C open() read() then write() to copy a file

[Xender1]

I know its possible as my friend has been able to copy a file using C but using the open() read() and write() C commands. I have attempted in doing the same thing to merely copy a file but it is not working. Here is what I got:

//i have a variable size which is an int and is the byte size of the file
//i got the byte size of file from stat
int fileread = open("original.txt",'r');
void *buffer;
buffer = malloc(sizeof(void) * size);

int nread = read(fileread,buffer,size);

int filewrite = open("original.txt.backup",'w');

write(filewrite,buffer,size);

close(filewrite);
close(fileread);

This is creating the file original.txt.backup with some strange permissions which I can address later but its not a copy of original.txt it is just a empty file. Most of the info I have from read write and open is from http://rabbit.eng.miami.edu/info/functions/unixio.html

Thanks!

[annoyingrob]

I don't have any of my code ni front of me, but I do remember having to fflush() before closing to commit any changes currently cached to disk.

[worksofcraft]

int fileread = open("original.txt",'r');

ah you confuse two different ways of doing it.
the open, read and write functions use a file handle and you need to open it with an integer access code like so:

int fileread = open("original.txt", O_RDONLY);

otoh there are functions fopen, fread and fwrite that are perhaps slightly higher level and they use struct FILE and you open them with a string like "r" or "w" which is not a single character code but a null terminated string.

[Some Penguin]

In addition to the O_FLAG problems, read() and write() are permitted to read and write fewer bytes than you ask them to.

[psusi]

In addition to using the wrong argument with the wrong function, you are multiplying your size by sizeof( void ) which I think would be zero, since void means NOTHING.

[worksofcraft]

In addition to using the wrong argument with the wrong function, you are multiplying your size by sizeof( void ) which I think would be zero, since void means NOTHING.

lol - that is true, however the new ANSI standard doesn't guarantee size of char to be one anymore so people need to use sizeof(void) to get smallest addressable unit

If you use the C++ compiler it will tell you:

void.c:4: warning: invalid application of ‘sizeof’ to a void type

but if you use the C compiler it is quite happy and says:

$> gcc -Wall void.c
$> ./a.out
sizeof(void)=1

[Xender1]

Ah thank you all very much for your help. Following your advice I was able to fix it perfectly! Here is what I have now:

//i have a variable size which is an int and is the byte size of the file
//i got the byte size of file from stat
int fileread = open("original.txt", O_RDONLY);
void *buffer;
buffer = malloc(sizeof(void) * size);

int nread = read(fileread,buffer,size);

int filewrite = open("original.txt.backup",O_CREAT | O_RDWR, 0644);

write(filewrite,buffer,size);

close(filewrite);
close(fileread);

0644 for rw-r--r--

Thanks!

[psusi]

lol - that is true, however the new ANSI standard doesn't guarantee size of char to be one anymore so people need to use sizeof(void) to get smallest addressable unit

Ok, but why bother multiplying by one?

[spjackson]

lol - that is true, however the new ANSI standard doesn't guarantee size of char to be one anymore so people need to use sizeof(void) to get smallest addressable unit

Which issue of which standard are you referring to here?