为什么fseek使用read()系统调用? [英] Why does fseek use read() system call?
问题描述
我试图了解fseek的glibc实现.为此，我下载了glibc源代码并尝试了解其功能执行顺序.
I'm trying to understand the glibc implementation of fseek. To do so, I downloaded the glibc source code and tried to understand its function execution order.
我在libio/fseek.c中找到了fseek实现.基本上，它使用相同的参数调用函数(或宏)_IO_fseek().此宏在libio/iolibio.h中实现.
I found the fseek implementation in libio/fseek.c. Basically, it calls the function (or rather the macro) _IO_fseek() using the same parameters. This macro is implemented in libio/iolibio.h.
它定义为_IO_seekoff_unlocked (__fp, __offset, __whence, _IOS_INPUT|_IOS_OUTPUT)(在libio/ioseekoff.c中实现).执行此操作的下一步对我来说很令人困惑:
It is defined as _IO_seekoff_unlocked (__fp, __offset, __whence, _IOS_INPUT|_IOS_OUTPUT) (implemented in libio/ioseekoff.c). The next step in its execution is rather confusing for me:
_IO_seekoff_unlocked基本上返回_IO_SEEKOFF (fp, offset, dir, mode);，它返回_IO_seekoff_unlocked (fp, offset, dir, mode);，这应该创建一个调用循环.
_IO_seekoff_unlocked basically returns _IO_SEEKOFF (fp, offset, dir, mode);, which returns _IO_seekoff_unlocked (fp, offset, dir, mode);, which should create a call loop.
此外，在示例程序(seek.c)上使用strace时:
Also, when using strace on an example program (seek.c):
#include <stdio.h>
int main(void) {
printf("[Fseek] Executing fseek\n");
FILE *f = fopen("./seek.c", "rb");
fseek(f, 0L, SEEK_END);
}
它表明fseek将调用read系统调用，即使我在glibc实现中找不到它.
it shows that fseek will call the read system call, even though I could not find it in the glibc implementation.
...
write(1, "[Fseek] Executing fseek\n", 24[Fseek] Executing fseek
) = 24
openat(AT_FDCWD, "./seek.c", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=146, ...}) = 0
fstat(3, {st_mode=S_IFREG|0644, st_size=146, ...}) = 0
lseek(3, 0, SEEK_SET) = 0
read(3, "#include <stdio.h>\n\nint main(voi"..., 146) = 146
exit_group(0) = ?
+++ exited with 0 +++
我的目标是了解此处如何使用读取系统调用.我有自己的read系统调用实现，对于我编写的其他测试来说效果很好，但是通过fseek调用时由于某些原因会失败.
My goal is to understand how the read system call is used here. I have my own implementation of the read system call, which works well for other tests I wrote but will fail for some reason when it is called via fseek.
作为示例，我在函数中使用fseek来获取文件的大小:
As an example, I use fseek in a function to get the size of a file:
long get_file_size(const char *name)
{
FILE *temp_file = fopen(name, "rb");
if (temp_file == NULL)
{
return -1;
}
fseek(temp_file, 0L, SEEK_END);
long sz = ftell(temp_file);
fclose(temp_file);
return sz;
}
此函数将使用正常" read实现返回正确的大小，但由于我的原因将失败.因此，如果有人能告诉我如何理解fseek中read的用法(我在源代码中找不到)，我将不胜感激.
This function will return the correct size with the "normal" read implementation but will fail with mine. So, if anybody can tell me how I can understand the use of read within fseek (which I could not find in the source), I would highly appreciate it.
推荐答案
JUMP3 (__seekoff, FP, OFF, DIR, MODE) . JUMP3 是调用FILE 跳转"表/vtable .
打开默认情况下分配 _IO_file_jumps (或者类似的东西，因为文件可以被映射等)作为新FILE的跳转表.它是FILE的跳转表/虚拟表的实现.
fopen by default assigns _IO_file_jumps (or something like that, because the file can be mmap-ed etc. etc.) as the jump table for new FILEs. It is the implementation of the jump table/virtual table for a FILE.
因此_IO_SEEKOFF调用 _IO_file_jumps->__seekoff .它指向 _IO_new_file_seekoff ，最后在其中函数会调用 _IO_SYSREAD . _IO_SYSREAD 从跳转表，依次调用 _IO_file_read ，它调用 __read 最终执行
So _IO_SEEKOFF calls _IO_file_jumps->__seekoff. It points to _IO_new_file_seekoff and finally inside that function a call is made to _IO_SYSREAD. _IO_SYSREAD calls _read from the jump table, which in turn calls _IO_file_read, which calls __read which finally executes SYSCALL_CANCEL (read).
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