保存文件位置并在 Fortran 90 中返回
我正在编辑 fortran 90 来读取文件。一个特定的文件碰巧被一些额外的信息“污染”，所以我想尝试读取然后倒回；如果错误，请重读：
open(filenum,file=filename,form="unformatted",iostat=ierr) //'direct' access
...lots of stuff...
here = ftell(filenum)
read(filenum,iostat=ierr) var1, var2 //try reading as var1, var2
if(iswrong(var1, var2)) then //check if correct
call fseek(filenum,here-ftell(filenum),1) //rewind
read(filenum,iostat=ierr) vara, varb, varc //read as different type
endif
但是，当我编译此程序时，我得到
Undefined symbols:
"_fseek_", referenced from:
___myreader__subroutine_name in myreader.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
（我正在尝试在 gfortran (i686-apple-darwin8-gfortran-4.2) 上编译）。据我了解，fseek 不是标准的 Fortran 例程。
我想知道是否还有其他选择。我知道我可以做类似 read(filenum,rec=somevalue) 的事情，但是我如何以类似的方式使用它？我还考虑尝试使用 read(,advance='no') 进行读取以进行测试，然后使用 advance='yes' 再次读取（如果格式正确），但这需要特定的格式表达，这个没有指定。谢谢。
I am editing a fortran 90 to read a file. A particular file happens to be 'contaminated' with some extra information, so I wanted to attempt a read and then rewind; reread if in error:
open(filenum,file=filename,form="unformatted",iostat=ierr) //'direct' access
...lots of stuff...
here = ftell(filenum)
read(filenum,iostat=ierr) var1, var2 //try reading as var1, var2
if(iswrong(var1, var2)) then //check if correct
call fseek(filenum,here-ftell(filenum),1) //rewind
read(filenum,iostat=ierr) vara, varb, varc //read as different type
endif
However, when I compile this program, I get
Undefined symbols:
"_fseek_", referenced from:
___myreader__subroutine_name in myreader.o
ld: symbol(s) not found
collect2: ld returned 1 exit status
(I am trying to compile on gfortran (i686-apple-darwin8-gfortran-4.2)). I understand that fseek is not a standard fortran routine.
I wonder if there is an alternative. I understand I can do something like read(filenum,rec=somevalue) but how can I use this in a similar fashion? I also thought to attempt reading with read(,advance='no') for testing and then reading again with advance='yes' if it is in the correct format, but this requires a specific format expression, which is not specified. Thank you.
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您必须使用退格键返回到上一条记录。传统的 Fortran IO 是基于记录的，而不是基于流的（如 C 语言）
You have to use backspace to go back to the previous record. Traditional Fortran IO is record based, non stream based (like in C)
在 Linux 和 Windows 上，通常计算寻道偏移量是可以的。
然而，在面向记录的文件系统（OpenVMS、OS/370、NOS 等）上，来自
ftell()的值是一个神奇的 cookie，无法使用简单的算术来检查或处理计算新的文件位置。相反，请使用rewind语句返回到文件的开头（如果确实可查找）。On Linux and Windows, it is okay in general to compute seek offsets.
However, on record-oriented file systems (OpenVMS, OS/370, NOS, etc.), the value which comes from
ftell()is a magic cookie and cannot be inspected or processed using trivial arithmetic to compute a new file location. Instead, use therewindstatement to go back to the beginning of a file (if it is indeed seekable).如果您不确定文件内容（在这种情况下有时文件内容不正确或不同），还有另一种处理输入的技术，即读入字符串，然后检查字符串的内容。如果内容有效，您可以使用读取语句来读取字符串。或者，如果内容“不同”，请使用不同的读取语句，或采取适当的操作。无需倒带或退格。您必须猜测最长的可能行长度才能声明字符串的长度。
There is another technique for processing input if you are uncertain about the file contents -- in this case sometimes the files contents are incorrect or different -- which is to read into a string, then examine the contents of the string. If the contents are valid you can use a read statement to read from the string. Or if the contents are "different", use a different read statement, or take appropriate action. No rewind or backspace is necessary. You do have to guess the longest possible line length to declare the length of the string.