[SOLVED] Error in accessing to the content of the memory a pointer points to

sanfter · 01-23-2022, 11:20 PM

Dear friends,

I have some arrays of various data types and after converting them into a generic form I want to write them into a file byte by byte.
To do that, I refer to them using a void pointer and then I try
to write the content of it using a character pointer.

Actually I've written something like the following code but I get the segmentation fault error.

Code:

long int  *li_ptr;
char  *c_ptr;
void  *v_ptr;

char buffer[100];

*(li_ptr+0)=100;
*(li_ptr+1)=200;
*(li_ptr+2)=300;



v_ptr=li_ptr;   //  point to the content of long int pointer

c_ptr =(char *)v_ptr;  // point to the content of void pointer

buffer[0]= *(c_ptr+0); //  I get error on this line

Could you please give me a suggestion and help me solve the issue?

regards

NevemTeve · 01-23-2022, 11:45 PM

In this example you don't assign value to your pointers so they don't point to any data. Provide a more complete example. (Also please use [code] and [/code] tags.)

sanfter · 01-23-2022, 11:53 PM

Quote:

Originally Posted by NevemTeve

In this example you don't assign value to your pointers so they don't point to any data. Provide a more complete example. (Also please use [code] and [/code] tags.)

Thank you for the guidance. I edited and updated my question.

astrogeek · 01-24-2022, 12:06 AM

Welcome to LQ and the Programming forum!

Please wrap your code and data snippets inside [CODE]...[/CODE] tags. Doing so will preserve indentation and provide other visual clues which make it easier for others to comprehend. You may write those yourself as shown, or use the # button available with Advanced edit options. (A complete list of BBCode tags is always available via a link near the bottom of every thread view).

Quote:

Originally Posted by sanfter

I have some arrays of various data types and after converting them into a generic form I want to write them into a file byte by byte.

Please explain exactly what you mean by this statement, particularly "generic form".

Quote:

Originally Posted by sanfter

To do that, I refer to them using a void pointer and then I try
to write the content of it using a character pointer.

You can only access the data by its actual type, so casting a void pointer to a char * will only work it the pointer actually points to a char object.

Code:

long int  *li_ptr;
char  *c_ptr;
void  *v_ptr;

char buffer[100];

v_ptr=li_ptr;   //  point to the content of long int pointer

c_ptr =(char *)v_ptr;  // point to the content of void pointer

buffer[0]= *(c_ptr+0); //  I get error on this line

The first problem is that you never initialized any of those pointers, so any attempt to dereference them will result in undefined behavior - usually resulting a segmentation fault - attempt to reference illegal memory!

But even assuming that you had initialized li_ptr to point to an actual long integer type object, you cannot simply cast it to a char * and dereference it and get a meaningful result.

Please try to explain what you really want to accomplish with example data.

sanfter · 01-24-2022, 12:22 AM

Quote:

Originally Posted by astrogeek

Welcome to LQ and the Programming forum!

Please wrap your code and data snippets inside [CODE]...[/CODE] tags. Doing so will preserve indentation and provide other visual clues which make it easier for others to comprehend. You may write those yourself as shown, or use the # button available with Advanced edit options. (A complete list of BBCode tags is always available via a link near the bottom of every thread view).

Please explain exactly what you mean by this statement, particularly "generic form".

You can only access the data by its actual type, so casting a void pointer to a char * will only work it the pointer actually points to a char object.

Code:

long int  *li_ptr;
char  *c_ptr;
void  *v_ptr;

char buffer[100];

v_ptr=li_ptr;   //  point to the content of long int pointer

c_ptr =(char *)v_ptr;  // point to the content of void pointer

buffer[0]= *(c_ptr+0); //  I get error on this line

The first problem is that you never initialized any of those pointers, so any attempt to dereference them will result in undefined behavior - usually resulting a segmentation fault - attempt to reference illegal memory!

But even assuming that you had initialized li_ptr to point to an actual long integer type object, you cannot simply cast it to a char * and dereference it and get a meaningful result.

Please try to explain what you really want to accomplish with example data.

Dear Astrogeek

after updating my question, the li_ptr is pointing to some real data and my aim is to get various data types in a function in form of an array (pointer to long int, int, double and ... )and write the input data byte by byte in a file.

astrogeek · 01-24-2022, 12:23 AM

Quote:

Originally Posted by sanfter

Code:

long int  *li_ptr;
char  *c_ptr;
void  *v_ptr;

char buffer[100];

*(li_ptr+0)=100;
*(li_ptr+1)=200;
*(li_ptr+2)=300;

That is not a valid use of the li_ptr pointer - it still has never been initialized to point to any object of type long int, but you are trying to dereference it to initialze some value. You also appear to be treating it as an array, but it was not defined to be an array.

Here is a simple example which defines two objects of type long int named li_val and li_val2, and a pointer to long int named li_ptr. It then initializes li_val with a value of 100, and initializes the pointer with the address of the object. It then dereferences the pointer to get the value of li_val to assign it to li_val2.

Code:

long int li_val, li_val2;
long int *li_ptr;

li_val = 100;
li_ptr = &li_val;

li_val2 = *li_ptr;

But before we make too any guesses about what you are trying to accomplish, an example would be helpful!

sanfter · 01-24-2022, 02:01 AM

Quote:

Originally Posted by sanfter

Dear friends,

I have some arrays of various data types and after converting them into a generic form I want to write them into a file byte by byte.
To do that, I refer to them using a void pointer and then I try
to write the content of it using a character pointer.

Actually I've written something like the following code but I get the segmentation fault error.

Code:

long int  *li_ptr;
char  *c_ptr;
void  *v_ptr;

char buffer[100];

*(li_ptr+0)=100;
*(li_ptr+1)=200;
*(li_ptr+2)=300;



v_ptr=li_ptr;   //  point to the content of long int pointer

c_ptr =(char *)v_ptr;  // point to the content of void pointer

buffer[0]= *(c_ptr+0); //  I get error on this line

Could you please give me a suggestion and help me solve the issue?

regards

Dear Friends,

using your suggestions I managed to solve the problem.

Thank you all so much

astrogeek · 01-24-2022, 02:13 AM

Glad that helped!

LQ is all about sharing, so it would be in the spirit of things to share your solution with others who might land on this thread when looking for help with similar problems.

Good luck and welcome to LQ!

dugan · 01-24-2022, 04:23 PM

I have no idea what you're trying to do or why you're trying to do it, but I wonder if a standard serialization format (like Protocol Buffers or MessagePack) might be worth looking into.