write system call basics....

bisDude · 10-17-2005, 05:06 AM

hi guys....im new at unix programming and have a question on the write() system call....
how do I write integers?

Code:

int x = 2;
write(1, (const void*)&x, sizeof(x));

it compiles fine...
I just want to write the value 2 to the screen, but nothing is displayed....
its no problems writing a char buffer:

char *str = "hey";
write(1, (const void*)str, strlen(str));

I know I can use other functions to do this, but I need to use system calls for another purpose...just thought I'd try to write a int to the screen first

.....hopefully someone can help me with this...
Thanks a bunch.......

bigearsbilly · 10-17-2005, 05:49 AM

you can't write an 'integer' to the screen, you can only write a string.
which you must null terminate yourself in the normal way.
So you will need to convert the int to an ascii string
representation. itoa or sprintf.
try this:

Code:

int main (int argc, char ** argv, char ** envp)
{
    int x = 012;

    write(1, (const void*)&x, sizeof(x));
    x = 0110;
    write(1, (const void*)&x, sizeof(x));
    x = 0105;
    write(1, (const void*)&x, sizeof(x));
    x = 0114;
    write(1, (const void*)&x, sizeof(x));
    x = 0114;
    write(1, (const void*)&x, sizeof(x));
    x = 0117;
    write(1, (const void*)&x, sizeof(x));
    x = 0; /* null terminate */
    write(1, (const void*)&x, sizeof(x));
    x = 012; /* new line */
    write(1, (const void*)&x, sizeof(x));

}

bisDude · 10-17-2005, 06:33 AM

thanks for the reply.....

so if I have a int array that I want to write to a pipe...
int x[] = {1, 2, 3};

write(pipe[1], ...., ....);

i suppose I first need to convert the int array to a string array first, Im I right?

bigearsbilly · 10-17-2005, 08:05 AM

No of course not.
If you want to send raw data that's fine.

It's just earlier you were sending to a terminal device
(the screen). That's a different matter because it needs ASCII
codes to print what you expect.

bisDude · 10-17-2005, 08:58 AM

ok thanks for clearing that up for me....

acornish · 11-04-2005, 06:36 PM

write( .., &x, sizeof(x) ) will write 4 bytes, 3 of which will be NUL.
This is due to sizeof(x) being 4.

On an Intel architecture machine, the first byte will be non-NUL.
On other architectures, the last byte written might be non-NUL.

int x;

should really be

char x;

Then the sample code would write one byte per system call,
since sizeof(x) is 1.

qais · 11-04-2005, 06:48 PM

If you need to convert double then you can use gcvt() function .

write(fd,gcvt(double,sizeof(double),buff),sizeof(double));

just do man on gcvt.