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 _______________________________________________ Redhat-devel-list mailing list Redhat-devel-list@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/redhat-devel-list