The Big Programming Thread - Page 673

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Wrath

3174 Posts

October 31 2015 14:21 GMT

#13441

Hi all, sorry for the noob question but it seems there is no already function in C to read line by line? Do I need to implement one by myself?

The one I found is "getLine". but that is not portable is it GNU function and not a standard.

Ropid

Germany3557 Posts

October 31 2015 14:30 GMT

#13442

There's fgets().

The man-page of getline() says it was added to POSIX at some point, so maybe Microsoft does also have it. Did you check?

Manit0u

Poland17539 Posts

October 31 2015 14:38 GMT

#13443

http://rosettacode.org/wiki/Read_a_file_line_by_line#C
https://www.gnu.org/software/libc/manual/html_node/Line-Input.html

Wrath

3174 Posts

October 31 2015 14:51 GMT

#13444

On October 31 2015 23:30 Ropid wrote:
There's fgets().

The man-page of getline() says it was added to POSIX at some point, so maybe Microsoft does also have it. Did you check?

I checked on VS and it did not identify it. And I can't find on google that microsoft included it. Beside, the fgets() won't hellp much because it will read a defined number of characters instead of reading the whole line. If I have a file with different lines length, it won't help me.

Blitzkrieg0

United States13132 Posts

October 31 2015 14:53 GMT

#13445

On October 31 2015 23:51 WrathSCII wrote:

Show nested quote +

Make a while loop on the condition that the char isn't carriage return/line feed.

Wrath

3174 Posts

October 31 2015 15:08 GMT

#13446

On October 31 2015 23:53 Blitzkrieg0 wrote:

Show nested quote +

Make a while loop on the condition that the char isn't carriage return/line feed.

I did that and here is my function that reads line by line (It is not perfect but it is doing the job... so far...)


void  read_line(FILE *fp)
{
	char* buffer;
	char  temp;
	buffer = (char*)calloc(100, sizeof(char));
	char *p = buffer;
	*p = (char)fgetc(fp);
	while (*p != '\n')
	{
		p++;
		*p = (char)fgetc(fp);
	}
	printf("%s", buffer);

}

This is how I use the function in the main:


int main()
{
	FILE *fp = fopen("C:\\enable.txt", "r");
	read_line(fp);
	read_line(fp);
	
	return 0;
}

I thought I'm doing pass by value here. so it should print the first line twice. But it is printing the 1st time it prints the 1st line and the 2nd time it prints the 2nd line.

Any thoughts on why this is happening?

Blitzkrieg0

United States13132 Posts

October 31 2015 15:15 GMT

#13447

You're creating the buffer when you enter the function so a new buffer is created for the second line. If you want a single buffer you should create it in main and pass it to the function.

Maybe nitpicky, but you should be giving that memory back to the operating system at the end of the function as well.

Wrath

3174 Posts

October 31 2015 15:21 GMT

#13448

On November 01 2015 00:15 Blitzkrieg0 wrote:
You're creating the buffer when you enter the function so a new buffer is created for the second line. If you want a single buffer you should create it in main and pass it to the function.

Maybe nitpicky, but you should be giving that memory back to the operating system at the end of the function as well.

Not sure how creating the buffer is related to my question here. I mean I pass the file pointer by value when I call it in the main. So the second I time I call it, the file pointer should not have changed in the first call as what was passed there is a copy of it. So theoretically it should once again print the first line.

Blitzkrieg0

United States13132 Posts

October 31 2015 15:31 GMT

#13449

On November 01 2015 00:21 WrathSCII wrote:

Show nested quote +

fgetc increments the file pointer.

netherh

United Kingdom333 Posts

October 31 2015 15:32 GMT

#13450

On November 01 2015 00:21 WrathSCII wrote:
Not sure how creating the buffer is related to my question here. I mean I pass the file pointer by value when I call it in the main. So the second I time I call it, the file pointer should not have changed in the first call as what was passed there is a copy of it. So theoretically it should once again print the first line.

That's not how it works: http://en.cppreference.com/w/c/io

While you pass the file pointer by value (so the value of the pointer in main is the same afterwards), there's nothing to stop fgetc from changing the underlying object that the pointer points to.

The file pointer isn't really a "pointer to a file" anyway. It represents a file input stream, that contains a current position in the file. When you call fgetc you read a character from the stream, and increment the position. So the next thing you read from the file continues from that point.

netherh

United Kingdom333 Posts

October 31 2015 15:32 GMT

#13451

Oops

Ropid

Germany3557 Posts

October 31 2015 17:32 GMT

#13452

On October 31 2015 23:51 WrathSCII wrote:

Show nested quote +

No, fgets() will stop after it finds a new-line. Its size parameter is a limit where you can tell fgets() the size of your buffer so that it won't overwrite your stuff.

Manit0u

Poland17539 Posts

October 31 2015 18:03 GMT

#13453

Why can't you do it like that?

http://www.tutorialspoint.com/c_standard_library/c_function_fgets.htm


#include <stdio.h>

int main ()
{
   FILE *handle;
   char buffer[4096];
  
   handle = fopen("file.txt","r");

   if (handle == NULL) 
   {
      perror("Error opening file");

      return(-1);
   }
   
   while (fgets(buffer, 4096, handle) != NULL)
   {
      puts(buffer);
   }

   if (!feof(handle))
   {
      perror("Error: unexpected fgets() fail.");
   }

   fclose(handle);
   
   return(0);
}

Something like that...

Wrath

3174 Posts

October 31 2015 19:07 GMT

#13454

^ I really did not know that it is possible with fgets that it stops reading once it reaches the "\n"

Thanks!

WarSame

Canada1950 Posts

October 31 2015 21:42 GMT

#13455

In regards to cron, I can't seem to find this anywhere. You should only use crontab when your job doesn't fit nicely into the folders, right? So if your job should run daily at midnight then you put it in cron.daily, but if it runs at midnight and 0600, then it should be in the crontab?

Shield

Bulgaria4824 Posts

October 31 2015 22:25 GMT

#13456

On November 01 2015 04:07 WrathSCII wrote:
^ I really did not know that it is possible with fgets that it stops reading once it reaches the "\n"

Thanks!

Same applies to std::cin I think.

Edit: Actually, it may be EOF not \n.

Ropid

Germany3557 Posts

October 31 2015 22:46 GMT

#13457

On November 01 2015 06:42 WarSame wrote:
In regards to cron, I can't seem to find this anywhere. You should only use crontab when your job doesn't fit nicely into the folders, right? So if your job should run daily at midnight then you put it in cron.daily, but if it runs at midnight and 0600, then it should be in the crontab?

Yup, that's how I understood it as well. If you need special rules, you use crontab.

There's also a thingy named "anacron" which you might want to look at. It's what deals with what happens when the machine was off at the time when a rule would have been hit. Normal cron simply doesn't do anything about that and scripts might never run. I think the daily, weekly, etc. folders are looked at by that anacron tool, but I don't know what's up with things that are in crontab.

Another thing is your machine might use "systemd" as the init system. That has "timers" that can be used as an alternative to cron+anacron.