Hi, well, it is almost impossible to give a usefull answer without knowing the source code. The best would be, if you could send a small demo-file, which shows the problem. But already from your errors: > I have #include <string.h> , but why am I still can't declare string ? > > > When I compile my code ,show the error msg as follows : > root@acer:~/probeServer# gcc probec.c -o probec -lm -lpthread > probec.c:1:18: error: string: No such file or directory Here gcc claims, that he did not find the file "string". Are you sure, that you did not tried to include "string" instead of "string.h" > probec.c: In function 'main': > probec.c:458: error: 'string' undeclared (first use in this function) If I remember correctly, "string" is not defined in the header-File "string.h" (which is a C header)? Do you mean the C++-class std::string? If yes, you should: - tell the compiler that you are using C++ (e.g. naming your file "probec.cc") - compile with "g++" instead of "gcc" to guarantee he uses the correct libraries for linking - use "include <string>" to include the C++-header (containing the definition of std::string), neither "include <string.h>" (that's deprecated in C++) nor "include <cstring>" (that includes the C-header, which is called "string.h" in C, but does NOT contain the class std::string ...) - add "using namespace std;" if you want to use the class std::string by simply writing "string". But - maybe you want to do something else? Axel
