Whats the difference between between save, static, global, local and parameter variables?<\/p>\n
Save and static are two words for the same thing<\/strong>. Within a function or subroutine subprogram, an entity specified by a SAVE statement does not become undefined as a result of the execution of a RETURN or END statement in the subprogram<\/p><\/blockquote>\n 1.00000000 You can count how often a subroutine is called. If you ever want to do this... I can not recommend to do it in this way. 101.000000 The save<\/strong> specifier is not needed by using the data<\/strong> keyword. The greatest pitfall with static<\/strong> variables is this:<\/p>\n If you initialize a variable by the declaration, it become a static<\/strong> variable! That really really hurts. You want be a good programer and always zero your variables and then you crash the hole program :D<\/p>\n Lets talk about parameters<\/strong>. Many programmers declare they constans with the save<\/strong> attribute. All fine ;)<\/p>\n If you use parameter<\/strong> instead of save<\/strong> the compiler can detect this issue.<\/p>\n PI = 4 Second: Fortran is a high performance language. You can easily vectorize<\/strong> assigments using the forall<\/strong> constrcut. forall<\/strong>( i = 1:100 ) A(i) = func() And a pure<\/strong> function can not have static variables<\/p>\n real, save :: PI = 3.14 Using parameter<\/strong> solves the issue<\/p>\n $ ifort -O2 -c save.f90 -vec-report3
\nI found a nice definition of what save do in the Fortran 77 Standard [1] <\/p>\n\r\nsubroutine func()\r\n implicit none\r\n real, save<\/strong> :: time\r\n time = time + 1\r\n write(*,*) time\r\nend subroutine\r\n\r\nprogram test\r\n implicit none\r\n integer :: i\r\n do i = 1, 10\r\n call func()\r\n end do\r\nend\r\n<\/code><\/pre>\n
\n 2.00000000
\n 3.00000000
\n 4.00000000
\n 5.00000000
\n 6.00000000
\n 7.00000000
\n 8.00000000
\n 9.00000000
\n 10.0000000 <\/p><\/blockquote>\n
\nAs you may have noticed, I have not initialized the time variable with an start value.
\nYou can do this with the data<\/strong> instruction.<\/p>\n\r\nsubroutine func()\r\n implicit none\r\n real :: time\r\n data<\/strong> time\/100\/ \r\n time = time + 1\r\n write(*,*) time\r\nend subroutine\r\n<\/code><\/pre>\n
\n 102.000000
\n 103.000000
\n 104.000000
\n 105.000000
\n 106.000000
\n 107.000000
\n 108.000000
\n 109.000000
\n 110.000000 <\/p><\/blockquote>\n
\nThis may result in confusion, because not everybody knows exactly what the data<\/strong> specifier does.
\nAgain: do not use it.<\/p>\n\r\nreal :: time = 0\r\n<\/code><\/pre>\n
\nI can not recommend this.
\nFirst: Constants does not change. Variables with save<\/strong> attribute can change. <\/p>\n\r\nreal, save<\/strong> :: PI = 3.14...\r\nPI = 4\r\n<\/code><\/pre>\n\r\nreal, parameter<\/strong> :: PI = 3.14...\r\nPI = 4\r\n<\/code><\/pre>\n
\n 1
\nError: Named constant 'pi' in variable definition context (assignment) at (1)<\/p><\/blockquote>\n
\nAll functions called from the forall constrcut must be declared as pure<\/strong>. to ensure that they have no side effects.<\/p>\n\r\n function func() result(val)\r\n implicit none\r\n real, save<\/strong> :: PI = 3.14\r\n real :: val\r\n val = PI\r\n end function\r\n\r\nforall( i = 1:100 ) A(i) = func()\r\n<\/code><\/pre>\n
\n 1
\nError: Reference to non-PURE function 'func' at (1) inside a FORALL block\n<\/p><\/blockquote>\n\r\n pure<\/strong> function func() result(val)\r\n implicit none\r\n real, save<\/strong> :: PI = 3.14\r\n real :: val\r\n val = PI\r\n end function\r\n<\/code><\/pre>\n
\n 1
\nError: SAVE attribute at (1) cannot be specified in a PURE procedure\n<\/p><\/blockquote>\n\r\n pure<\/strong> function func() result(val)\r\n implicit none\r\n real, parameter<\/strong> :: PI = 3.14\r\n real :: val\r\n val = PI\r\n end function\r\n\r\nforall<\/strong>( i = 1:100 ) A(i) = func()\r\n<\/code><\/pre>\n
\nsave.f90(19): (col. 25) remark: LOOP WAS VECTORIZED<\/strong>\n<\/p><\/blockquote>\n