Question

socket从内核角度讲一次最大允许读取多少字节

Answer 1

1、从socket里一次最大读取字节数取决于接收buffer的大小，亦即socket的SO_RCVBUF对应的数值的2倍，这个2倍关系是内核决定的。
2、所以关键在于SO_RCVBUF的大小了。SO_RCVBUF大小，每个系统可能不一样，比如我使用linux，rmem_max存储了它的最大值：
[root@localhost sockopt]# cat /proc/sys/net/core/rmem_max
124928
3、所以对于我这台电脑，最大接收buffer是124928*2的大小，也就是说最大允许一次读取124928*2大小了。
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以下是linux对该buffer的描述
SO_RCVBUF
Sets or gets the maximum socket receive buffer in bytes. The kernel doubles
this value (to allow space for bookkeeping overhead) when it is set using set-
sockopt(2), and this doubled value is returned by getsockopt(2). The default
value is set by the /proc/sys/net/core/rmem_default file, and the maximum
allowed value is set by the /proc/sys/net/core/rmem_max file. The minimum
(doubled) value for this option is 256.
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