Segmentation fault block - c

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!

Answer 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);
}