Problems getting ftell() in binary append

Question

I've got some code that was working on a server until it was moved to a different system. The problem seems to be here:

given a structure I've defined elsewhere:

user1_type user1;              /* structure containing user data */
user1_type *user1_ptr=&user1;

this routine appends the record to the end of the file

if ((dfile=fopen(filename,"ab+"))==NULL)  
  error_message("Unable to open file for append.",filename,1);
 else { /* append data */
 user1.recid=ftell(dfile);               /* update file position */

 fwrite(user1_ptr,sizeof(user1),1,dfile);
 fflush(dfile);
 fclose(dfile);

I can confirm the data gets appended in the file, but the value of user1.recid always returns 0 - any ideas why?

UPDATE: Looks like the issue is not all implementations require a fseek() after fopen(). I obviously need an fseek(dfile,0,SEEK_END); before I do a ftell() when appending. But if I want to read from the beginning of a text or binary file, is it also customary to place a fseek() right after fopen? Does it vary depending upon the file type?

typedef struct { long recid; char dataid[5]; /* USER1 */ .. } user1_type; — Trent Three
From the docs for ftell() that I use: "Note that when a file is opened for appending data, the current file position is determined by the last I/O operation, not by where the next write would occur." You must call fseek() first. — Hans Passant
apparently on some implementations the file pos for a+b is set at EOF without a seek being needed (FreeBSD) — Trent Three

Biruk Abebe Biruk Abebe · Accepted Answer · 2016-04-27T17:03:47

From MSDN's documentation of ftell

The position returned by ftell() is expressed as an offset relative to the beginning of the stream

If no I/O operation has yet occurred on a file opened for appending, the file position is the beginning of the file.

This gives you an offset of 0 relative to the beginning.

So when you call user1.recid=ftell(dfile); no I/O operation has occurred on the stream yet so ftell() returns 0 indicating the file pointer position is at the begining.

Problems getting ftell() in binary append

3 Answers