fopen() behavior: File positioning in append mode

Stefan Heinzmann
Fri Feb 3 10:59:00 GMT 2017


I am unsure what the behavior of fopen() should be when the "a+" mode is 
given. The source for this is in file newlib/libc/stdio/fopen.c

Mode a+ translates to open() flags O_RDWR|O_CREAT|O_APPEND according to 
the POSIX standard. This would set the file position to the beginning of 
the file. The file position would be relevant only to read operations, 
since the O_APPEND flag causes writes to happen at the file end in any case.

However, fopen() seems to set the file position to the end in this case, 
by calling fseek() explicitly. The consequence is that read operations 
also happen at the file end. I don't understand why this is done. 
Wouldn't it be more useful to omit the call to fseek(), or to restrict 
it to the write-only case?

The C standard doesn't seem to be clear here. Does it actually specify 
what the result of ftell() should be immediately after calling fopen()? 
I seem to be unable to find conclusive text.


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