Databases
StackOverflow2013
Postsdbo
POLinux kernel system call returns -1 instead of {-1, -256}
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POLinux kernel system call returns -1 instead of {-1, -256}
    primarykey
    data
    text
    <p>I'm a kernel newbie and am facing a weird issue. I have written a proof-of-concept calculator syscall and while it works fine for most computations, it is returning -1 when the SUBTRACTION result is between -1 to -256. If someone can throw some light on what could be happening, would appreciate it. Below is the syscall code.</p> <pre><code> SYSCALL_DEFINE3(calc, int, a, int, b , char, op) { int res_int; switch(op) { case '+': res_int = a + b; break; case '-': res_int = a - b; break; case '*': res_int = a * b; break; case '/': res_int = (a*1000) / b; break; } printk(KERN_INFO "KERNEL CALC RESULT : %d %c %d = %ld",a, op, b, res_int); return res_int; } </code></pre> <p>EDIT: Kernel version: Android Linux Kernel 3.10.xxx. Platform: Nexus7 ARM EABI. What I don't understand is why it is failing. The errno is not useful at all since it is setting -res_int to errno. Also, I don't understand why it is failing only when res_int is {-1, -256}.</p> <p>a=1200, b=1300 op='-' => res_int=-100 is an example case where printk prints -100, but in my userspace app, I receive -1.</p> <p>It looks like when res_int is {-1, -256}, errno is being set to -res_int.</p> <pre><code>root@android:/data/local # ./calc Please enter request in 'num1 oper num2' format: 2.45 - 2.2 returned from syscall with res_int = 250 errno = 22, strerror(errno) = Invalid argument Calculator result = 0.250000 root@android:/data/local # ./calc Please enter request in 'num1 oper num2' format: 2.2 - 2.45 returned from syscall with res_int = -1 errno = 250, strerror(errno) = Unknown error 250 Calculator result = -0.001000 root@android:/data/local # </code></pre>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. USstackoverflow
    1. USstackoverflow
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POLinux kernel system call returns -1 instead of {-1, -256}
      1. This table or related slice is empty.
      1. VTUpMod
    1. COMost (all?) system calls return `-1` on failure and store the actual result code into `errno`. Maybe your result is getting put into `errno` too?
      singulars
      1. POLinux kernel system call returns -1 instead of {-1, -256}
      1. USAdam Rosenfield
    2. COWhat are the arguments that yield the erroneous results?
      singulars
      1. POLinux kernel system call returns -1 instead of {-1, -256}
      1. USFreddie
    3. CO@Freddie added example case.
      singulars
      1. POLinux kernel system call returns -1 instead of {-1, -256}
      1. USstackoverflow
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload