C / C++ / MFC

Re: What am I doing wrong defining single bit?

7-Jan-18 15:23

You can only have bit fields in a struct or union, it would be physically stupid to have them outside a struct or union because they couldn't be packed anyhow you might as well pick any normal type. Your somewhat pointless definition (it saves zero memory space) would be

typedef struct {
    unsigned bit : 1; // single bit
} BIT;

The initialize would be

static BIT bit = { 0 };
or  
static BIT bit = {.bit = 0}; // C11 and above standard

The only point to bit fields is to save space or make naming and offsets easier in particular for hardware registers.
The other recommendation I make is only define them as sign or unsigned you don't have to assign a normal C type the
compiler internally knows how to cast bit fields to do operations on them.

Usually what you are doing on hardware registers is assigning the bits to match something like an 8 bit definition

typedef struct __attribute__((__packed__, aligned(1))) {
    unsigned Clk : 1; // clk bit
    unsigned Bit : 1; // Data bit
    unsigned CS : 1;  // Chip select
    unsigned _reserved: 5; // 5 unused bits
} MYREGISTER;

In the above you have 8 bits packed into a byte so it saves space carrying them.

In vino veritas

modified 7-Jan-18 21:41pm.

Vaclav_8-Jan-18 5:37

Vaclav_

8-Jan-18 5:37

Thanks

C program to reverse the order of the words entered.

Tarun Jha7-Jan-18 6:45

Re: C program to reverse the order of the words entered.

7-Jan-18 6:45

for ex,
input: My name is Bay Max.
output: Max Bay is name My.

below is my code, but its too long and messy. How do i make it more concise and readable?
Any alternative would be appreciated.

C++

#include <stdio.h>
#include <string.h>

#define EOL '\n'
#define MAX_SIZE 100

int main(){

    char text[MAX_SIZE], back[MAX_SIZE], temp[MAX_SIZE];
    int count, i, mark[50];

	//reading the text
    for(count=0; (text[count] = getc(stdin))!= EOL; ++count){
        mark[count]=0;
    }
    text[count] = '\0';

	//reversing the text
    for(i = count-1; i>=0; --i){
        back[(count-1)-i] = text[i];
    }
    back[count] = '\0';

	//noting the frequencies of the occurences of space
	for(i=count-1; i>=0; --i){
		if(back[i]== ' '){
            mark[i] = i;
        }
	}

	//irr-reversing each words at their respective place
    int j, k=0, l=0;
    for(i=0; i<count-1; i++){
        if(mark[i] != 0){
            for(j=mark[i]-1; j>=k; j--){
                temp[l] = back[j];					//it reverse the words btw two space entries
                l++;
            }           
            temp[mark[i]] = ' ';
            l= l+1;
            k = mark[i]+1;
        }
    }
    for(i=count-1; i>=k; i--){
    	temp[l] = back[i];				//it reverse the word after the last space entry. 
    	l++;
	}
    
    //printing the string.
    printf("\n%s", temp);
    return 0;
}

Thank you.

PIEBALDconsult7-Jan-18 7:01

PIEBALDconsult

7-Jan-18 7:01

I'd make a map of the words (array of pointers to char), then iterate it backward.

Re: C program to reverse the order of the words entered.

David Crow7-Jan-18 15:50

Write data to file which cannot be viewed

7-Jan-18 15:50

I would be tempted to use strtok(). Then just read the returned value in reverse order.

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

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

manoharbalu5-Jan-18 2:09

manoharbalu

5-Jan-18 2:09

I am developing a c++ application which deals secured information which should not be viewed but to be saved to a file.

Is there a way to write to a file which cannot be viewed directly by the user from disk whereas it could be read by the program.

David Crow5-Jan-18 2:16

5-Jan-18 2:16

manoharbalu wrote:

Is there a way to write to a file which cannot be viewed directly by the user from disk whereas it could be read by the program.

Encrypt it?

Richard MacCutchan5-Jan-18 2:35

Richard MacCutchan

5-Jan-18 2:35

Don't save it on the user's system.

jschell5-Jan-18 13:45

jschell

5-Jan-18 13:45

manoharbalu wrote:
Is there a way to write to a file which cannot be viewed directly by the user

no.

leon de boer6-Jan-18 4:45

Writing a huge structure data to file

6-Jan-18 4:45

Do what Richard says encrypt it when your write and decrypt when you read it.

You can go from simple XOR encryption if it isn't that important to the real heavy stuff and there is a pile of libraries and code out there.

In vino veritas

manoharbalu4-Jan-18 22:29

manoharbalu

4-Jan-18 22:29

I am using huge structure template and size is around 100kb with various member variables like char array, int, long and short. I am writing this entire structure using fwrite. But when I retreive the structure data from file using fread, some of the structure members especially long variable shows incorrect value.

Kindly suggest me if writing a full structure is wrong or how to work around.

Re: Writing a huge structure data to file

Richard MacCutchan4-Jan-18 23:27

Richard MacCutchan

4-Jan-18 23:27

Without seeing your code it is impossible to guess what you are doing wrong. How are you writing it, with a single fwrite call, or separate calls for each structure item?

