如果程序传递了无效的指针值，或者如果缓冲区的长度太小会发生此错误。

This error occurs if a program passes an invalid pointer value or if the length of the buffer is too small.

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如果参数为NULL，则openssl会将字符串写入到一个长度为256字节的静态缓冲区中，并返回指向该缓冲区的指针。

If this parameter is NULL, OpenSSL writes the string to a static buffer that is 256 bytes in length, and returns a pointer to that buffer.

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dxferp：指向数据传输时长度至少为dxfer_len字节的用户内存的指针。

Dxferp: a pointer to user memory of at least dxfer_len bytes in length for data transfer.

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