Trouble with fscanf() in C

Question

I'm reading and writing (append mode) into a file, and something very strange happens. The first time I run it, it reads the file perfectly, and appends the new line perfectly. The second time, it gets stuck reading the file.

What am I doing wrong?

The reading:

fptr = fopen("log.txt", "r");
fseek(fptr, -70, SEEK_END); 
for(i = 0; i < 11; i++)
{
    fscanf(fptr, "%d, ", llast + i);
    printf("%d ", llast[i]);
}

/* read the last review */    
for(i = 0; i < 11; i++)
{
    fscanf(fptr, "%d, ", last + i);
    printf("%d ", last[i]);
}

fclose(fptr);

The appending:

fptr = fopen("log.txt", "a");
for(i = 0; i < 11; i++)
{
    fprintf(fptr, "%d, ", *(new + i));
}
fprintf(fptr, "\n");
fclose(fptr); 

The log.txt file last two lines contain:

7, 4, 0, 10, 2, 8, 9, 5, 6, 3, 1, 
3, 7, 5, 6, 9, 2, 4, 10, 0, 1, 8, 

The Full Code:

#include <stdio.h>
#include <stdlib.h> /* for the rand function */

int main()
{
int last[11] = {0};
int llast[11] = {0};
int new1[11] = {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1};
int i = 0, j = 0;
int rev = 0;
int flag = 0;
int big_flag = 0;
FILE *fptr;

printf("\nThe last review:\n");
PrintRev();
printf("\n\n");

/* open the log file of the reviews, and load the last two into arrays */
fptr = fopen("log.txt", "r");

/* read the review before the last review*/
fseek(fptr, -70, SEEK_END); 

for(i = 0; i < 11; i++)
{
    fscanf(fptr, "%d, ", llast + i);
    printf("%d ", llast[i]);
}

/* read the last review */    
for(i = 0; i < 11; i++)
{
    fscanf(fptr, "%d, ", last + i);
    printf("%d ", last[i]);
}

fclose(fptr);

/* create a new arrangement of reviewing */

/* the big flag is used for the extreme case when the first 10 reviewers are assigned
   but a problem occurs in the last remaining option for the last reviewer -
   so instead of running into a dead end of endless-loop, if there is a problem,
   we start over again ... */
while(!big_flag)
{
    for(i = 0; i < 10; i++)
    {
        flag = 0;
        /* the small flag is used to make sure the first 10 reviewers follow the rules */
        while(!flag)
        {
            rev = (rand() % 11);
            if((rev != i) && /* you can't review yourself */
               (rev != last[i]) &&  /* you can't review the person you did last time */
               (rev != llast[i]) && /* let's try without the last-last time */
               (rev != (i - 1)) && (rev != (i + 1)) && /* you can't review your neighbour */
               (new1[rev] != i)) /* you can't review the person reviewing you */
            {
                flag = 1;                
                /* a reviewer can't review more than one person... so we check if he wasn't
                   chosen before */
                for(j = 0; j < i; j++)
                {
                    if(rev == new1[j])
                    {
                        flag = 0;
                    }
                }
            }
        }
        new1[i] = rev;
    }

    /* find the only remaining id of reviewer */
    rev = 55; /* sum of 0,1, ... 10 */
    for(i = 0; i < 10; i++)
    {
        rev -= new1[i];
    }
    /* check if the remaining id is valid */ 
    if((rev != i) && 
       (rev != last[i]) &&  
       (rev != llast[i]) && /* let's try without the last-last time */
       (rev != (i - 1)) && 
       (new1[rev] != i))
    /* if so, appoint it, and raise the big flag to exit the loop */
    {
        new1[10] = rev;
        big_flag = 1;
    } 
}

/* append the new arrangement to file */
fptr = fopen("log.txt", "a");
for(i = 0; i < 11; i++)
{
    fprintf(fptr, "%d, ", *(new1 + i));
}

fclose(fptr); 




return 0;
}

Answer 1

There is at least one problem in your code. You say that log.txt contains lines and that you add a new line at the end, but your code never output a single '\n', so you never write the EOL marker. As you say you use Linux, the end of line on disk is indeed a single character '\n', but your code would not work on Windows where it is the pair "\r\n" => said differently the present code is not portable.

But if you only want to run in on a Unix system, just add a new line after writing the 11 values:

/* append the new arrangement to file */
fptr = fopen("log.txt", "a");
for(i = 0; i < 11; i++)
{
    fprintf(fptr, "%d, ", *(new1 + i));
}
fputc('\n', fptr);
fclose(fptr); 

I could run it 5 times and it did add 5 lines (of exactly 35 characters...)

But beware: Here you are essentially processing a text file as if it was a binary file with records of size 35. That is not portable because end of lines are different across architecture, but it is dangerous for another reason: as it is a text file, it can be opened with a text editor, say vi. What would you expect if someone opens the file and inadvertently add some spaces at the end of one of the 2 last lines? The records will not be exactly 35 characters long and you will read wrong values. And as you fail to test any result of you input functions (neither open, nor fscanf) and you do not test that you have read values from 0 to 10 either, your program is likely to crash without any useful message.

TL/DR: do add tests for all your open and fscanf calls, and do control that every line contains all values from 0 to 10