updating text files in C(is it possible?)

Question

Do any of you guys know if it's possible to update a text file(e.g something.txt) in C? I was expecting to find a function with similar syntax as update_txt(something.txt), but I haven't found anything while browsing the internet for the last 2 hours..... The thing is that I would like some data to be stored and displayed in real time in an already opened text file. I can store the data but I am unable to find a way to display it without manually closing the text file and then open it again...

Do someone know how to solve this issue? Or do you have another way to solve it? I have read something about transferring data to a new text document and then renaming it, but I am quite sure that this wouldn't solve my problem. I have also read something about macros that could detect changes in the document and then somehow refresh it. I have never worked with macros and I have absolutely no idea of how they are implemented....

But please tell me if it is a fact that it is impossible to update an already opened text document?

I am thankful for any suggestions or tutorials that you guys may provide! :)

Answer 1

That's outside the scope of C; it will require some system-specific filesystem monitoring mechanism. For example, inotify offers this functionality

Answer 2

First off, you can use the rewind(), fseek(), ftell() or fgetpos() and fsetpos() functions to locate the read pointer in a file. If you record the start position where the updated record was written (the start offset) using ftell() or fgetpos(), you could jump back to that position later with fseek() or fsetpos() and read in the changed data.

The other gotcha lurking here is that in general, you can't simply 'update' a text file. Specifically, if the replacement text is not the same length as the original text, you have problems. You either need to expand or contract the file. This is normally done by making a copy with the desired edit in the correct location, and then copying or moving the modified copy of the file over the original file.

Detecting when some other process modifies the file is harder still. There are different mechanisms in different operating systems. For Linux, it is the inotify system, for example.

Answer 3

Based upon your statement that you 'can't display it without manually closing the text file and open it again', it may be a buffer issue. When using the C standard library calls (fopen, fread, fwrite, fclose, etc ...) the data you write may be buffered in user-space until the buffer is full or the file is closed.

To force the C library to flush the buffer, use the fflush(fp) call where fp is your file pointer.

Answer 4

Regarding: But please tell me if it is a fact that it is impossible to update an already opened text document? Yes, it is not possible, unless you own the handle to the file (i.e. FILE *fp = fopen("someFilePath", "w+");)

Regarding: if it's possible to update a text file(e.g something.txt) in C?
Yes. If you know the location of the file, (someFileLocation, eg. "c:\dev\somefile.txt"), then open it and write to it.

A simple function that uses FILE *fp = fopen(someFileLocation, "w+"); (open existing file for append) and fclose(fp); will do that: Here is an example that I use for logging:

(Note, you will have to comment out, or create the other functions this one refers to, but the general concept is shown)

int WriteToLog(char* str)
{
    FILE* log;
    char *tmStr;
    ssize_t size;
    char pn[MAX_PATHNAME_LEN];
    char path[MAX_PATHNAME_LEN], base[50], ext[5];
    char LocationKeep[MAX_PATHNAME_LEN];
    static unsigned long long index = 0;

    if(str)
    {
        if(FileExists(LOGFILE, &size))
        {
            strcpy(pn,LOGFILE);
            ManageLogs(pn, LOGSIZE);
            tmStr = calloc(25, sizeof(char));
            log = fopen(LOGFILE, "a+");
            if (log == NULL)
            {
                free(tmStr);
                return -1;
            }
            //fprintf(log, "%10llu %s: %s - %d\n", index++, GetTimeString(tmStr), str, GetClockCycles());
            fprintf(log, "%s: %s - %d\n", GetTimeString(tmStr), str, GetClockCycles());
            //fprintf(log, "%s: %s\n",  GetTimeString(tmStr), str);
            fclose(log);
            free(tmStr);
        }
        else
        {
            strcpy(LocationKeep, LOGFILE);
            GetFileParts(LocationKeep, path, base, ext);
            CheckAndOrCreateDirectories(path);
            tmStr = calloc(25, sizeof(char));
            log = fopen(LOGFILE, "a+");
            if (log == NULL)
            {
                free(tmStr);
                return -1;
            }
            fprintf(log, "%s: %s - %d\n", GetTimeString(tmStr), str, GetClockCycles());
            //fprintf(log, "%s: %s\n",  GetTimeString(tmStr), str);
            fclose(log);
            free(tmStr);
        }
    }
    return 0;
}  

Regarding: browsing the internet for the last 2 hours. Next time try
"tutorial on writing to a file in C" in Google, it lists lots of links, including: This one... More On The Topic.