c/c++ linux programming by examples my programming tips and techniques

To write formatted string to files, the easiest way is uses fprintf. It works exactly the same way like printf.

File * fp=fopen("filename","w");

fprintf(fp,"%s.%s = %d\n", szTag,szSubTag,nValue);

fclose(fp);

In case you want to create your own fprintf, can uses stdarg’s function - vfprintf

//Assume you have access to file pointer at outside
File * fp;
int WriteLog(char *str,...)
{
    //fp is null, file is not open.
    if (fp==NULL)
          return -1;

    fprintf(fp,"%d : ",time(NULL));
    va_list arglist;
    va_start(arglist,str);
    vfprintf(fp,str,arglist);
    va_end(arglist);
    fprintf(fp," \n");

    return 1;
}

I simply uses time() to returns me a time value in seconds since the Epoch, it is just a simple example of how you can use fprintf and also vfprint to write text based file such as logs and config files.

I have tried to put just “\n” in fprintf to leave a blank line but it fails, but with a blank space together with “\n”, it works!

For va_list usage, refers back to va_list examples.