字符串反转时链接出现问题，求助~

char *revstr(char *str,size_t len)
{
    char *start=str;
    char *end=str+len-1;
    char ch;
    if (start<end)
    {
        ch=*start;
        *start=*end;
        *end=ch;
        start++;
        end--;
    }
    return str;
}链接问题
--------------------Configuration: reversal - Win32 Debug--------------------
Linking...
LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main
Debug/reversal.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

reversal.exe - 2 error(s), 0 warning(s)