warning: incompatible implicit declaration of built-in function ‘strlen’

warning: incompatible implicit declaration of built-in function ‘printf’  出现这样的警告一般是缺少头文件或者头文件不正确。

我的程序用到了函数strlen，以前在redhat9.0下编译可以通过，为什么在fedora8下面却出现错误？
源代码如下：

C/C++ code

    
    
    
    

     
     
     
      #include 
     
     
     
     <
     
     
     
     stdio.h
     
     
     
     >
     
     
     
      #include 
     
     
     
     <
     
     
     
     sys
     
     
     
     /
     
     
     
     socket.h
     
     
     
     >
     
     
     
      #include 
     
     
     
     <
     
     
     
     netinet
     
     
     
     /
     
     
     
     in
     
     
     
     .h
     
     
     
     >
    
    
    
    

    
    
    
    

     
     
     
     #include <string.h>
     
     
     
      
     
     
     
     int
     
     
     
      main() { 
     
     
     
     int
     
     
     
      s; 
     
     
     
     char
     
     
     
      dgram[
     
     
     
     512
     
     
     
     ]; 
     
     
     
     char
     
     
     
      dd[
     
     
     
     512
     
     
     
     ] 
     
     
     
     =
     
     
     
      
     
     
     
     "
     
     
     
     hello client!
     
     
     
     "
     
     
     
     ; 
     
     
     
     struct
     
     
     
      sockaddr_in adr_ser; 
     
     
     
     struct
     
     
     
      sockaddr_in adr_cli; adr_ser.sin_family 
     
     
     
     =
     
     
     
      AF_INET; adr_ser.sin_port 
     
     
     
     =
     
     
     
      htons(
     
     
     
     6666
     
     
     
     ); adr_ser.sin_addr.s_addr 
     
     
     
     =
     
     
     
      inet_addr(
     
     
     
     "
     
     
     
     192.168.1.137
     
     
     
     "
     
     
     
     ); s 
     
     
     
     =
     
     
     
      socket(AF_INET,SOCK_DGRAM,
     
     
     
     0
     
     
     
     ); bind(s,(
     
     
     
     struct
     
     
     
      sockaddr
     
     
     
     *
     
     
     
     )
     
     
     
     &
     
     
     
     adr_ser,
     
     
     
     sizeof
     
     
     
     (adr_ser)); 
     
     
     
     int
     
     
     
      len 
     
     
     
     =
     
     
     
      
     
     
     
     sizeof
     
     
     
     (adr_cli); recvfrom(s,dgram,
     
     
     
     sizeof
     
     
     
     (dgram),
     
     
     
     0
     
     
     
     ,(
     
     
     
     struct
     
     
     
      sockaddr
     
     
     
     *
     
     
     
     )
     
     
     
     &
     
     
     
     adr_cli,
     
     
     
     &
     
     
     
     len); printf(
     
     
     
     "
     
     
     
     From client:%s\n
     
     
     
     "
     
     
     
     ,dgram); sendto(s,
     
     
     
     "
     
     
     
     Hello
     
     
     
     "
     
     
     
     ,strlen(
     
     
     
     "
     
     
     
     Hello
     
     
     
     "
     
     
     
     )
     
     
     
     +
     
     
     
     1
     
     
     
     ,
     
     
     
     0
     
     
     
     ,(
     
     
     
     struct
     
     
     
      sockaddr
     
     
     
     *
     
     
     
     )
     
     
     
     &
     
     
     
     adr_cli,len); close(s); 
     
     
     
     return
     
     
     
      
     
     
     
     0
     
     
     
     ; }