easy way to parse a text file? - c

Question

I'm making a load balancer (a very simple one). It looks at how long the user has been idle, and the load on the system to determine if a process can run, and it goes through processes in a round-robin fashion.
All of the data needed to control the processes are stored in a text file.
The file might look like this:
PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4397 IDLE=3.000000 BUSY=1.500000 USER=4.000000
PID=4405 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4412 IDLE=0.000000 BUSY=2.000000 USER=2.000000
PID=4420 IDLE=3.000000 BUSY=1.500000 USER=4.000000
This is a university assignment, however parsing the text file isn't supposed to be a big part of it, which means I can use whatever way is the quickest for me to implement.
Entries in this file will be added and removed as processes finish or are added under control.
Any ideas on how to parse this?
Thanks.

Answer 1

Here is a code that will parse your file, and also account for the fact that your file might be unavailable (that is, fopen might fail), or being written while you read it (that is, fscanf might fail). Note that infinite loop, which you might not want to use (that's more pseudo-code than actual code to be copy-pasted in your project, I didn't try to run it). Note also that it might be quite slow given the duration of the sleep there: you might want to use a more advanced approach, that's more sort of a hack.
int pid;
float idle, busy, user;
FILE* fid;
fpos_t pos;
int pos_init = 0;
while (1)
{
// try to open the file
if ((fid = fopen("myfile.txt","rw+")) == NULL)
{
sleep(1); // sleep for a little while, and try again
continue;
}
// reset position in file (if initialized)
if (pos_init)
fsetpos (pFile,&pos);
// read as many line as you can
while (!feof(fid))
{
if (fscanf(fid,"PID=%d IDLE=%f BUSY=%f USER=%f",&pid, &idle, &busy, &user))
{
// found a line that does match this pattern: try again later, the file might be currently written
break;
}
// add here your code processing data
fgetpos (pFile,&pos); // remember current position
pos_init = 1; // position has been initialized
}
fclose(fid);
}

Answer 2

As far as just parsing is concerned, something like this in a loop:
int pid;
float idle, busy, user;
if(fscanf(inputStream, "PID=%d IDLE=%f BUSY=%f USER=%f", %pid, &idle, &busy, &user)!=4)
{
/* handle the error */
}
But as #Blrfl pointed out, the big problem is to avoid mixups when your application is reading the file and the others are writing to it. To solve this problem you should use a lock or something like that; see e.g. the flock syscall.

Answer 3

Use fscanf in a loop. Here's a GNU C tutorial on using fscanf.
/* fscanf example */
#include <stdio.h>
typedef struct lbCfgData {
int pid;
double idle;
double busy;
double user;
} lbCfgData_t ;
int main ()
{
// PID=4390 IDLE=0.000000 BUSY=2.000000 USER=2.000000
lbCfgData_t cfgData[128];
FILE *f;
f = fopen ("myfile.txt","rw+");
for ( int i = 0;
i != 128 // Make sure we don't overflow the array
&& fscanf(f, "PID=%u IDLE=%f BUSY=%f USER=%f", &cfgData[i].pid,
&cfgData[i].idle, &cfgData[i].busy, cfgData[i].user ) != EOF;
i++
);
fclose (f);
return 0;
}