File Pointers

[ram_47]

Hi,

1) I declare 2 file pointers -
FILE *fp1, *fp2;

2) Using 'fp1' I open a file for reading in binary mode -
fp1=fopen("test","rb");

3) I use 'fp2' to point to same memory what fp1 is pointing to -
fp2=fp1;

4) Using 'fp2' I read a record from the file 'test' using a structure -
fread(&rec,sizeof(struct st),1,fp2);


Question -
What is the impact on pointer 'fp1' after step #4 is executed? Where will 'fp1' point to?


My Observation -
After step #4 is executed, 'fp2' should be at the end of 1st record that is read. and since 'fp1' is never used it should still be pointing to the beginning of the file. but it does not work that way. what i find is though 'fp1' is not used, it too is at the end of the 1st record along with 'fp2'.

i want 'fp1' to remain at the beginning of the file untill i position it. is this something that i could achieve? pl help.

[Kris Clippeleyr]

Hi Ram,

Since something of the type FILE * is a pointer to something (in this case an object of the opaque type FILE), it is only natural that if an operation changes something in that object, both pointers to the same object reflect those changes.

You might want to read on the functions fseek() and ftell(); these can be used to (re)position the read/write marker in a file.

Regards,
Kris (aka Qkcl)

[Antoniov. · Accepted solution]

Hi Ram,
if you want to keep two different file pointer on the same file you have to open file two times.

FILE *fp1, *fp2;
fp1=fopen("test","rb");
fp2=fopen("test","rb");

Antonio Vigliotti

[John Gillings]

fp1 and fp2 are pointers to the same data structure. Just as multiple pointers to the same variable will all "see" the same value, regardless of which pointer was used to set the value, both file pointers will always "see" the same view of the file.

Opening the file twice will do what you want. HOWEVER, you need to open the file with shared access. The two opens from the same process are seen in the same way as opens from multiple processes - hence shared access is required.

If you used RMS I/O you could OPEN the file once and CONNECT multiple streams. This would allow you to maintain multiple pointers into the file, but with exclusive access. Native C I/O doesn't have a way to access multiple RMS streams (BASIC and COBOL do).

[ram_47]

Am not quite familiar with RMS. Where could I find documentation on RMS, if possible with examples.

[David B Sneddon]

ram,

There are a number of examples in SYS$EXAMPLES showing various
features of RMS.

The documentation can be found by following the link at the
bottom of the main page for this forum.

http://h71000.www7.hp.com/doc/index.html

Dave

[David B Sneddon]

ram,

The RMS reference manual is at

http://h71000.www7.hp.com/doc/731FINAL/4523/4523PRO.HTML

which covers all the services and has various
examples, some in C.

Dave

[Antoniov.]

Hi
RMS programming is not simple and (in this case) can't help.
Neither fopen(), neither fread() of standard c-runtime doesn't lock record.
Using of RMS is useful to manage indexed files, not binary/stream flushes.
Full working example is attached.

Antonio Vigliotti

[ram_47]

How can I have a look at SYS$EXAMPLES. Am sorry but I am quite new to VMS.

[David B Sneddon]

$ dir sys$examples:*.c

will list all the C programs.

$ type/page sys$examples:alpha_logger.c

will list alpha_logger.c

You may find it easier to look at the examples
in the documentation.

Dave