struct protoent结构，getprotobyname 及相关函数
在我的嵌入式设备中写了一个抓包程序，获取协议号的信息，结果在调用getprotobynumber时老是出错返回的protoent结构指针始终为空，打印了调试信息

/* Open protocol data base files and mark them as staying open even   after a later search if STAY_OPEN is non-zero.   This function is a possible cancellation point and therefore not   marked with __THROW.  */extern void setprotoent (int __stay_open);/* Close protocol data base files and clear `stay open' flag.   This function is a possible cancellation point and therefore not   marked with __THROW.  */extern void endprotoent (void);/* Get next entry from protocol data base file.  Open data base if   necessary.   This function is a possible cancellation point and therefore not   marked with __THROW.  */extern struct protoent *getprotoent (void);/* Return entry from protocol data base for network with NAME.   This function is a possible cancellation point and therefore not   marked with __THROW.  */extern struct protoent *getprotobyname (__const char *__name);/* Return entry from protocol data base which number is PROTO.   This function is a possible cancellation point and therefore not   marked with __THROW.  */extern struct protoent *getprotobynumber (int __proto);

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