Thread: What mean filedescriptor error-codes?

Hi,
I have written the following code:

Code:

#include <stdio.h>

int main(void) {

        char *myfile = "/home/username/Desktop/testfile";
        FILE *fd;
 
        fd=fopen(myfile, "r");
        printf("%i\n",fd);
        fclose(fd);
        return 0;

}

Executed I get this here: 134520840, which looks like a valid filedescriptor.

If I try now the same with syscalls:

Code:

#include </usr/include/linux/unistd.h>
#include <stdio.h>

int main(void) {

        char *myfile = "/home/username/Desktop/testfile";
        int fd;

        fd = syscall(__NR_open,myfile,"O_RDONLY") ;
        printf("%i\n",fd);
  
        return 0;
}

I always get -1 and if I use &myfile in the syscall I get 3 ? What are -1 and 3 meaning?

Can someone see the mistake in my second code?

bye

okay I did so now like I would do the same in assembler:

Code:

fd = syscall(__NR_open,myfile,00,04000) ;

in asm I do this an it works:

Code:

	mov eax,5
        mov ebx, myfile
	mov ecx,00
	mov edx,04000
	int 80h

But in my C-code I now always get 3 printed out, which is an error-code or? 3 is no pointer to a filedescriptor..

Ahh thank you I thought FILE would be a fd type or something like this. 3 was really the correct fd. This here is working fine for me:

Code:

#include </usr/include/linux/unistd.h>
#include <string.h>

int main(void) {

        char *myfile = "testfile",buffer[100];
        int fd;

        fd = syscall(__NR_open,myfile,00,04000) ;
        syscall(__NR_read,fd,buffer,100);
        syscall(__NR_write,1,&buffer,strlen(buffer));
    
        return 0;
}

But I have one last question. The syscall_write expects:

eax 4
ebx Device descriptor.
ecx Pointer to the buffer containing the data to be written.
edx Number of bytes to be written.

A pointer is an adress pointing to the value of a variable. Now the &buffer above in my code snippet gives us the adress where the content of buffer starts on the stack. But why is buffer without the & also working exactly the same way? So the code above also works without the & before the buffer in the syscall_write?

And why is *buffer not working? I mean k buffer is no pointer typ or, but at least a compiler-error should be given for using it as such or?

greets

Originally Posted by lilcoder

I mean k buffer is no pointer typ or, but at least a compiler-error should be given for using it as such or?

The name of an array becomes a pointer to it's first element -> buffer is a pointer to char -> *buffer is the same as buffer[0] -> not a pointer -> compile error
Kurt

Originally Posted by ZuK

The name of an array becomes a pointer to it's first element

Ahh so buffer and &buffer are exactly the same? Because also &buffer points to the first element of this char array.

Originally Posted by ZuK

*buffer is the same as buffer[0] -> not a pointer -> compile error

But is * not the symbol for the pointer-type? And next to that I don't get a compiler error for using it.

Code:

int *x = NULL; // * used such that x is a pointer.

makes sense

Code:

int a = *b;    // * used such that *b is an int.

Aha this makes a containing the hexcode of the first field of b[]:

Code:

        char *b="test";
        int a;
        a = *b;
        printf("%i",a) ;

Prints: 116 so is it that char *b="test" tells push test on the stack and make b a pointer to it. While *b points to the content which start with 116, b and &b are just the memory adresses ?

Originally Posted by laserlight

How did you use it?

In this code:

Code:

#include </usr/include/linux/unistd.h>

int main(void) {

        char *myfile = "testfile",buffer[100];
        int fd,buffer_len;
   
        fd = syscall(__NR_open,myfile,00,04000) ;
        syscall(__NR_read,fd,buffer,100);
        syscall(__NR_close,fd);
        for ( buffer_len=1; buffer[buffer_len] != '\0'; buffer_len++);
        syscall(__NR_write,1,*buffer,buffer_len);
    
        return 0;
}

I don't get any errors for *buffer. Normally compiled with gcc -o test test.c and gcc version 4.1.2 . The program simply doesn't work but compiles nice. *buffer exchanged with buffer or &buffer and the program works fine.

Originally Posted by lilcoder

I don't get any errors for *buffer.

That is because syscall is a function that takes a variable number of arguments. That's why the compiler can't check the type of the arguments.
Kurt

ah okay thank you for your answers

Your mistake is believing that a FILE object and a file descriptor have anything to do with each other. They don't. The only problem I see here is attempting to print a FILE pointer as an integer -- why would you do that?

EDIT: I'm still clueless as to why you are calling system calls like that. Why not just call open()? Calling a syscall directly is not guaranteed to behave anything like the manual says it should.