I need help reading from and writing to files (C)

[FaiL___]

So, I've been trying to (just at the minute) print out a text file line by line. I will eventually need to get it reading into an array, but I'll be able to do that once I get the printout to work. I haven't even attempted writing to it yet, but here's what I have so far...

void ReadToArrayFromTextFile()
{
	FILE *fptr;
	fptr = fopen("keys.txt", "r");
	char line[256];

	while (fgets(line, sizeof(line), fptr))
	{
		printf("%s", line);
	}
	fclose(fptr);
}

 

[reniat]

What issue are you having? Are you just asking for help on writing, or are you getting incorrect behavior when reading?

[FaiL___]

7 hours ago, reniat said:

What issue are you having? Are you just asking for help on writing, or are you getting incorrect behavior when reading?

When I call the function, nothing get's printed. Sorry, I thought i said that, was pretty tired writing this post!

[JoostinOnline]

I've got a few notes, partly just about coding practices.

 

1) Get in the habit of defining your variables.  It saves headaches.  For example:

char line[256] = {0};

2) The while should be an if-else, and you should give output in case it fails:

if (fgets(line, sizeof(line), fptr) != NULL)
{
	printf("%s\n", line);
} else {
	printf("Error: failed to read file\n");
}

3) In situations like this, I suggest using a macro I wrote for debugging.  Add this below the includes:

#include <stdio.h>
#include <stdarg.h>

#define ENABLE_DEBUG

#ifdef ENABLE_DEBUG
void debug(const char *format, ...) {
	va_list args;
	va_start(args, format);
	vprintf(format, args);
	va_end(args);
}
#else
#define debug(...)
#endif

You can now use the "debug" function just like printf.  If you comment out

#define ENABLE_DEBUG

then all "debug" functions will be ignored by the compiler.  It's a great way to give a bunch of output for testing, while allowing you to take it all out with two forward slashes.

 

If you don't want the error to show up in the final product, you can do this:

if (fgets(line, sizeof(line), fptr) != NULL)
{
	printf("%s\n", line);
} else {
	debug("Error: failed to read file\n");
}

If "keys.txt" doesn't exist, you'll only see an error if ENABLE_DEBUG is defined.

 

My guess is that it's failing to open keys.txt.

[JacobFW]

On 3/22/2018 at 4:35 PM, FaiL___ said:

So, I've been trying to (just at the minute) print out a text file line by line. I will eventually need to get it reading into an array, but I'll be able to do that once I get the printout to work. I haven't even attempted writing to it yet, but here's what I have so far...

void ReadToArrayFromTextFile()
{
	FILE *fptr;
	fptr = fopen("keys.txt", "r");
	char line[256];

	while (fgets(line, sizeof(line), fptr))
	{
		printf("%s", line);
	}
	fclose(fptr);
}

 

1)  Make sure the file you use for testing this is guaranteed to work.  This is of course, unrealistic for a working program, but for testing the code and getting it to work, it's fine.

In your case, since your buffer is at most 255 chars + EOS mark, make sure each line is less than 255 characters.

 

2)  Make sure you're in the correct directory.  I'd recommend commenting out everything in the function, and just run put in place something like

void ReadToArrayFromTextFile()
{
	FILE *fptr;
  
	fptr = fopen("TestFile_FindMe.txt", "w");
  
	//fptr = fopen("keys.txt", "r");
	// char line[256];

	//while (fgets(line, sizeof(line), fptr))
	//{
	//	printf("%s", line);
	//}
	fclose(fptr);
}

Then make sure the keys.txt file is in the same directory as the TestFile_FindMe.txt file.

 

3)  I'd recommend adding in some more checks.

For instance, after opening the file, check for the pointer to be null.  If it is, then you know it failed to open the file at all, instead of it being an error during the reads.

 

4)  And I'd also recommend in the while loop going ahead and being more specific in the condition.

while(fgets(...) != NULL)

 

Probably not necessary, but the goal is to limit sources of error.