Thread: reading long integer from binary file

hi all,

I'm having trouble reading a long integer from a binary file. The long int is 4 bytes long.

i have, for example:

Code:

byte_t bytes_read;
long temp;

bytes_read = fread(&temp, sizeof(long)*4, 1, filePtr);

that didn't work so i tried:

Code:

byte_t bytes_read;
long* temp;
temp = (long*)malloc(sizeof(temp));

bytes_read = fread(temp, sizeof(long)*4, 1, filePtr);

that still doesn't work

any ideas? i think it's trivial but i can't wrap my head around it...thanks!

1) read what tabstop wrote.
2) bytes_read is a misnomer here -- fread returns 'the number of items successfully read' -- the third argument. Here, it will return 1 on success. Read thy man pages.
3) Finally, if this is a binary file you're reading, are you aware of endianness? Using your code, the endianness of the file and the host machine must match. There are better ways to write I/O code that will read a binary integer from a file, regardless of host machine endianness.

Originally Posted by Cactus_Hugger

There are better ways to write I/O code that will read a binary integer from a file, regardless of host machine endianness.

The important factor isn't the endianness of the machine, but the endianness of the quantity within the file. That needs to be specified before anything else can take place. There are three possibilities: big endian, little endian, and native. If the specification is for "native," then it is perfectly acceptable (and more efficient) to read the value directly, as is happening here.

Originally Posted by tabstop

Why are you multiplying sizeof(long) by 4? Do you have four longs that you somehow want to read into one number?

the sizeof(long)*4 is just telling fread() to read 4 bytes at a time. so: fread(test, sizeof(long)*4 , 1, file) means it will read 1 block of 4 bytes once...unless i'm confused with this part. Regardless, fread(test, sizeof(long), 4, file) which reads a long type 4 times doesn't work as well...

Originally Posted by jason_m

Double check how many bytes are are telling fread() to read in.

i've specified fread to read 4 bytes at one time

Originally Posted by Cactus_Hugger

1) read what tabstop wrote.
2) bytes_read is a misnomer here -- fread returns 'the number of items successfully read' -- the third argument. Here, it will return 1 on success. Read thy man pages.
3) Finally, if this is a binary file you're reading, are you aware of endianness? Using your code, the endianness of the file and the host machine must match. There are better ways to write I/O code that will read a binary integer from a file, regardless of host machine endianness.

i know bytes_read returns the number of bytes read..i was referring to the memory address of test...sorry for the confusion!

In the file format...the number is only 4 bytes long and that's what i've specified fread to do, read once 4 bytes (of type long) at a time...unless i'm completely misunderstanding what i just wrote...

thanks for all your help!!

the sizeof(long)*4 is just telling fread() to read 4 bytes at a time

No. A long probably is 4 bytes on your system, so sizeof(long) is probably 4. Thus you're asking for 1 item of 16 bytes, which is what tabstop was originally trying to point out to you.

i know bytes_read returns the number of bytes read

I don't think you understand me -- fread() does not return the number of bytes read. It returns (as the manual says) the number of items read. If an 'item' is a 'byte' then in that case only will it return the number of bytes read. That case is not yours, given how you're calling fread(). You are saying (currently) "read 1 sizeof(long)*4 bytes long item" -- it will return 1 on success, 0 on failure in this case for the arguments you are passing it. (but you should probably be reading just sizeof(long), if you just want 1 integer.)

The important factor isn't the endianness of the machine, but the endianness of the quantity within the file.

I apparently wasn't clear, but yes -- that is my point. The OP must know the endianness of his file. I would rule native out -- it's fairly rare. If the OP is writing the file himself as 'native', it is only because he is unaware of endianness in the first place.

No one said sizeof(long *) == sizeof(long).

A few suggestions:
* Error checking
* Don't cast malloc()
* Free when you're done.

In fact, I wouldn't be using malloc() for 4 bytes :\

My suggestion: Stop guessing.

Originally Posted by tabstop

Yes, but sizeof(temp) is not necessarily going to be the size of a long -- sizeof(long) is going to be the size of a long, and temp is not a long variable -- it's a pointer. (Now granted on many machines in current use these are the same size, but they don't have to be.) The typical idiom is to use sizeof(*temp) -- the size of the thing temp points to -- in your malloc and your read.

oh ic...that's right...that makes a lot more sense but it still doesn't work...i'm still having problems:

Code:

int main(int argc, char* argv[])
{
	if(!InitCUDA()) {
		return 0;
	}

	char* fileID;
	long tempIntReader;
	size_t bytes_read;

	/* File Input Variables */
	int lenFileName;
	char *fileName;
	FILE *whichFile;
	char* buf;

	buf = (char*)malloc(sizeof(char)*17);
	//tempIntReader = (long*)malloc(sizeof(long));

	/* # pixels and # bytes in a frame*/
	int frm_size;

	INT16_T *h_idata;	/* input data on host */
	float *h_odata;		/* image data on host */

    //frm_size = 512 x FFT_LENGTH;	/* #pixels = image width x image length */

	printf("Please enter the full file name (i.e. imagefile.raw)\n-> ");
	scanf("%c", &fileName);
		
	if((whichFile = fopen("1.RAW", "r"))== NULL) {
		printf("Error while loading data from file %s.\n", fileName);
		exit(0);
	} else {
		bytes_read = fread(buf, sizeof(buf), 17, whichFile);
		
		for(int i = 0; i < 17; i++){
			printf("%c", buf[i]); 
		}
		
		memset(buf, '\0', sizeof(buf));

        printf("\n");
        bytes_read = fread(&tempIntReader, sizeof(long), 1, whichFile);
        printf("%d", tempIntReader);

i tried that first, just reading in a long int and saving it to tempIntReader

then i tried:

Code:

long* tempIntReader;
tempIntReader = (long*)malloc(sizeof(long));
fread(tempIntReader, sizeof(long), 1, whichFile);

then i tried:

Code:

long* tempIntReader;
tempIntReader = (long*)malloc(sizeof(long));
fread(tempIntReader, sizeof(*tempIntReader), 1, whichFile);

i continously get this error:
Unhandled exception at 0x00403508 in CUDAWinApp4.exe: 0xC0000005: Access violation reading location 0xffffff31.

i'm more confused now than ever because I'm reading the right about of bytes, I now have the right size to save what was read in and I have the right type. What am I missing?

I tried resetting the iterator for the filePtr by:
fseek(whichFile, 17, SEEK_SET);

which sets the iterator at the exact spot of my long int