C fread to read 512k binary file

Fossi Bluman

I use the bellow code to read binary file but it throws an exceptions at run time and sometimes gives a wrong results , please tell me what's the wrong ?

#include
#include

unsigned long size,n;
int main()
{
unsigned char *buf;
FILE* fp;
fp=fopen ("myfile.bin","rb");
if (fp == NULL) {
fprintf(stderr, "Can't open the file");
exit(1);}

fseek (fp,0,SEEK\_END);	//
size=ftell (fp);		//	get the size of the file
fseek (fp,0,SEEK\_SET);	//

if ((buf=(unsigned char\*) malloc (size+1))==0)
	printf("\\n%s","allocation Error");

n=fread (buf,sizeof (buf),size,fp);
printf ("\\n%02x %lu bytes read",buf\[15\],n);

free (buf);
fclose (fp);
return 0;

}

David Crow

Fossi Bluman wrote:

...it throws an exceptions at run time...

What exception? Where?

n=fread (buf,sizeof (buf),size,fp);
printf ("\n%02x %lu bytes read",buf[15],size);

I can tell you right now that these two lines will raise eyebrows. Among other things, size is the number of bytes to read, whereas n is the number of bytes actually read.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles

Daniel Pfeffer

in your fread() statement, you have: sizeof(buf) - the size of the pointer, at least 2 (on 16-bit processors), but typically 4 (32-bit) or 8 (64-bit) bytes these days. size - size of file in characters (each character is defined to be 1 unit in size) You are reading at least twice (four times / eight times) the data that you expected, overflowing the buffer. Perhaps you meant to write: n = fread(buf, sizeof(*buf), size, fp); As another poster mentioned, your printf() statement also looks funny.

If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack. --Winston Churchill

Fossi Bluman

this funny "fread" doing the job , exactly as i need , any way ,, how to make it not funny ?

Daniel Pfeffer

Examine the printf() statement closely. You will see the following formatting: %02x for buf[15] %lu for size The %02x formatting is meant for printing an unsigned int, but you are printing an unsigned char. In order to be certain that this works on all systems, cast buf[15] to unsigned int.

If you have an important point to make, don't try to be subtle or clever. Use a pile driver. Hit the point once. Then come back and hit it again. Then hit it a third time - a tremendous whack. --Winston Churchill

Fossi Bluman

then how to print the hexadecimal values in a format of two digits ?

k5054

DavidCrow wrote:

Among other things, size is the number of bytes to read, whereas n is the number of bytes actually read.

Just to clarify - the signature of fread() is

size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream);

where size is the size of each data element and nmemb is the number of elements read. The returned value is the number of data elements actually read. Thus

FILE *f;
struct mystruct foo[10];
size_t n;
n = fread(foo, sizeof(struct mystruct), 10, f);

after the call to fread(), n will contain the number of (struct mystruct) items read, not the number of bytes read. So if sizeof(struct mystruct) = 42 and the stream f contains 4 items between the current file pointer and the end of file, then fread() will return 4 (the number of items), not 168 (the number of bytes [42*4]).

David Crow

All true, but that doesn't change the issue that the OP is having. I was simply hinting at the fact that size and n may differ, and the parameter being sent to printf() was incorrect.

"One man's wage rise is another man's price increase." - Harold Wilson

"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons

"You can easily judge the character of a man by how he treats those who can do nothing for him." - James D. Miles

Software_Developer

Hello. Here is another solution.

//******************************************
// reading an entire binary file the c++ way

#include #include using namespace std;

int main (){

streampos size;
char * memblock;
cout<<" \n\n";

// open file as binary
ifstream file ("myfile.bin", ios::in|ios::binary|ios::ate);
if ( file.is_open() ){

// get file size
size = file.tellg();

// alloocate appropiate file size + 1 for '\\0'
memblock = new char \[size+1\];

// go to begining of file
file.seekg (0, ios::beg);

// read entire file into ram memory
file.read (memblock, size);
file.close();

// display size of file
cout << "the entire file content of "<< size <<" bytes is in memory  \\n\\n\\n";

// release memory
delete\[\] memblock;

}
else cout << "Unable to open file\n\n";

cout<<"Press ANY key to close.\n\n";
cin.ignore(); cin.get();
return 0;
}

Fossi Bluman

thank you for reply , but I use C language .