fread, fwrite and fclose

Guys, please help me understand what wrong with the below code. I just trying a simple read from an existing file and output it to terminal.

Code:

#include<stdio.h>

int main() {
   FILE *filed;
   char *buffer;

   filed = fopen("ffile.txt", "r");
   fread(buffer, 64, 5, filed);
   fwrite(buffer, 64, 5, stdout);
   fclose(filed);
      exit(0);
}

When i compile and execute the above code I get the following:

Code:

Segmentation fault (core dumped)

Definately its something with syntax of fwrite and fread.. but what?

I don't know if this is the problem but are you sure the file is opened successfully? you should validate it first.

Guys I am able to find out what it was.... it was syntax in fread and fwrite. I changed the following lines of code and it worked great:

Code:

#include<stdio.h>

int main() {
   FILE *filed;
   char *buffer;

   filed = fopen("ffile.txt", "r");
   fread(&buffer, 64, 5, filed);
   fwrite(&buffer, 64, 5, stdout);
   fclose(filed);
      exit(0);
}

Originally Posted by jas_atwal

Guys I am able to find out what it was.... it was syntax in fread and fwrite. I changed the following lines of code and it worked great:

Really? It's not meant to work that well. You probably should, rather, allocate some memory of 5 * 64 for your buffer - right now, you are storing 320 bytes into the location that holds buffer, which is definitely not the right thing to do.

If you put

Code:

char abc[] = "abc";

after char *buffer;
and

Code:

printf("abc = %s", abc);

after your fclose, I bet you that you won't get "abc" printed!

--
Mats

Originally Posted by matsp

Really? It's not meant to work that well. You probably should, rather, allocate some memory of 5 * 64 for your buffer - right now, you are storing 320 bytes into the location that holds buffer, which is definitely not the right thing to do.

If you put

Code:

char abc[] = "abc";

after char *buffer;
and

Code:

printf("abc = %s", abc);

after your fclose, I bet you that you won't get "abc" printed!

--
Mats

That is an educative input. Thanks Mats!


And you are correct. I get Segmentation fault as output when i use the above two lines of code.

Originally Posted by jas_atwal

Code:

#include<stdio.h>

int main() {
   FILE *filed;
   char *buffer;

   filed = fopen("ffile.txt", "r");
   fread(&buffer, 64, 5, filed);
   fwrite(&buffer, 64, 5, stdout);
   fclose(filed);
      exit(0);
}

That code is even more flawed than the first.
Do you even have compiler warnings enabled?
fread and fwrite does not take a char**, they take const char* and char* respectively.

Originally Posted by Elysia

That code is even more flawed than the first.
Do you even have compiler warnings enabled?
fread and fwrite does not take a char**, they take const char* and char* respectively.

Ok.. some changes I can think of

Code:

#include<stdio.h>

int main() {
   FILE *filed;
   char buffer[3200];

   filed = fopen("ffile.txt", "r");
   fread(&buffer, 64, 5, filed);
   fwrite(&buffer, 64, 5, stdout);
   fclose(filed);

      exit(0);
}

Better, but you're still passing char** to the functions. Do you have warnings enabled? It should warn you.

Originally Posted by Elysia

Better, but you're still passing char** to the functions. Do you have warnings enabled? It should warn you.

strange I am getting no warnings:

Code:

[jaspreet@td189 C]$ gcc -o ffile ffile.c
[jaspreet@td189 C]$

how do I check if warnings are enabled or not.?

and when I use &buffer in fread is it similar to char **? I am confused.

People suggest you use -Wall.

Originally Posted by Elysia

People suggest you use -Wall.

I used -Wall. I get a lot of output this time while compiling. But none seems related to char**. I'll appreciate if you will have a look at this.

Code:

collect2: ld terminated with signal 11 [Segmentation fault]
ffile(.data+0x8): In function `__data_start':
: multiple definition of `__dso_handle'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/crtbegin.o(.data+0x0): first defined here
ffile(.init+0x0): In function `_init':
: multiple definition of `_init'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crti.o(.init+0x0): first defined here
ffile(.text+0x0): In function `_start':
: multiple definition of `_start'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crt1.o(.text+0x0): first defined here
ffile(.fini+0x0): In function `_fini':
: multiple definition of `_fini'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crti.o(.fini+0x0): first defined here
ffile(.got.plt+0x0): multiple definition of `_GLOBAL_OFFSET_TABLE_'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crt1.o(.got.plt+0x0): first defined here
ffile(.rodata+0x0): multiple definition of `_IO_stdin_used'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crt1.o(.rodata+0x0): first defined here
ffile(.data+0x0): In function `__data_start':
: multiple definition of `__data_start'
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crt1.o(.data+0x0): first defined here
/tmp/ccFXIQ0i.o(.text+0x0): In function `main':
: multiple definition of `main'
ffile(.text+0xb8): first defined here
/usr/lib/gcc/x86_64-redhat-linux/3.4.6/../../../../lib64/crt1.o(.dynamic+0x0): multiple definition of `_DYNAMIC'
ffile(.dynamic+0x0): first defined here

I'm not well versed with that compiler. I don't understand much of its errors and warnings and I don't have any experience with Linux either.
But I don't get why it doesn't complain about char**, since it should.

> Better, but you're still passing char** to the functions. Do you have warnings enabled? It should warn you.
1. It isn't a char **, it is a pointer to an array. Given the code in post #7, the type of &buffer is actually char (*)[3020], not char **.

2. There's no reason why it would ever warn you, because fread/fwrite accept a void* as the first parameter. So long as it's a pointer (of any type), then all would be well.

Also, 64*5 is 320, not 3020

> I get a lot of output this time while compiling.
It looks like you might have forgotten the "-o" option on the command line.

Originally Posted by Salem

> Better, but you're still passing char** to the functions. Do you have warnings enabled? It should warn you.
1. It isn't a char **, it is a pointer to an array. Given the code in post #7, the type of &buffer is actually char (*)[3020], not char **.

2. There's no reason why it would ever warn you, because fread/fwrite accept a void* as the first parameter. So long as it's a pointer (of any type), then all would be well.

Also, 64*5 is 320, not 3020

> I get a lot of output this time while compiling.
It looks like you might have forgotten the "-o" option on the command line.

Thank you Salem

You can make your life easier by enabling warnings all of the time . . . .

Code:

$ alias gcc='gcc -W -Wall'

That should work, in bash at least. Then, whenever you type "gcc", you're actually executing "gcc -W -Wall". (You might even want to use "gcc -W -Wall -ansi -pedantic".)

You can even insert that into your ~/.bashrc so you don't have to type it all of the time . . . .