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Nov 23rd, 2008

help about fread and fwrite

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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.

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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Whilliam

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Nov 23rd, 2008

Re: help about fread and fwrite

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:

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:

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.

ArkM

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Nov 23rd, 2008

Re: help about fread and fwrite

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.

Clockowl

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Nov 23rd, 2008

Re: help about fread and fwrite

Click to Expand / Collapse Quote 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

...

ArkM

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Nov 23rd, 2008

Re: help about fread and fwrite

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.

Clockowl

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Posting Whiz

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Nov 24th, 2008

Re: help about fread and fwrite

Click to Expand / Collapse Quote 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:
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:
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..

Last edited by Whilliam; Nov 24th, 2008 at 7:19 am.

Whilliam

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