你没做错什么 在设置时，Linux将值加倍（在内核内），并在查询时返回加倍的值。
man 7 socket说：
[...] SO_SNDBUF Sets or gets the maximum socket send buffer in bytes. The ker- nel doubles this value (to allow space for bookkeeping overhead) when it is set using setsockopt(), and this doubled value is returned by getsockopt(). The default value is set by the wmem_default sysctl and the maximum allowed value is set by the wmem_max sysctl. The minimum (doubled) value for this option is 2048.[...]NOTES Linux assumes that half of the send/receive buffer is used for internal kernel structures; thus the sysctls are twice what can be observed on the wire.[...]
