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Michael Schwendt <ms0301rh@xxxxxxxx> writes:
> On Thu, 10 Apr 2003 18:03:02 +0530, Ajay Bansal wrote:
>
> > Problem was not that I _have_ to read a file somehow. Problem was.. How
> > do read using fgets. & why it bbehaves differently on Solaris/Linux
> > with same compiler.
> >
> > If I open the file with "a+", the on Solaris file pointer is at the
> > beginning of the file whereas on Linux it is at the end of the file. Why
> > is this difference???
>
> 'a' = append, but appending makes sense only at the end of a file.
> 'a' without positioning the file pointer at the end of the file
> would be like 'w' (writing) and would NOT append, but overwrite
> (truncate) the file. 'a+' is like 'a' but also gives 'r'
> read-access.
>
> What does the manual on Solaris say about 'a' and 'a+'?
Here's what the (Draft) C99 standard has to say about it:
7.19.5.3 The fopen function
Synopsis
[#1]
#include <stdio.h>
FILE *fopen(const char * filename,
const char * mode);
Description
[#2] The fopen function opens the file whose name is the
string pointed to by filename, and associates a stream with
it.
[#3] The argument mode points to a string. If the string is
one of the following, the file is open in the indicated
mode. Otherwise, the behavior is undefined.212)
WG14/N843 Committee Draft -- August 3, 1998 305
r open text file for reading
w truncate to zero length or create text file for
writing
a append; open or create text file for writing at
end-of-file
rb open binary file for reading
wb truncate to zero length or create binary file for
writing
ab append; open or create binary file for writing at
end-of-file
r+ open text file for update (reading and writing)
w+ truncate to zero length or create text file for update
a+ append; open or create text file for update, writing
at end-of-file
r+b or rb+ open binary file for update (reading and writing)
w+b or wb+ truncate to zero length or create binary file for
update
a+b or ab+ append; open or create binary file for update, writing
at end-of-file
[#4] Opening a file with read mode ('r' as the first
character in the mode argument) fails if the file does not
exist or cannot be read.
[#5] Opening a file with append mode ('a' as the first
character in the mode argument) causes all subsequent writes
to the file to be forced to the then current end-of-file,
regardless of intervening calls to the fseek function. In
some implementations, opening a binary file with append mode
('b' as the second or third character in the above list of
mode argument values) may initially position the file
position indicator for the stream beyond the last data
written, because of null character padding.
[#6] When a file is opened with update mode ('+' as the
second or third character in the above list of mode argument
values), both input and output may be performed on the
associated stream. However, output shall not be directly
followed by input without an intervening call to the fflush
function or to a file positioning function (fseek, fsetpos,
or rewind), and input shall not be directly followed by
output without an intervening call to a file positioning
function, unless the input operation encounters end-of-file.
Opening (or creating) a text file with update mode may
instead open (or create) a binary stream in some
implementations.
[#7] When opened, a stream is fully buffered if and only if
it can be determined not to refer to an interactive device.
The error and end-of-file indicators for the stream are
cleared.
Returns
____________________
212If the string begins with one of the above sequences, the
implementation might choose to ignore the remaining
characters, or it might use them to select different
kinds of a file (some of which might not conform to the
properties in 7.19.2).
7.19.5.3 Library 7.19.5.3
306 Committee Draft -- August 3, 1998 WG14/N843
[#8] The fopen function returns a pointer to the object
controlling the stream. If the open operation fails, fopen
returns a null pointer.
IOW, it doesn't say. It says only that writes are to the end of the file.
It says nothing about reads.
> > And if some code is already written using fgets on solaris, this
> > difference makes it non-portable!!!!!
>
> Remains to be seen whether the API is non-portable or whether the
> source code is.
The source code isn't portable. Portable code must use fseek(), fsetpos(),
or rewind() to position the file for reading.
--
tim writer <tim@xxxxxxxxxxx> starnix inc.
tollfree: 1-87-pro-linux thornhill, ontario, canada
http://www.starnix.com professional linux services & products
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