lseek()后面跟着new open()时返回0

我有以下代码(它是“示例”代码,所以没有什么特别之处):

#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <unistd.h>

int main()
{
    char buffer[9];
    int fp = open("test.txt", O_RDONLY);

    if (fp != -1) // If file opened successfully
    {
        off_t offset = lseek(fp, 2, SEEK_SET); // Seek from start of file
        ssize_t count = read(fp, buffer, strlen(buffer));
        if (count > 0) // No errors (-1) and at least one byte (not 0) was read
        {
            printf("Read test.txt %d characters from start: %s\n", offset, buffer);
        }

        close(fp);
    }

    int fp2 = open("test.txt", O_WRONLY);
    if (fp2 != -1)
    {
        off_t offset = lseek(fp2, 2, SEEK_CUR); // Seek fraom current position (0) - same result as above in this case
        ssize_t count = write(fp2, buffer, strlen(buffer));
        if (count == strlen(buffer)) // We successfully wrote all the bytes
        {
             printf("Wrote to test.txt %d characters from current (0): %s\n", offset, buffer);
        }

        close(fp2);
    }
}

此代码不会按原样返回第一个打印输出(读取),第二个打印输出为:“write test.txt 0 characters from current(0):”,表示它没有在文件中查找任何位置,缓冲区为空。

奇怪的是,如果我从 fp2 = open("test.txt", O_WRONLY); ,第一个打印输出返回我期望的结果。只要我包括第二个 open 声明(即使没有其他内容)也写不出来。它是以某种方式重新排序公开声明还是其他什么?