories searched, see Section 3.48.

• You can specify the pathname and file name of an object library as you would specify any file. Specifying each object library that resides in special directories in this manner is an alternative to specifying the library by using the -l string or -l dir option. Specifying the pathname and file name of an object library can reduce the amount of searching the linker must do to locate all the needed object files.
  For instance, instead of specifying the options -l/usr/jones and -lfft , this example specifies the directory path and file name of the library file:

  % f90 main.o more.o rest.o /usr/jones/libfft.a

  
  In certain cases, you may need to specify the pathname and file name instead of using the -l string or -l dir options for the linker to resolve global symbols with shared libraries.

• You can indicate that ld should not search its list of standard directories at all by specifying the -l option. When you do so, you must specify all libraries on the f90 command line in some form, including the directory for f90 standard libraries.
  To specify all libraries, you might use the -l option in combination with the -l dir option on the same f90 command line.

External references found in an archive library result in the referenced routine being included in the resulting executable program file at link time.

External references found in a shared object library result in a special link to that library being included in the resulting executable program file, instead of the actual routine itself. When you run the program, this link gets resolved by either using the shared library in memory (if it already exists) or loading it into memory from disk.

Certain f90 options influence whether ld searches for an archive ( .a ) or shared object ( .so ) library on the standard list of f90 libraries, as well as any additional libraries specified by using the -l string or -l dir options:

• The -call_shared option is the default. It indicates that .so files are searched before .a files. As ld attempts to resolve external symbols, it looks at the shared library before the corresponding archive library. For instance, /usr/shlib/libc.so is searched before /usr/lib/libc.a .
  References to symbols found in .so libraries are dynamically loaded into memory at run time.
  References to symbols found in .a libraries are loaded into the executable program file at link time.
• The -non_shared option indicates that only .a files are searched, so the object file created contains static references to external routines. These static references are loaded into the executable program at link time, not at run time. Corresponding .so files are not searched.
  The following example requests that the standard f90 .a files be searched instead of the corresponding .so files:

  % f90 -non_shared main.f rest.o

• The -shared option requests the creation of an executable program to be included in a shared library.
  • If you specify the -c option to inhibit linking, an object file ( .o file) is created that can subsequently be processed by ld to create a shared library.
  • If you omit the -c option, the f90 command creates a shared library ( .so file).
  
  In either case, use the -o option to name the resulting object file or shared library with the correct file name and suffix.
  For more information about creating a shared library using either the f90 command or the ld command, see Section 2.6.

When you link your program with a shared library, all symbols must be referenced before ld searches the shared library. You should always specify libraries at the end of the f90 command line, after all file names. Unless you specify the -non_shared option, shared libraries will be searched before the corresponding archive libraries.

For instance, the following command generates an error if the file rest.o references routines in the library libx :

% f90 -call_shared test.f -lX rest.o

The correct command specifies the library at the end of the line, as follows:

% f90 -call_shared test.f rest.o -lX

Link errors can occur with symbols that are defined twice, as when both an archive and a shared object library are specified on the same command line. In general, specify any archive libraries after the last file name, followed by any shared libraries at the end of the command line.

Before you reference a shared library at run time, it must be installed. For further information on creating or installing a shared library, see Section 2.6.

2.6 Creating Shared Libraries

To create a shared library from a Fortran source file, process the files using the f90 command:

• You must specify the -shared option to create the .so file.
• You can specify the -o output option to name the output file.
• If you omit the -c option, you will create a shared library ( .so file) directly from the f90 command line in a single step.
  If you also omit the -o output option, the file name of the first Fortran file on the command line is used to create the file name of the .so file. You can specify additional options associated with shared library creation.
• If you specify the -c option, you will create an object file ( .o file) that you can name with the -o option. To create a shared library, process the .o file with ld , specifying certain options associated with shared library creation.

You can specify multiple source and object files when creating a shared library by using the f90 command.

2.6.1 Creating a Shared Library with a Single f90 Command

You can create a shared library ( .so ) file with a single f90 command:

% f90 -shared octagon.f90

The -shared option is required to create a shared library. The name of the source file is octagon.f90 . You can specify multiple source files and object files.

The -o option was omitted, so the name of the shared library file is octagon.so .

2.6.2 Creating a Shared Library with f90 and ld Commands

You first must create the .o file, such as octagon.o in the following example:

% f90 -O3 -c octagon.f90

The file octagon.o is then used as input to the ld command to create the shared library named octagon.so :

% ld -shared -no_archive octagon.o \ -lUfor -lfor -lFutil -lm -lots -lc

• The -shared option is required to create a shared library.
• The -no_archive option indicates that ld should not search archive libraries to resolve external names (only shared libraries).
• The name of the object file is octagon.o . You can specify multiple object ( .o ) files.
• The -lufor and subsequent options are the standard list of libraries that the f90 command would have otherwise passed to ld . When you create a shared library, all symbols must be resolved. For more information about the standard list of libraries used by DIGITAL Fortran 90, see Section 2.5.

Consider the following when deciding whether to use a single f90 command ( -c omitted) or both the f90 ( -c present) and ld commands to create a shared library:

• Certain ld options may not be available from the f90 command line. If you need to use those options, use the two-command method (specify f90 -c and subsequently use ld ). Such options include -check_registry and -update_registry (see ld(1)).
• If you use a single f90 command with -shared and omit -c , you do not need to specify the standard list of f90 libraries by using the -lstring option.

In addition to the options shown in Section 2.6.1 and Section 2.6.2, certain other ld options may be needed. For instance, to optimize shared library startup, use the -update_registry and -check_registry options, which preassigns a starting address in virtual memory to a shared library using the file /usr/shlib/so_locations .

For additional information on the relevant ld options, see the ld(1) reference page.

For more information about the standard list of libraries used by DIGITAL Fortran 90, see Section 2.5.

2.6.4 Shared Library Restrictions

When creating a shared library with ld , be aware of the following restrictions:

• Shared libraries must not be linked with archive libraries.
  When creating a shared library, you can only depend on other shared libraries for resolving external references. If you need to reference a routine that currently resides in an archive library, either put that routine in a separate shared library or include it in the shared library being created. You can specify multiple object ( .o ) files when creating a shared library.
  To put a routine in a separate shared library, obtain the source or object file for that routine, recompile if necessary, and create a separate shared library. You can specify an object file when recompiling with the f90 command or when creating the shared library with the ld command.
  To include a routine in the shared library being created, put the routine (source or object file) with other source files that make up the shared library and recompile if necessary.
  Then create the shared library, making sure that you specify the file containing that routine either during recompilation or when creating the shared library. You can specify an object file when recompiling with the f90 command or when creating the shared library with the ld command.
• When creating shared libraries, all symbols must be defined (resolved).
  Because all symbols must be defined to ld when you create a shared library, you must specify the shared libraries on the ld command line, including all standard DIGITAL Fortran 90 libraries. The list of standard DIGITAL Fortran 90 libraries might be specified by using the -lstring option, as in the previous example in this section.

For other restrictions imposed by the operating system, see the DIGITAL UNIX Programmer's Guide.

2.6.5 Installing Shared Libraries

Once the shared library is created, it must be installed for private or system-wide use before you run a program that refers to it:

• To install a private shared library (when you are testing, for example), set the environment variable LD_LIBRARY_PATH, as described in loader(5).
• To install a system-wide shared library, place the shared library file in one of the standard directory paths used by ld (see loader(5) or Section 3.48).

For complete information on installing shared libraries, see the DIGITAL UNIX Programmer's Guide.