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Jan 20th, 2009

make fgets() wait for new lines.

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Hi.
am using fgets() to read lines from a text file.
the file is being updated after a few seconds.

i want to read and process the last appended line at the end of the file.

i tried the following:

c Syntax (Toggle Plain Text)
while(true){ //infinite loop
 
if(fgets(line,size,fp)!=NULL)
    process line.
 
}
while(true){ //infinite loop

if(fgets(line,size,fp)!=NULL)
    process line.

}

plz help.

Last edited by Narue; Jan 20th, 2009 at 9:23 am. Reason: added code tags

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CoolAtt

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Jan 20th, 2009

Re: make fgets() wait for new lines.

Something like this (untested)

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while ( 1 ) {
  while ( fgets(line,size,fp)!=NULL) {
    // process lines
  }
  clearerr( fp );  // reset end of file flag
  sleep( 1 );  // wait for more lines to arrive
}
while ( 1 ) {
  while ( fgets(line,size,fp)!=NULL) {
    // process lines
  }
  clearerr( fp );  // reset end of file flag
  sleep( 1 );  // wait for more lines to arrive
}

Salem

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Jan 21st, 2009

Re: make fgets() wait for new lines.

i tried it. its not working.

CoolAtt

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Jan 21st, 2009

Re: make fgets() wait for new lines.

Oh, thanks for the update, be sure to add more information later.

Salem

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Jan 21st, 2009

Re: make fgets() wait for new lines.

does fgets() gets aware when a file is updated ?

will clearerr( fp ) tell fgets() that EOF has not been reached?

Last edited by CoolAtt; Jan 21st, 2009 at 3:18 am.

CoolAtt

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Jan 21st, 2009

Re: make fgets() wait for new lines.

Click to Expand / Collapse Quote originally posted by CoolAtt ...

does fgets() gets aware when a file is updated ?

will clearerr( fp ) tell fgets() that EOF has not been reached?

If the above method is not working then you must go for closing and reopening of the file when it gets updated

ajay.krish123

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Jan 21st, 2009

Re: make fgets() wait for new lines.

Okay....I think, the following prototype might help you...
- I made this on Linux and compiled with gcc
- 'test.txt' was in the same directory as my '.c' file

C Syntax (Toggle Plain Text)
#include<stdio.h>
#include<string.h>
 
int main(void)
{
    char buff[100];
    FILE* fp_1;
 
    fp_1 = fopen("./test.txt","r");
    memset(buff,0,sizeof(buff));
 
    while(1)
    {
        if(fgets(buff,100,fp_1))
              ;
        else
        {
            printf("\n %s \n",buff);  //process the last appended line here
            fclose(fp_1);
            fp_1 = fopen("./test.txt","r");
        }
 
    }
 
    return 0;
}
#include<stdio.h>
#include<string.h>

int main(void)
{
    char buff[100];
    FILE* fp_1;

    fp_1 = fopen("./test.txt","r");
    memset(buff,0,sizeof(buff));

    while(1)
    {
        if(fgets(buff,100,fp_1))
              ;
        else
        {
            printf("\n %s \n",buff);  //process the last appended line here
            fclose(fp_1);
            fp_1 = fopen("./test.txt","r");
        }

    }

    return 0;
}

me_ansh

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Jan 23rd, 2009

Re: make fgets() wait for new lines.

thx for the code.
but because there is an infinite loop cpu usage is 100%.
do you know how to prevent this ?

CoolAtt

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Jan 23rd, 2009

Re: make fgets() wait for new lines.

You could try using sleep(n) in the loop body. This will make your code defer to some other thread for at least n milliseconds, and free up the CPU. Note: is _sleep() under Win32

sma245

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Jan 25th, 2009

Re: make fgets() wait for new lines.

You may check out that are there any kind of signals that are generated and can be handled when a text file gets updated....if yes, then you can use signal handling and make your program wait until a signal is received and then process the data in file....that will definitely reduce your CPU usage...

me_ansh

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