How can I get the latest changes in a file using ifstream? - c++

Question

It's a real-time capture system, I need to get the latest changes from a file which is occasionally edited(mostly add content) by other applications.
In other words, how can I get content that added in the period when I open it without reopening the file?
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main(){
ifstream tfile("temp.txt",ios::in);
if(!tfile){
cout<<"open failed"<<endl;
return 0;
}
string str;
while(1){
if(tfile.eof())
continue;
getline(tfile,str);
cout<<str<<endl;
}
tfile.close();
}

Answer 1

C++ / C Solution
If you are looking for a c++ solution you can use the following functions that I had created a while back:
#include <iostream>
#include <string>
// For sleep function
#ifdef _WIN32
#include <Windows.h>
#else
#include <unistd.h>
#endif
using namespace std;
void watchLogs(const char *FILENAME) {
FILE * f;
unsigned size = 0;
f = fopen(FILENAME , "r");
char c;
while (true) {
if (!size) { // will print content of your log file. If you just want the updates you can remove the current content except the first two lines;
fseek(f, 0, SEEK_END);
size =(unsigned long)ftell(f) ;
fseek (f, 0, SEEK_SET);
char buffer[size + 1];
fread ( buffer, 1, size, f );
buffer[size] = '\0';
cout << buffer << "\n";
}
else if ((c = (char)fgetc(f)) >= 0) {
fseek(f, 0, SEEK_END); // reach end of file
int BUFFER_SIZE =(unsigned long)ftell(f) - size; // save the length of the update to your logs
char buffer[BUFFER_SIZE + 1]; // prepare a buffer to print the characters
fseek(f,-BUFFER_SIZE,SEEK_END); // rewind BUFFER_SIZE characters before the EOF
int i = 0;
do {buffer[i++] = (char)fgetc(f);} while(i < BUFFER_SIZE); // copy to buffer
buffer[i] = '\0'; // don't forget to NULL terminate your buffer
cout << buffer << "\n";
size += i; // increment the size of the current file
}
}
sleep(3); // updates are checked every 3 seconds to avoid running the cpu at fullspeed, you could set the new logs to show up every minutes or every seconds, up to you.
fclose(f);
}
And you can test it with:
int main(int argc, char **argv) {
if (argc < 2)
return 1;
const char *FILENAME = argv[1];
watchLogs(FILENAME);
return 0;
}
./a.out mysql_binary.log
I could have used stringstreamer but I like that this version would also work with c files with some minor tweaks (can't use string).
I hope you will find it helpful!
NB: This assume that your file will only grow and that the changes will be appended to the end of your file.
NB2: This program is not segfault proof, you may want to check the return of fopen etc
Inotify
If you use Linux you could also potentially go for inotify:
Download inotify: sudo apt-get install -y inotify-tools
Then create the following script mywatch.sh
while inotifywait -e close_write $1; do ./$1; done
Give permission to execute:
add chmox +x mywatch.sh
and call it with ./watchit.sh mysql_binary.log