3 Controlling data storage
This chapter describes the use of
•Disabling implicit typing
•Automatic and static variables
•Increasing the precision of constants
•Sharing data among programs
•Modules vs. common blocks
NOTE: For information about how HP Fortran aligns data, see HP Fortran Programmer's Reference.
Disabling implicit typing
By default, HP Fortran uses implicit typingto determine the type of a variable or function that has not been declared with a type declaration statement. That is, the type of an undeclared entity is determined by the first letter of its name: if the letter is in the rangeI - N , the entity is of type integer; otherwise, it is of type real.
Although implicit typing is mandated by the Standard, its use can become a source of runtime bugs because implicit typing allows the inadvertent use of undeclared variables or functions. For the sake of illustration, consider a program that calls a nonintrinsic library function named foo. Assume that:
•The default typing rules are in effect.
•fooreturns an integer.
•The programmer has not declared the return type offoo and has assigned its return value to a variable of type real.
Experience has shown that this is not an unlikely scenario and that it can produce unexpected results
The Standard provides the IMPLICIT NONEstatement to override implicit typing. But the IMPLICIT NONEstatement is limited in scope to the program unit in which it appears. To force explicit typing for all files specified on the command line, use the +implicit_noneoption. This option disables implicit typing; that is, all variables, arrays, named constants, function subprograms, ENTRY names, and statement functions (but not intrinsic functions) must be explicitly declared.
Using this option is equivalent to specifyingIMPLICIT NONE for each program unit in each file specified on the f90command line. However, the +implicit_noneoption does not override any IMPLICITstatements in the source file. The HP Fortran Programmer's Reference describes the implicit typing rules, the IMPLICIT NONE statement, and the +implicit_noneoption.
Automatic and static variables
By default, HP Fortran allocates stack storage for program variables. Such variables are called automatic variables because they are allocated at each invocation of the program unit in which they are declared.
Static variables are allocated storage from static memory when the program is first loaded into memory.
They remain allocated for the life of the program.
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