Segmentation fault block

Question

FILE *fp;

fp = fopen(filen, "wb");

const char tok[2] = ",";
char str[340];

while (fgets(str, 340, stdin) != NULL)
{
  struct test loadTest;
  printf("You entered: %s", str);
  strncpy(loadTest.level, strtok(str, tok), 20);
  strncpy(loadTest.first, strtok(NULL, tok), 30);
  fwrite(&loadTest, sizeof(struct test), 1, fp);
}

fclose(fp);

Hello all,

For some reason I'm getting a segmentation fault error in my code.

I'm almost positive the error is somewhere within the small code block above (since that's all I modified for the seg fault), but I can't seem to pinpoint it.

I know segmentation faults have to do with accessing memory I shouldn't be accessing, but I'm not sure where I am even doing that in the code.

Any help would be greatly appreciated!

Are you sure that length of tokens is fixed to specified 20 and 30? — Lemonov, Aug 30 '17 at 05:35
Where do you check that you successfully opened the output file? — Jonathan Leffler, Aug 30 '17 at 05:37
Why not use `sizeof(str)` instead of `340` in the call to `fgets()`? Why not use `sizeof(loadTest.level)` instead of `20` in the call to `strncpy()`? Similarly for `loadTest.first`? Why no check on whether the write succeeds? — Jonathan Leffler, Aug 30 '17 at 05:40
@Lemonov, yes I double checked the token lengths. yes I have tried, it just leads me to segfault after first step for some reason. I have just added those but I'm still getting the error. Thanks for your help though! — dooder, Aug 30 '17 at 05:48
Your debugger will tell you where exactly the segfault occurs. Use it. If you don't know how to use your debugger it's probably a good occasion to start learning now. Also read this please: [mcve]. And don't use `strncpy`, the destination string may be not NUL terminated, read [this](https://stackoverflow.com/questions/1453876/why-does-strncpy-not-null-terminate) — Jabberwocky, Aug 30 '17 at 06:56
show the whole code, not just a part. what is `filen`? it is initialized? why you not check if the file is open without error? `if (fp == NULL) return;` 99% that you haven't open a file and `fp` is NULL. — vadim_hr, Aug 30 '17 at 07:26
Maybe there is no second `,` and `strtok(NULL, tok)` returns NULL. — i486, Aug 30 '17 at 08:02
See whether output file is created and there is something inside or it is empty. — i486, Aug 30 '17 at 08:26

vadim_hr · Answer 1 · 2017-08-30T10:08:33.437

-1

Some improvements of your code

check result of fopen before read/write to file
initialize variables before using their values
use sizeof instead of constant (as mentioned in comments)
strtok() can return NULL and this must be checked (see here why)
you must use strncpy() carefully because of this

Here is corrected version of your code

FILE *fp;

fp = fopen(filen, "wb");

if (fp == NULL) 
{
    printf("Error opening file: %s", filen);
}
else
{

    const char tok[2] = ",";
    char str[340];

    while (fgets(str, sizeof(str), stdin) != NULL)
    {
        struct test loadTest;
        char *level;
        char *first;
        memset(&loadTest, 0, sizeof(loadTest)); 
        printf("You entered: %s", str);
        level = strtok(str, tok);
        if (level == NULL)
        {
            continue; // bad input ?
        }
        first  = strtok(NULL, tok);
        if (first == NULL)
        {
            continue;
        }
        strncpy(loadTest.level, level, sizeof(loadTest.level)-sizeof(char));
        strncpy(loadTest.first, first, sizeof(loadTest.first)-sizeof(char));
        fwrite(&loadTest, sizeof(loadTest), 1, fp);
    }

    fclose(fp);
}

edited Aug 30 '17 at 10:08

answered Aug 30 '17 at 07:51

vadim_hr

533
5
11

These are improvements but don't think they will stop the seg fault. Post them like comment. – i486 Aug 30 '17 at 08:03
@i486 what i need to post like comment? I already added comment to the question before posting answer. I think segfault appear because the file isn't opened successfully. `fp` is NULL after `fopen`. – vadim_hr Aug 30 '17 at 08:10
You can only suspect that `fopen` is the problem. It is not 100% sure => the solution is for comment, it is not answer. – i486 Aug 30 '17 at 08:24
@i486 I can't be 100% sure without see the whole code. only OP can tell why he get segfault error. thanks for your opinion, but i can't delete answer after posting. – vadim_hr Aug 30 '17 at 10:20

Segmentation fault block

1 Answers1