Linux 的 pwrite() 有个 BUG: 当文件以 O_APPEND 模式打开时，虽然 file offset
不会变，但是写入的内容会追加到文件尾部。
此外，pwrite() 和 pwritev() 的 offset 参数不能为 -1, 否则会 EINVAL.
但是 pwritev2() 可以，表示使用当前的 file offset 并且会更新之。
man(2) pwrite
BUG
POSIX requires that opening a file with the O_APPEND flag should have
no effect on the location at which pwrite() writes data. However, on
Linux, if a file is opened with O_APPEND, pwrite() appends data to the
end of the file, regardless of the value of offset.
下面是个示例程序：
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
int main(int argc, char* argv[])
{
int flags, fd;
if (argc != 2) {
printf("usage: %s file\n", argv[0]);
return 1;
}
flags = O_RDWR | O_CREAT | O_TRUNC;
if (strstr(argv[1], "append") != NULL) {
flags |= O_APPEND;
}
fd = open(argv[1], flags, 0600);
if (fd < 0) {
perror("open");
return 1;
}
if (write(fd, "hello\n", 6) < 0) {
perror("write");
}
printf("offset: %ld\n", lseek(fd, 0, SEEK_CUR));
/* offset = -1 => EINVAL */
if (pwrite(fd, "world\n", 6, 0) < 0) {
perror("pwrite");
}
printf("offset: %ld\n", lseek(fd, 0, SEEK_CUR));
close(fd);
return 0;
}
运行结果：
$ ./a.out hello; cat hello
offset: 6
offset: 6
world
$ ./a.out append; cat append
offset: 6
offset: 6
hello
world