Having few issues with my copy program which creates a copy of a file user enteres. I decided not to use (size_t) structure instead just assigned (int) and (char) types variables so I know exact value of bytes to read() out. ie I know start at beggining of file and read 4 bytes(int) to get value of lenght of filename, which I use as size in next read()
So, when I am writing (copying file exactly with same name) users inputted file to the output file (copied file) I writing it in long string, without spaces obviously just to make it readable here,
filenamesize filename filecontentsize filecontent
ie 10 myfile.txt 5 hello
So when come to reading that data out I start at begining of file using lseek() and I know the first 4 bytes are (int) which is lenght of filename so I put that into value int namelen using the read function.
My problem is I want to use that value read for the filenamesize(first 4 bytes) to declare my array to store filename with the right lenght. How do I put this array into read() so the read stores value inside that char array specified, see below please
int namelen; //value read from first 4 bytes of file lenght of filename to go in nxt read()
char filename[namelen];
read(fd, filename[namelen], namelen);//filename should have 'myfile.txt' if user entered that filename
So my question is once I read that first 4 bytes from file giving me lenght of filename stored in namelen, I then want to read namelen amount of bytes to give me the filename of originally file so I can create copied file inside directory?
Thanks
int namelen; //value read from first 4 bytes of file lenght of filename to go in nxt read()
char* filename = new char[namelen+1];
read(fd, filename, namelen);
filename[namelen]=0; // Just to keep readed buffer c-string compatible
do something with filename
delete[] filename;
In your previous question here, you did not upvote a single answer or accept any of them. You do appear to have used those answers though.
People who answered that earlier question might be inclined to help you here if you could be bothered to show a little gratitude for their earlier help by upvoting their answers and accepting the one that you found most helpful.
Related
I've been trying to write a binary file with some information in a program but I can't make it work. I write it and try to read it to see if it worked. This is the struct I'm trying to write inside the file:
typedef struct{
int puntuacio;
int posicio_x;
int posicio_y;
int vides;
int direccio;
}Jugador;
I've got a variable called player of type Jugador. In the function I work with the binary file I recieve player as a pointer (so Jugador *player). This is the code I've written (I only give the relevant parts):
f=fopen("whatever.bin","wb+");
fwrite(nom,sizeof(char),strlen(nom),f); //nom is a string containing the player's name
fwrite(&player,sizeof(Jugador*),1,f);
auxint=player->direccio; //just doing this to see if I pass the info correctly
fwrite(&auxint,sizeof(int),1,f);
//auxp, auxjug and auxint are auxiliar variables I declared inside the function
fseek(f,0,SEEK_SET); //go to the start of the file before reading
fread(auxp,sizeof(char),20,f);
fread(&auxjug,sizeof(Jugador),1,f);
fread(&auxint,sizeof(int),1,f);
printf("auxp:%s--\n",auxp);
printf("puntuacio:%d--\n",auxjug.puntuacio);
printf("dir:%d--\n",auxjug.direccio);
printf("posx:%d--\n",auxjug.posicio_x);
printf("posy:%d--\n",auxjug.posicio_y);
printf("vids:%d--\n",auxjug.vides);
printf("auxint:%d--",auxint);
auxp prints the name correctly but I get an additional garbage character in the last position of the string, but that's easy to solve. auxint prints perfect. But I get what I guess are memory adresses when I print the parameters of auxjug.
fwrite(&player,sizeof(Jugador*),1,f);
is only writing a pointer size element (4 or 8 bytes) to the file. You need to have:
fwrite(player,sizeof(Jugador),1,f);
without the extra & and the extra *.
Another problem is that you only output strlen(nom) bytes to the file. But when you read the file, you read exactly 20 bytes. So you should probably pad your nom string to 20 bytes and fwrite exactly 20 bytes to the file:
fwrite(nom,sizeof(char),20,f);
...
fread(auxp,sizeof(char),20,f);
I am having trouble reading a few lines of text from a file using fgets. The file is some basic user data that is written to a file within the bundle the first time the plugin is launched. Any subsequent launch of the plugin should result in the user data being read and cross referenced to check the users authenticity.
The data is always 3 lines long and is written with frwite exactly as it should be and is opened with fopen.
My original theory was to just call fgets 3 times reading each line into it's own char array which is part of a data struct. The problem is the first line is read correctly, the second line is read as though the position indicator starts on the next line but offset by the number of characters read from line 1. The third line is then not read at all.
fgets is not returning any errors and is behaving as though it has read the data it should have so i'm obviously missing something.
Anyway here's a portion of my code hopefully someone can some shed some light on my mistakes!
int length;
fgets(var.n, 128, regFile);
length = strlen(var.n);
var.n[length-1] = NULL;
fgets(var.em, 128, regFile);
length = strlen(var.em);
var.em[length-1] = NULL;
fgets(var.k, 128, regFile);
length = strlen(var.k);
var.k[length-1] = NULL;
fclose(regFile);
Setting the last character in each string to NULL is just to remove the /n
This sequence of code outputs the whole of line 1, the second half of line 2 and none of line 3.
