Segmentation fault reported in mac terminal - c

Question

We were supposed to extract strings from a provided file, the output matches the expect, but it reports segmentation fault in the end and I don't know why.
#include<stdio.h>
#include<string.h>
int main(int argc, char *argv[]){
char str[100];
char f;
int len = 0;
FILE *file;
file = fopen(argv[1],"r");
//read only here, so use "r"
if(file==NULL){
printf("The file doesn't exist.\n");
return 1;
}
while(feof(file)==0){
//if feof returns 0 it means it havent reaches the end yet
fread(&f,sizeof(f),1,file);//read in the file
//printabel character between 32 and 126
if(f>=32&&f<=126){
str[len] = f;
len++;
continue;//keep doing it(for ->while)
}
if(strlen(str)>3){
//a string is a run of at least 4
printf("The output is:%s\n",str);
len=0;//reset
memset(str, 0, sizeof(str));
//reset the str so it wont get too big(overflow)
}
}
//close the file and return
fclose(file);
return 0;
}

Answer 1

This is not true
while(feof(file)==0){
//if feof returns 0 it means it havent reaches the end yet
And a very common mistake.
This returns 0 if you have Not read past the end of file. Its s subtle but important detail. Your last read may have read up-to the end of file but not past it. This means there is actually no data left to read but feof() will still return 0.
This is why you must test the result of the read operation.
fread(&f,sizeof(f),1,file);
If this returns zero then you failed to read anything.
Which is why you should structure your loop to test the result of the read (not feof()).
while (fread(&f,sizeof(f),1,file) == 1)
{
// You have successfully read an object from the stream
}

Answer 2

Your code has some fundamental errors:
See Why is while ( !feof (file) ) always wrong?
You don't check if fread returns 0, meaning that no more character could be
read, yet you continue with your algorithm
str is not '\0'-terminated, the strlen(str)>3 yields undefined
behaviour in the first iteration and will likely be evaluated as true right in the first iteration.
Then the printf would also yield undefined behaviour for the same reason.
Don't use the ASCII code directly, it's hard to read, you have to look up in
the ASCII table to see what 32 is and what 126. Better use the character
constants
if(f>= ' ' && f <= '~'){
...
}
This is easier to read and you get the intention of the code immediately.
So the program can be rewritten like this:
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]){
char str[100];
char f;
int len = 0;
FILE *file;
file = fopen(argv[1],"r");
//read only here, so use "r"
if(file==NULL){
printf("The file doesn't exist.\n");
return 1;
}
memset(str, 0, sizeof str);
while(fread(&f, sizeof f, 1, file) == 1)
{
if(f >= ' ' && f <= '~')
{
str[len++] = f;
continue;
}
if(strlen(str) > 3) // or if(len > 3)
{
printf("The output is: %s\n", str);
len = 0;
memset(str, 0, sizeof str);
}
}
fclose(file);
return 0;
}