How do I get proper output from fgets when called from a separate thread?

Question

I would like to read a file line by line in a thread that I have created, but for some reason, the result that I'm getting from fgets is just garbage. What is the cause of this problem and how would I go about fixing it?

Here is the problem area, which is contained in its own thread:

    FILE *orders = fopen("orders.txt","r");

    int numOrders;
    numOrders = getNumLines(orders);

    int orderLineSize = getMaxLineCount(orders);
    char Line[orderLineSize];


    fgets(Line,orderLineSize,orders); // This is the call that gives me a garbage output
    printf("Line is: %s\n",Line);

This is what is contained in Line: ;3+. However, this changes every time I run the program

And this is what orders.txt contains:

"I Could Pee on This: And Other Poems by Cats"|7.49|1|HOUSING01
"Across Islands and Oceans"|1.99|1|SPORTS01
"Soul Surfer"|7.99|3|SPORTS01
"The Immortal Life of Henrietta Lacks"|8.91|4|POLITICS01
"The Handbuilt Home: 34 Simple Stylish and Budget-Friendly Woodworking Projects for    Every Room"|15.63|1|HOUSING01
"The History of Surfing"|31.5|2|SPORTS01

Here is getMaxLineCount:

int getMaxLineCount(FILE *filePtr)
{
    char c;
    int lineCount;
    int maxCount;
    lineCount = 0;
    maxCount = 0;
    while(c != EOF)
    {
            c = fgetc(filePtr);
            lineCount++;
            if((c == '\n') && (lineCount > maxCount))
            {
                    maxCount = lineCount;
                    lineCount = 0;
            }
    }

    rewind(filePtr);
    return maxCount;
}

getNumLines:

int getNumLines(FILE *filePtr)
{
    char c;
    int lineCount;
    int maxCount;
    lineCount = 0;
    maxCount = 0;

    int peopleCount;
    peopleCount = 0;
    while(c != EOF)
    {
            c = fgetc(filePtr);
            if(c == '\n')
                    peopleCount++;
    }

    rewind(filePtr);
    return peopleCount;
}

Answer 1

My guess is that getMaxLineCount is consuming the file, so you have to rewind it.

Before the call to fgets, try adding

rewind(orders);

Unless multiple threads are trying to read the same file, this issue should have nothing to do with threading.

Update: The following short program works fine for me.

Update2: I have fixed at least some of the undefined behavior mentioned by @chux.

Update3: by chux changed to size_t. Re-order to allow last line to be tested if longest. Move lineCount = 0; (it had only happened when a new max was found.)

#include <stdio.h>
#include <stdlib.h>


size_t getMaxLineCount(FILE *filePtr)
{
    int c; // Changed `char` to int
    size_t lineCount;
    size_t maxCount;
    lineCount = 0;
    maxCount = 0;
    while(1)
    {
        c = fgetc(filePtr);
        if (c == EOF) break;
        lineCount++;
        if (c == '\n')
        {
          if (lineCount > maxCount) maxCount = lineCount;
          lineCount = 0;
        }
    }
    if (lineCount > maxCount) maxCount = lineCount;

    rewind(filePtr);
    return maxCount;
}

int main(){
    FILE *orders = fopen("orders.txt","r");

    size_t orderLineSize = getMaxLineCount(orders);
    char Line[orderLineSize+1]; // Extra space for the buffer.


    fgets(Line,orderLineSize,orders); // This works fine for me.
    printf("Line is: %s\n",Line);
}