C 在二进制文件中间写入而不覆盖任何现有内容 [英] C write in the middle of a binary file without overwriting any existing content
问题描述
今天的问题是我需要在一个二进制文件的起始位置写一个数字数组.我有它应该开始的位置，之后我不想覆盖值，只想在文件的起始位置插入数组.例如:
Today's problem is that I need to write an array of numbers in a binary file at a starting position. I have the position where it should start, and I don't want to overwrite values after that, just want to insert the array at the starting position in the file. E.g:
12345
让我们将 456 推到位置 2:
Let's push 456 at position 2:
12456345
我知道可能我必须自己实现它，但我想知道您对如何尽可能高效地实现它有什么看法.
I know that probably I'll have to implement it by myself, but I want to know what's your opinion on how to implement that as efficiently as possible.
推荐答案
这里有一个函数 extend_file_and_insert() 可以或多或少地完成工作.
Here's a function extend_file_and_insert() that does the job, more or less.
#include <sys/stat.h>
#include <unistd.h>
enum { BUFFERSIZE = 64 * 1024 };
#define MIN(x, y) (((x) < (y)) ? (x) : (y))
/*
off_t is signed
ssize_t is signed
size_t is unsigned
off_t for lseek() offset and return
size_t for read()/write() length
ssize_t for read()/write() return
off_t for st_size
*/
static int extend_file_and_insert(int fd, off_t offset, char const *insert, size_t inslen)
{
char buffer[BUFFERSIZE];
struct stat sb;
int rc = -1;
if (fstat(fd, &sb) == 0)
{
if (sb.st_size > offset)
{
/* Move data after offset up by inslen bytes */
size_t bytes_to_move = sb.st_size - offset;
off_t read_end_offset = sb.st_size;
while (bytes_to_move != 0)
{
ssize_t bytes_this_time = MIN(BUFFERSIZE, bytes_to_move);
ssize_t rd_off = read_end_offset - bytes_this_time;
ssize_t wr_off = rd_off + inslen;
lseek(fd, rd_off, SEEK_SET);
if (read(fd, buffer, bytes_this_time) != bytes_this_time)
return -1;
lseek(fd, wr_off, SEEK_SET);
if (write(fd, buffer, bytes_this_time) != bytes_this_time)
return -1;
bytes_to_move -= bytes_this_time;
read_end_offset -= bytes_this_time; /* Added 2013-07-19 */
}
}
lseek(fd, offset, SEEK_SET);
write(fd, insert, inslen);
rc = 0;
}
return rc;
}
(注意 2013-07-19 添加的附加行；这是一个错误，仅当缓冲区大小小于要复制到文件的数据量时才会显示.感谢 malat 指出错误.代码现在用 BUFFERSIZE = 4 测试.)
(Note the additional line added 2013-07-19; it was a bug that only shows when the buffer size is smaller than the amount of data to be copied up the file. Thanks to malat for pointing out the error. Code now tested with BUFFERSIZE = 4.)
这是一些小规模的测试代码:
This is some small-scale test code:
#include <fcntl.h>
#include <string.h>
static const char base_data[] = "12345";
typedef struct Data
{
off_t posn;
const char *data;
} Data;
static const Data insert[] =
{
{ 2, "456" },
{ 4, "XxxxxxX" },
{ 12, "ZzzzzzzzzzzzzzzzzzzzzzzzX" },
{ 22, "YyyyyyyyyyyyyyyY" },
};
enum { NUM_INSERT = sizeof(insert) / sizeof(insert[0]) };
int main(void)
{
int fd = open("test.dat", O_RDWR | O_TRUNC | O_CREAT, 0644);
if (fd > 0)
{
ssize_t base_len = sizeof(base_data) - 1;
if (write(fd, base_data, base_len) == base_len)
{
for (int i = 0; i < NUM_INSERT; i++)
{
off_t length = strlen(insert[i].data);
if (extend_file_and_insert(fd, insert[i].posn, insert[i].data, length) != 0)
break;
lseek(fd, 0, SEEK_SET);
char buffer[BUFFERSIZE];
ssize_t nbytes;
while ((nbytes = read(fd, buffer, sizeof(buffer))) > 0)
write(1, buffer, nbytes);
write(1, "
", 1);
}
}
close(fd);
}
return(0);
}
它产生输出:
12456345
1245XxxxxxX6345
1245XxxxxxX6ZzzzzzzzzzzzzzzzzzzzzzzzZ345
1245XxxxxxX6ZzzzzzzzzzYyyyyyyyyyyyyyyYzzzzzzzzzzzzzzZ345
它应该在一些更大的文件上进行测试(比 BUFFERSIZE 大的文件，但使用比 64 KiB 小很多的 BUFFERSIZE 进行测试是明智的；我使用了 32 个字节，它似乎没问题).我只观察了结果，但设计的模式是为了让人们很容易看出它们是正确的.该代码不检查任何 lseek() 调用；这是一个小风险.
It should be tested on some larger files (ones bigger than BUFFERSIZE, but it would be sensible to test with a BUFFERSIZE a lot smaller than 64 KiB; I used 32 bytes and it seemed to be OK). I've only eyeballed the results but the patterns are designed to make it easy to see that they are correct. The code does not check any of the lseek() calls; that's a minor risk.
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