c。在一个二进制文件的中间乱写覆盖任何现有内容 [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，我需要在起始位置写数字的二进制文件的数组。我有中应开始的位置，我不希望覆盖后的价值，只是想在文件中的起始位置插入阵列。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
我们推456在位置2：
Let's push 456 at position 2:
12456345
我知道，可能我会通过自己来实现它，但我想知道什么是你如何尽可能有效实施尽可能意见。
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
下面是一个函数 extend_file_and_insert（），没有工作，或多或少。
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;
}
（注意附加线加入2013年7月19日，它是一个错误时，缓冲器大小比数据的要被复制了文件由于malat 您指出的错误。code现在 BUFFERSIZE = 4 测试。 ）的
(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.)
这是一些小规模的试code：
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, "\n", 1);
}
}
close(fd);
}
return(0);
}
它产生的输出：
12456345
1245XxxxxxX6345
1245XxxxxxX6ZzzzzzzzzzzzzzzzzzzzzzzzZ345
1245XxxxxxX6ZzzzzzzzzzYyyyyyyyyyyyyyyYzzzzzzzzzzzzzzZ345
应该在一些较大的文件进行测试（那些比BUFFERSIZE大，但它是明智的一个BUFFERSIZE超过64昆明植物小了很多测试，我用了32个字节，它似乎是确定）。我只直勾勾的结果，但图案的设计可以很容易地看到，他们是正确的。在code不检查任何的lseek的（）通话;这是一个小的风险。
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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