Re: Writing a huge structure data to file

jschell5-Jan-18 13:49

jschell

5-Jan-18 13:49

manoharbalu wrote:
Kindly suggest me if writing a full structure is wrong

As described probably it is incorrect. Even the design is suspect. Such a large collection must be divisible and as such it should be programmed that way. Note of course that this doesn't mean it cannot be handled as a serialized data stream. That is in fact what all files on disk are exactly.

Other than that it is likely that you have a bug in your code that reads or writes the data. Probably you have a size wrong. If the process was broken into smaller pieces (as I said above) then you could individually test each of those and thus insure that the entire thing was valid.

Re: Writing a huge structure data to file

leon de boer6-Jan-18 4:11

How to return a string from a user defined function to main function ?

6-Jan-18 4:11

Have you made sure you open the file for write in binary mode.

FILE * binfile = fopen("somefilename.bin", "wb"); //Open writable bin file

You open it with just "w" command and it's in text mode some of the raw struct characters will be translated to tabs, CR, LF etc.

In vino veritas

Tarun Jha3-Jan-18 3:21

Re: How to return a string from a user defined function to main function ?

3-Jan-18 3:21

#include <stdio.h>
#include <string.h>

int input();

int main()
{
	char name[100];
	
	printf("Enter your name:\n");
	strcpy(name,input());
	
	printf("\n%s", name);
	
	return 0;
}

int input()
{
	char c[100];
	gets(c);
	
	return(c);
}

i am getting this error while running this code.
[Warning] passing argument 2 of 'strcpy' makes pointer from integer without a cast.

I have tried this code. I know I can do it similarly as that of returning an array from a function to main, but that's too long is there any alternative short method.

David Crow3-Jan-18 6:24

Re: How to return a string from a user defined function to main function ?

3-Jan-18 6:24

input() returns an int. strcpy() is expecting a char pointer.

Tarun Jha3-Jan-18 7:20

Re: How to return a string from a user defined function to main function ?

3-Jan-18 7:20

how do we use char *function(), and does it returns character, like int function() returns an integer ?

David Crow3-Jan-18 7:29

Re: How to return a string from a user defined function to main function ?

3-Jan-18 7:29

Tarun Jha wrote:
how do we use char *function(), and does it returns character...

It returns a pointer to a character.

However, C is not a strong-typed language so you can coerce that into many things. Pointers are a prime example of C being known for giving you just enough rope to hang yourself. Just sayin...

Jochen Arndt3-Jan-18 12:48

Jochen Arndt

3-Jan-18 12:48

There is no simple method in C to return a string from a function.

Possible methods are:

1. Passing a char* pointer to the function and the function fills the passed buffer. In such cases the size of the buffer should be also passed and the return value indicates failure (usually -1) or success (usually length of copied string):

int input(char *buf, int buf_size)
{
    char local_buf[100] = "";
    // Use fgets here instead of gets to avoid buffer overruns
    fgets(local_buf, sizeof(local_buf), stdin);
    int len = strlen(local_buf);
    if (buf_size <= len)
        return -1;
    strcpy(buf, local_buf);
    return len;
}

2. Letting the function allocate a buffer. The calling function must free the buffer when no longer used:

char *input()
{
    char local_buf[100] = "";
    // Use fgets here instead of gets to avoid buffer overruns
    fgets(local_buf, sizeof(local_buf), stdin);
    return strdup(local_buf);
}

// Usage
char *input_str;
input_str = input();
// Use string here
// ...
// Finally free it
free(input_str);

However, in your case there is no need for an own function because it is already provided by the standard C library: fgets - C++ Reference[^].

Re: How to return a string from a user defined function to main function ?

Member 115604903-Jan-18 14:48

Member 11560490

3-Jan-18 14:48

You can't do this. Because array c is released when input() function execute completed. Maybe you can used "static" key word to prevent array c to released or pass a buffer to input() function as parameter.

Re: How to return a string from a user defined function to main function ?

leon de boer3-Jan-18 15:07

Re: How to return a string from a user defined function to main function ?

3-Jan-18 15:07

As per your last post lets fix my code from it into a function .. which is what I assume you are trying to do

int StringInput (char* name, int namemax)
{ 
   int c;
   int namelen = 0;  // initialize array position index to zero
   if (name && namemax > 1) {  // Lets be safe and check pointer valid and space > 1
      do {
         c = getc(stdin);  // fetch the integer keyboard read value .. EOF safe
         name[namelen++] = (char) c;  // convert the integer to a char and place in array .. increment position
      } while (c !=EOF && c != '\n' && namelen < namemax-1);  // repeat until EOF or new line or hit max length-1 (need space for '\0')
      temp.name[namelen-1] = '\0';  // Make C null terminated string (overwrite last EOF or '\n')
   }
   return(namelen); // return the name length
}

/** HOW to USE *//
int main(void)
{
   char name[100];
   int len = StringInput(&name[0], sizeof(name));
   // OR if you don't need length just
   StringInput(&name[0], sizeof(name));
}

Now if this is for uni or commercial then you probably want this form ... An excercise for you is to understand why and how to do it

int StringInput (const char* name, const int namemax, const FILE* source)

In vino veritas

modified 3-Jan-18 21:27pm.

Tarun Jha3-Jan-18 19:29