Thanks to #alvits for the answer to this one:
fwrite() is not compatible with fgets(). Files created using fwrite() should use fread() to read them ?>back in. Both fwrite() and fread() operates on binary streams unless explicitly converted to and from >strings. fgets() is compatible with fputs(), both operates on strings.
I used fputs() to write my data instead and it read back in perfectly.
In POSIX systems, including Linux, there is no differentiation between binary and text files. When opening a file stream, the b flag is ignored. This is described in fopen().
You might ask "how would you differentiate text from binary files?". The contents differentiate them. How the contents are written makes them a binary or text file.
Look at the signature size_t fwrite(const void *ptr, size_t size, size_t nmemb, FILE *stream). You'll notice that it writes the contents of *ptr with size describing the size of each members, nmemb. The written stream is not converted to string. If you were to write 97 it will write the binary 97 which in ascii is A. Binary data does not obey string terminations. Presence of \n and \0 in data is literally written as is.
Now look at the signature int fputs(const char *s, FILE *stream). It writes the string content of *s. If you were to write 97, it will have to be a string "97" which is not A. String termination is obeyed. \n is automatically converted to the O/S supported newline (CRLF or LF).
You can coerce fwrite() to behave like fputs() but not the other way around. For example, if you declare ptr as a pointer to string and calculate the size exactly as the length of the content excluding string terminator, you'll be able to write it out as text instead of binary. You will also need to handle \0 and \n and convert them to O/S supported newline. Writing the entire string buffer will write everything including and past the string terminators.
So what I'm trying to do is open a file and read it until the end in blocks that are 256 bytes long each time it is called. My dilemma is using fgets() or fread() to do it.
I was using fgets() initially, because it returns a string of the bytes that were read, which is great because I can store that data and work with it. However, in my particular file that I'm reading, the 256 bytes often happen over a more than 2 lines, which is a problem because fgets() stops reading when it hits a newline character or the end of the file.
I then thought of using fread(), but I don't know how to save the line that I'm referring to with it because fread() returns an int referring to the number of elements successfully read (according to its documentation).
I've searched and thought of solutions for a while now and can't find anything that works with my particular scenario. I would like some guidance on how to go about this issue, how would you go about this in my position?
You can use fread() to read each 256 bytes block and keep a lineCount variable to keep track of the number of new line characters you have encountered so far in the input. Since you have to process the blocks already this wouldn't mean much of an overhead in the processing.
To read a block of 256 chars, which is what I think you are doing, you just need to create a buffer of chars that can hold 256 of them, in other words a char array of size 256.
#define BLOCK_SIZE 256
char block[BLOCK_SIZE];
Then if you check the documentation for fread() it shows the following signature:
Following is the declaration for fread() function.
size_t fread(void *ptr, size_t size, size_t nmemb, FILE *stream)
Parameters
ptr -- This is the pointer to a block of memory with a minimum size of size*nmemb bytes.
size -- This is the size in bytes of each element to be read.
nmemb -- This is the number of elements, each one with a size of size bytes.
stream -- This is the pointer to a FILE object that an input stream.
So this means it takes a pointer to the buffer where it will write the read information, the size of each element it's supposed to read, the maximum amount of elements you want it to read and the file pointer. In your case it would be:
int read = fread(block, sizeof(char), BLOCK_SIZE, file);
This will copy the information from the file to the block array, which you can later process and keep track of the lines. The characters that were read by fread are in the block array, so the first char in the last read block would be block[0], the second block[1] and so on. The returned value in read indicates how many elements (in your case chars) were inserted in the array block when you call fread, this number will be equal to BLOCK_SIZE for every call, unless you reach the end of the file or there's an error.
I suggest you read some documentation for a full example, play a little with the code and do some reading on pointers in C to gain a better understanding of how everything works in general. If you still have questions after that, we can take it from there or you can create a new SO question.
The user should input some file names in the command line and the program will read each file name from argv[] array. I have to perform error checking etc.
I want to read each filename. For example, if argv[2] is 'myfile.txt', the program should read the content of 'myfile.txt' and store value in char buffer[BUFSIZ] and then write the content of buffer into another file.
However before the content is written, the program should also write the name of the file and the size. Such that the file can be easily extracted later. A bit like the tar function.
The file I write the content of buffer, depending on the number of files added by user, should be a string like:
myfile.txt256Thisisfilecontentmyfile2.txt156Thisisfile2content..............
My question is
1) How do I write value of argv[2] into file using write() statement, as having problems writing char array, what should I put as (sizeof(?)) inside write(). see below as I don't know the length of the file name entered by the user.
2) Do I use the '&' to write an integer value into file after name, for example write 4 bytes after file name for the size of file
Here is the code I have written,
char buffer[BUFSIZ];
int numfiles=5; //say this is no of files user entered at command
open(file.....
lseek(fdout, 0, SEEK_SET); //start begging of file and move along each file some for loop
for(i=0-; ......
