C program crashes with Seg Fault but work perfectly fine under LLDB

Question

I'm writing a Vanilla File read code.

Most of it look like this.

Firstly the header file file.h

// fheader.h
#ifndef __file_h__
#define __file_h__
// start from here
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
void readFile(FILE *fr);

FILE* openFile(char *file_name);
#endif

the main file fmain.c

#include "fheader.h"


void readFile(FILE *fr) {
    // Added Printf
    printf("\n...\n");
    printf("\n...\n");
    printf("\n...\n");
    printf("\n...\n");
  char *buffer = calloc(sizeof(char),1);
  while(!feof(fr)) {
    fread(buffer,sizeof(char),1,fr);
    printf("%s\n",buffer);
  }
  free(buffer);
  return;
}


FILE* openFile(char *file_name) {
  // printf("the file name that is going to be opened is %s",file_name);
  FILE *fr;
  fr = fopen(file_name,"r");
  return fr;
}

int main(int argc,char *argv[]) {
  if(argc < 2) {
    printf("USAGE: ./file test.txt\n");
    return 1;
  }

  if (argc > 2) {
    printf("ERROR: Too many argument\n");
    return 1;
  }
  FILE *fr;
  char *file_name = calloc(strlen(argv[1]),sizeof(char));

  strncpy(file_name,argv[1],strlen(argv[1]));
  fr = openFile(file_name);
  printf("\nReading from file\n");
  readFile(fr);
  fclose(fr);
  free(file_name);
  return 0;
}

I compiled the code using following command

gcc -g3 -Wall fmain.c -o file.o

When I ran the code

./file.o "~/workspaces/myWork/C_experiment/test.txt"

I see Segmentation fault: 11

But when I run the above program in lldb I work and exit with return code 0

lldb ./file.o
(lldb) run "~/workspaces/myWork/C_experiment/test.txt"
// output of the file 
Process 28806 exited with status = 0 (0x00000000)
(lldb) quit

Now, I'm clueless as to how to debug the code and find the Seg Fault reason.

Answer 1

you forgot to add the '\0' at the end of the file_name and when you do a fread to read 1 char, you don't add a '\0' at the end of the buffer. And if you read only one character you should use a char instead of an array of 1.

The errors are here :

 char *file_name = calloc(strlen(argv[1]),sizeof(char));
  strncpy(file_name,argv[1],strlen(argv[1]));
  fr = openFile(file_name);

You copy the argv[1] but you don't add a '\0' at the end. So how fopen will know where your string stop ? You should add a +1 to calloc and after the strncpy, you should add the '\0' like that:

file_name[strlen(argv[1])] = '\0';

The second error is here:

 char *buffer = calloc(sizeof(char),1);
  while(!feof(fr)) {
    fread(buffer,sizeof(char),1,fr);
    printf("%s\n",buffer);
  }

You allocate 1 to your buffer and read 1, that's ok but when you send it to printf, you didn't add a '\0' to your buffer so how can printf know where to stop ? you should calloc 2 and not one and then add a buffer[1] = '\0';

So with these fixes:

#include "fheader.h"


void readFile(FILE *fr) {
  char buffer;
  while(!feof(fr)) {
    fread(&buffer,sizeof(char),1,fr);
    printf("%c\n",buffer);
  }
  return;
}


FILE* openFile(char *file_name) {
  // printf("the file name that is going to be opened is %s",file_name);
  FILE *fr;
  fr = fopen(file_name,"r");
  return fr;
}

int main(int argc,char *argv[]) {
  if(argc < 2) {
    printf("USAGE: ./file test.txt\n");
    return 1;
  }

  if (argc > 2) {
    printf("ERROR: Too many argument\n");
    return 1;
  }
  FILE * fr = openFile(argv[1]);
  printf("\nReading from file\n");
  readFile(fr);
  fclose(fr);
  return 0;
}