"suif.liyuan" <Suif.liy...@gmail.com> wrote in message
news:1182502328.527667.240820@e9g2000prf.googlegroups.com
On 6 19 , 1 34 , "suif.liyuan" <Suif.liy...@gmail.com> wrote:
Env: winxp , ms vc++6.0, save string"1111111111111111" into 1.txt in
DiskC
#include <stdio.h>
void main()
{
byte byBuffer[1024];
FILE *fp = fopen("C:\\1.txt", "r+");
//check error skip!
fread(byBuffer, 4, 1, fp);
fwrite("hello", sizeof("hello")-1, 1, fp);
fflush();
fclose(fp);
}
result: write failed! need to insert fseek(Current), a logic bug?
this code does work well in linux, I read source in MS VC crt, logic
in the code does not right thing!
The bug is in your code. It may work on Linux, but only does so by
accident. The POSIX standard says the same thing MSVC documentation does
(which I pointed out earlier, but you chose to ignore):
http://www.opengroup.org/onlinepubs/009695399/functions/fopen.html
When a file is opened with update mode ( '+' as the second or third
character in the mode argument), both input and output may be performed
on the associated stream. However, the application shall ensure that
output is not directly followed by input without an intervening call to
fflush() or to a file positioning function ( fseek(), fsetpos(), or
rewind()), and input is not directly followed by output without an
intervening call to a file positioning function, unless the input
operation encounters end-of-file.
So does Linux man page on fopen(3):
http://www.die.net/doc/linux/man/man3/fopen.3.html
Reads and writes may be intermixed on read/write streams in any order.
Note that ANSI C requires that a file positioning function intervene
between output and input, unless an input operation encounters
end-of-file. (If this condition is not met, then a read is allowed to
return the result of writes other than the most recent.) Therefore it is
good practice (and indeed sometimes necessary under Linux) to put an
fseek() or fgetpos() operation between write and read operations on such
a stream. This operation may be an apparent no-op (as in fseek(..., 0L,
SEEK_CUR) called for its synchronizing side effect.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925- -
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