Question

The text file below is the standard I am dealing with.

Basically I want to record how many time steps are involved throughout processing the below text file.

The lines that are of significance are the first line, and lines start with ‘if’.
‘if’ lines are handled as follows:
if i < 3 i=i+1 goto 8
I am assuming i is initialised at 0.

This means that the control should jump to line 8 as long as i is less than 3

10
1fi
if i < 3 i=i+1 goto 8
3sdkfj
4ksdkk
5kdkfk
6kdkjf
7dkjkfd
if k < 2 k=k+1 goto 2
9dkkf
10dku
if j < 2 j=j+1 goto 2

My question being, using fopen to open the text file and fgets to gather the lines… how would I use fgets to go back to a line already processed by fgets i.e. doing what the if statement suggests in the above text file and go back to line 2. Without opening the text file again and doing whatever…

My code so far works to gather the first line and number of lines in any given text file within an in.file that looks like follows:

./JobA.txt
./JobB.txt
./JobC.txt
./JobD.txt

My code:

    #include <errno.h>
    #include <limits.h>
    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <unistd.h>
    #include <ctype.h>


    #include "projscheduler.h"



    /* I/O Files */
    //static char *inputFile;
    char * in;
    static FILE *input;
    static FILE *cur;
    /*Scheduled jobs indexed by PID*/
    struct job list[20];

    /* the next job to schedule */
    //static struct job *job_next = NULL;

    /* Time */
    time 

clock;

/*Initialises job list*/
static void initialise_list(void) {
    for(int i = 0; i < sizeof(list)/sizeof(list[0]); i++) {
        list[i].parameters.pid = -1;
    }
}

/*Order Jobs*/
/*=static void order_jobs(void)
{
    for(int i=0; i < sizeof(list)/sizeof(list[0]); i++)
    {


}
*/

/** Read and parse input from input file */
static void parse_input(void) 
{
    char    buffer[BUFSIZ];
    char    lines[BUFSIZ];
    int jobs = 0;
    struct  job *current;


    initialise_list();



    while( fgets(buffer, sizeof(buffer), input) )   
    {


        time start;
        char buf[BUFSIZ];
        sscanf(buffer,"./%s/", buf);
        cur = fopen(buf, "r" );

        int steps = 0;


        while( fgets(lines, sizeof(lines), cur) )
        {


            if( steps == 0 )
            {
                current = &list[jobs];
                strcpy(current->job_id, buf);
                sscanf(lines,"%ld", &start);
                current->parameters.start = start;              
            }



            steps++;
        } 
        current->parameters.scheduled = steps;

        jobs++;

        fclose(cur);

    } 

    for (int i = 0; i < jobs; i++)
    {
        printf("%s %ld  %ld\n", list[i].job_id, list[i].parameters.start, list[i].parameters.scheduled);
    }   

}   


int main(int argc, char **argv) 
{
    in = argv[1];
    if ( (input = fopen(in, "r")) == NULL ) {
        fprintf(stderr, "cannot open %s\n", argv[1]);
    }

    parse_input();

    fclose(input);

    return EXIT_SUCCESS;
}

Answer 1

There are several possibilities to go backwards in a file:

1. Store all the lines of the file in an array (this doesn’t scale very well!!)
2. Close the file and reopen it, skipping to the nth line (lots of unnecessary opening and closing of files)
3. Using fseek to go to the correct line (record the contents of ftell at the start of each line and then use fseek to go back to them). This scales a lot better.

Here’s some info in fseek and also on ftell