why the program give "Segmentation fault" after 1018 try

Question

I'm writing a C code to open txt file and read two lines on it then print the value it worked for 1018 time then it gives "Segmentation fault" I've tried to flush the buffer but it don't work

while(running) {
    i = 0;
    if ((fptr = fopen("pwm.txt", "r")) == NULL) {
        printf("Error! File cannot be opened.");
        // Program exits if the file pointer returns NULL.
        exit(1);
    }
    fptr = fopen("pwm.txt","r");
    while (fgets(line,sizeof(line), fptr)){
        ppp[i]=atoi(line);
        i++;
    }
    fclose(fptr);
    printf("%d %d\n",ppp[0],ppp[1]);
    rc_servo_send_pulse_us(ch, 2000);
    rc_usleep(1000000/frequency_hz);
}

This also seems like a good time to learn how to use a [*debugger*](https://stackoverflow.com/questions/25385173/what-is-a-debugger-and-how-can-it-help-me-diagnose-problems) to catch crashes, and locate where in your code they happen. It will also allow you to examine variables and their values at the time and location of the crash, to help you understand what might have gone wrong. — Some programmer dude, Sep 05 '22 at 12:13
And why do you open the second time? You already have a valid pointer in `fptr` — Gerhardh, Sep 05 '22 at 12:34
How large is `ppp` array? Can it hold enough numbers for the whole file? — Gerhardh, Sep 05 '22 at 12:35
I removed the first fopen and it worked fine than you very much all — Majd Zahra, Sep 05 '22 at 13:03
You should remove the second. `fopen` could still return `NULL`. — Gerhardh, Sep 05 '22 at 13:25

score 2 · Accepted Answer · answered Sep 05 '22 at 12:17

2

Actually, the file-opening is the likely culprit: On e.g. Linux there's a limit to how many files you can have open, and it typically defaults to 1024. A few files are used for other things, and your program probably uses some other file-handles elsewhere, leaving only around 1018 left over.

So when you open the file twice, you leak the file handle from the first fopen call, and then your second fopen call will fail and give you a NULL pointer in return. And since you don't check for NULL the second time you attempt to use this NULL pointer and have a crash.

Simple solution: Remove the second and unchecked call to fopen.

answered Sep 05 '22 at 12:17

Some programmer dude

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I removed the first one and it worked fine than you very much – Majd Zahra Sep 05 '22 at 13:03
@MajdZahra The `fopen` call can still fail. You really need to check for that like you did in the first call. – Some programmer dude Sep 05 '22 at 14:30

why the program give "Segmentation fault" after 1018 try

1 Answers1