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help about fread and fwrite

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Join Date: Oct 2008

Posts: 60

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Whilliam

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Junior Poster in Training

help about fread and fwrite

Nov 23rd, 2008

hello guys, Im trying to understand how fread and fwrite works. My teacher said that these functions can store/output binaries and texts in files. I know how to do it with string. Im confused with floats and integers.
I tried to make a program for it but Im getting errors. I can't run my program. Can anyone tell me whats wrong? Here is my code.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
int save(float d)
 {
	  FILE *fp;
	  char fn[50];
 
	  printf("What is the filename? ");
	  flushall();
	  gets(fn);
 
	  if((fp = fopen(fn, "wb")) == NULL)
	  {
			printf("Cannot save file");
			return 0;
	  }
 
	  fwrite(d, sizeof(float), 1, fp);
 
	  printf("\nFile saved");
	  fclose(fp);
	  return 0;
 }
 
 int load()
 {
	  FILE *fp;
	  float b;
	  char fn[50];
 
          printf("What is the filename? ");
          flushall();
	  gets(fn);
 
	  fp = fopen(fn, "rb");
	  fread(b, sizeof(float), 1, fp);
 
	  fclose(fp);
	  printf("%.2f", b);
	  return 0;
 }
int save(float d)
 {
	  FILE *fp;
	  char fn[50];

	  printf("What is the filename? ");
	  flushall();
	  gets(fn);

	  if((fp = fopen(fn, "wb")) == NULL)
	  {
			printf("Cannot save file");
			return 0;
	  }

	  fwrite(d, sizeof(float), 1, fp);

	  printf("\nFile saved");
	  fclose(fp);
	  return 0;
 }

 int load()
 {
	  FILE *fp;
	  float b;
	  char fn[50];

          printf("What is the filename? ");
          flushall();
	  gets(fn);

	  fp = fopen(fn, "rb");
	  fread(b, sizeof(float), 1, fp);

	  fclose(fp);
	  printf("%.2f", b);
	  return 0;
 }

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Join Date: Jul 2008

Posts: 2,001

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ArkM

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Postaholic

Re: help about fread and fwrite

Nov 23rd, 2008

Is it so hard job to correct obvious errors: you get compiler messages on wrong function arguments: fread and fwrite want POINTERS to target/source data areas (&d - not d, &b - not b))!

Use code tag properly:
[code=c]
source
[/code]

Never use dangerous gets function: it does not check target buffer size. Use fgets instead of gets:

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
fgets(fn,sizeof fn,sdtin);
fgets(fn,sizeof fn,sdtin);

With gets you program crashed if the file name length is greater than 49.

Don't forget to append carriage return:

 Help with Code Tags 
 c Syntax (Toggle Plain Text) 
printf("\nFile saved\n");
printf("\nFile saved\n");

Avoid using of non-standard function flushall().

Last edited by ArkM; Nov 23rd, 2008 at 10:45 am.

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Join Date: May 2008

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Clockowl

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Posting Whiz

Re: help about fread and fwrite

Nov 23rd, 2008

I'm more of a newb but feeling cocky as always:

Don't AVOID compiler specific functions, but be sure to find a cross-compiler way to do it as well, as in: find it, use it a couple of time, memorize it.

However, I think it's a definite plus if you also know compiler specific tweaks.

Last edited by Clockowl; Nov 23rd, 2008 at 2:26 pm.

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Join Date: Jul 2008

Posts: 2,001

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ArkM

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Re: help about fread and fwrite

Nov 23rd, 2008

•

Originally Posted by Clockowl

I'm more of a newb but feeling cocky as always:

Don't AVOID compiler specific functions, but be sure to find a cross-compiler way to do it as well, as in: find it, use it a couple of time, memorize it.

However, I think it's a definite plus if you also know compiler specific tweaks.

Well, repeat your admonitions when you will port 500000 LOC with compiler-specific calls in the new environment

...

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Join Date: May 2008

Posts: 371

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Clockowl

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Posting Whiz

Re: help about fread and fwrite

Nov 23rd, 2008

Nah I just meant to say that if you're sure that you'll only use it on your system, why not use those compiler specific functions? I think it's overkill to make basically everything you create cross platform, but if you're coding for your job then you should I guess... I'm a hobby coder.

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Join Date: Oct 2008

Posts: 60

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Whilliam

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Junior Poster in Training

Re: help about fread and fwrite

Nov 24th, 2008

•

Originally Posted by ArkM

Is it so hard job to correct obvious errors: you get compiler messages on wrong function arguments: fread and fwrite want POINTERS to target/source data areas (&d - not d, &b - not b))!

Use code tag properly:
[code=c]
source
[/code]

Never use dangerous gets function: it does not check target buffer size. Use fgets instead of gets:
Help with Code Tags

c Syntax (Toggle Plain Text)
fgets(fn,sizeof fn,sdtin);
fgets(fn,sizeof fn,sdtin);
With gets you program crashed if the file name length is greater than 49.

Don't forget to append carriage return:
Help with Code Tags

c Syntax (Toggle Plain Text)
printf("\nFile saved\n");
printf("\nFile saved\n");
Avoid using of non-standard function flushall().

thanks ArkM. Now I can do storing and getting data from files with integers and floats. Thanks for the tips too. I'm really slow. It's hard for me to solve obvious errors that I haven't encountered before..