问题描述
我写了这个小code，以确定读的行为。
I wrote this small code to ascertain read behavior.
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <limits.h>
#include <errno.h>
int main ()
{
ssize_t ret;
int fd;
char str[30] = {0};
off_t lret
fd = open("./sample", O_RDWR);
printf("File descriptor = %d\n",fd);
lret = lseek(fd,LONG_MAX,SEEK_SET);
printf("%ld\n",lseek(fd, 0, SEEK_CUR));
ret = read(fd, str, 20);
if (ret == -1) {
perror("read error");
}
else {
printf("%ld\n",ret);
printf("%s\n",str);
}
ret = write(fd, "bye", 3);
if (ret == -1) {
perror("write error");
}
else
printf("%ld\n",ret);
printf("%ld\n",lseek(fd, 0, SEEK_CUR));
close (fd);
return 0;
}
下面是输出：
$ cat sample
HELLO$ ./a.out
File descriptor = 3
4294967295
read error: Invalid argument
write error: Invalid argument
4294967295
$ ll sample
-rw-r--r--. 1 bruce stud 5 Jan 14 17:25 sample
但是，如果我更改lseek的语句
But if I change lseek statement to
ret = lseek(fd,5,SEEK_SET);
read返回0
read returns 0
$ ./a.out
File descriptor = 3
5
0
3
8
$ cat sample
HELLObye$ ll sample
-rw-r--r--. 1 bruce stud 8 Jan 14 17:26 sample
为什么读这样的表现？
Why does read behave like this?
推荐答案
请注意，通过 lseek的返回的值是一个 off_t ，而不是为size_t 。所不同的是off_t签署。当你把一个符号值，使之无符号，它看起来像一个大的正数。
Note that the value returned by lseek is a off_t, not a size_t. The difference is that off_t is signed. When you take a signed value and make it unsigned, it appears like a large positive number.
我预计LONG_MAX其实不是 4294967295 ，但无论是 2147483647 （2 ^ 31-1 ），或者一个更大的数。因此 4294967295 来自-1 [它是2 ^ 32-1，这确实是在32位的运算一样-1]。
I would expect that "LONG_MAX" is not actually 4294967295, but either 2147483647 (2^31-1) or a much larger number. So the 4294967295 comes from a -1 [it is 2^32-1, which is indeed the same as -1 in 32-bit math].
在换句话说，你都可以从 lseek的错误。
In other words, you are getting an error from lseek.
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