C 将文件内容从 EOF 复制到 SOF

Question

我的程序几乎可以正常工作。预期目的是从末尾读取文件并将内容复制到目标文件。然而让我感到困惑的是 lseek() 方法，所以我应该如何设置偏移量。

目前我的src内容是:
第一行
2号线
第 3 行

目前我在目标文件中得到的是:
3号线
2
2...

据我所知，调用 int loc = lseek(src, -10, SEEK_END); 会将源文件中的“光标”移动到结尾，然后将其从 EOF 偏移到 SOF 10 个字节loc 的值将是我扣除偏移量后的文件大小。然而，在 C 的 7 小时之后，我在这里几乎脑死亡。

int main(int argc, char* argv[])
{
    // Open source & source file
    int src = open(argv[1], O_RDONLY, 0777);
    int dst = open(argv[2], O_CREAT|O_WRONLY, 0777);

    // Check if either reported an erro
    if(src == -1 || dst == -1)
    {
        perror("There was a problem with one of the files.");
    }

    // Set buffer & block size
    char buffer[1];
    int block;

    // Set offset from EOF
    int offset = -1;

    // Set file pointer location to the end of file
    int loc = lseek(src, offset, SEEK_END);

    // Read from source from EOF to SOF
    while( loc > 0 )
    {
        // Read bytes
        block = read(src, buffer, 1);

        // Write to output file
        write(dst, buffer, block);

        // Move the pointer again
        loc = lseek(src, loc-1, SEEK_SET);
    }

}

Answer 1

lseek() 不会更改或返回文件大小。它返回的是“光标”设置的位置。所以当你打电话时

loc = lseek(src, offset, SEEK_END);

两次它总是会再次将光标设置到相同的位置。我想你想做这样的事情:

while( loc > 0 )
{
    // Read bytes
    block = read(src, buffer, 5);

    // Write to output file
    write(dst, buffer, block);

    // Move the pointer again five bytes before the last offset
    loc = lseek(src, loc+offset, SEEK_SET);
}

如果行的长度是可变的，您可以改为执行以下操作:

// define an offset that exceeds the maximum line length
int offset = 256;
char buffer[256];
// determine the file size
off_t size = lseek( src, 0, SEEK_END );
off_t pos = size;
// read block of offset bytes from the end
while( pos > 0 ) {
    pos -= offset;
    if( pos < 0 ) {
        //pos must not be negative ...
        offset += pos;   // in fact decrements offset!!
        pos = 0;
    }
    lseek( src, pos, SEEK_SET );
    // add error checking here!!
    read(src, buffer, offset );
    // we expect the last byte read to be a newline but we are interested in the one BEFORE that
    char *p = memchr( buffer, '\n', offset-1 );
    p++;  // the beginning of the last line
    int len = offset - (p-buffer);  // and its length
    write( dst, p, len );
    pos -= len;            // repeat with offset bytes before the last line
}