//for each file write filename,filesize,data....filename,filesize,data......
int bytesread=read(argv[i],buffer,sizeof(buffer));
write(outputfile, argv[i], sizeof(argv)); //write filename size of enough to store value of filename
write(outputfile, &bytesread, sizeof(bytesread));
write(outputfile, buffer, sizeof(buffer));
But the code is not working as I expected.
Any suggestions?
Since argv consists of null-terminated arrays, the length you can write is strlen(argv[2])+1 to write both the argument and null terminator:
size_t sz = strlen (argv[2]);
write (fd, argv[2], sz + 1);
Alternatively, if you want the length followed by the characters, you can write the size_t itself returned from strlen followed by that many characters.
size_t sz = strlen (argv[2]);
write (fd, &sz, sizeof (size_t));
write (fd, argv[2], sz);
You probably also need to write the length of the file as well so that you can locate the next file when reading it back.
1., You can write the string the following way:
size_t size = strlen(string);
write(fd, string, size);
However, most of the time it's not this simple: you will need the size of the string so you'll know how much you need to read. So you should write the string size too.
2., An integer can be written the following way:
write(fd, &integer, sizeof(integer));
This is simple, but if you plan to use the file on different architectures, you'll need to deal with endianness too.
It sounds like your best bet is to use a binary format. In your example, is the file called myfile.txt with a content length of 256, or myfile.txt2 with a content length of 56, or myfile.txt25 with a content length of 6? There's no way to distinguish between the end of the filename and the start of the content length field. Similarly there is no way to distinguish between the end of the content length and the start of the content. If you must use a text format, fixed width fields will help with this. I.e. 32 characters of filename followed by 6 digits of content length. But binary format is more efficient.
You get the filename length using strlen(), don't use sizeof(argv) as you will get completely the wrong result. sizeof(argv[i]) will also give the wrong result.
So write 4 bytes of filename length followed by the filename then 4 bytes of content length followed by the content.
If you want the format to be portable you need to be aware of byte order issues.
Lastly, if the file won't all fit in your buffer then you are stuffed. You need to get the size of the file you are reading to write it to your output file first, and then make sure you read that number of bytes from the first file into the second file. There are various techniques to do this.
thanks for replies guys,
I decided not to use (size_t) structure instead just assigned (int) and (char) types so I know exact value of bytes to read() out. ie I know start at beggining of file and read 4 bytes(int) to get value of lenght of filename, which I use as size in next read()
So, when I am writing (copying file exactly with same name) users inputted file to the output file (copied file) I writing it in long string, without spaces obviously just to make it readable here,
filenamesize filename filecontentsize filecontent
ie 10 myfile.txt 5 hello
So when come to reading that data out I start at begining of file using lseek() and I know the first 4 bytes are (int) which is lenght of filename so I put that into value int namelen using the read function.
My problem is I want to use that value read for the filenamesize(first 4 bytes) to declare my array to store filename with the right lenght. How do I put this array into read() so the read stores value inside that char array specified, see below please
int namelen; //value read from first 4 bytes of file lenght of filename to go in nxt read()
char filename[namelen];
read(fd, filename[namelen], namelen);//filename should have 'myfile.txt' if user entered that filename
So my question is once I read that first 4 bytes from file giving me lenght of filename stored in namelen, I then want to read namelen amount of bytes to give me the filename of originally file so I can create copied file inside directory?
Thanks
Can you set any index of array as starting index i.e where to read from file? I was afraid if the buffer might get corrupted in the process.
#include <stdio.h>
int main()
{
FILE *f = fopen("C:\\dummy.txt", "rt");
char lines[30]; //large enough array depending on file size
fpos_t index = 0;
while(fgets(&lines[index], 10, f)) //line limit is 10 characters
{
fgetpos (f, &index );
}
fclose(f);
}
You can, but since your code is trying to read the full contents of the file, you can do that much more directly with fread:
char lines[30];
// Will read as much of the file as can fit into lines:
fread(lines, sizeof(*lines), sizeof(lines) / sizeof(*lines), f);
That said, if you really wanted to read line by line and do it safely, you should change your fgets line to:
// As long as index < sizeof(lines), guaranteed not to overflow buffer
fgets(&lines[index], sizeof(lines) - index, f);
Not like this no. There is a function called fseek that will take you to a different location in the file.
Your code will read the file into a different part of the buffer (rather than reading a different part of the file).
lines[index] is the index'th character of the array lines. Its address is not the index'th line.
If you want to skip to a particular line, say 5, then in order to read the 5th line, read 4 lines and do nothing with them, them read the next line and do something with it.
If you need to skip to a particular BYTE within a file, then what you want to use is fseek().
Also: be careful that the number of bytes that you tell fgets to read for you (10) is the same as the size of the array you are putting the line into (30) - so this is not the case right now.
If you need to read a part of a line starting from a certain character within that line, you still need to read the whole line, then just choose to use a chunk of it starting someplace other than the beginning.
Both of these examples are like requesting a part of a document from a website or a library - they're not going to tear out a page for you, you get the whole document, and you have to flip to what you want.