StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<p>You didnt' mention your kernel version and platform but fwiw, from kernel's perspective a syscall usually returns zero on success and a negative number on error. Generally this is the ABI convention between kernel folks and application developers so the code can understand each other.</p> <p>But user program always uses C library as a wrapper to make system calls and the C API should comply with the API standard. That is, the C library will check the return value from kernel space and set the errno based on that. (Note that errno is in user space and kernel doesn't know it.) Say, if syscall foo(), from user's view, returns -1 on failure and should set the errno to indicate the error (ERR1 for failure 1, ERR2 for failure2, etc), the kernel syscall implementation would return -ERR1 or -ERR2 accordingly and the C library would check the return value against zero and if it's negative, it will return -1 to user and also set errno to ERR1/ERR2. Hope this helps.</p> <p>Update: I checked the x86_64 code of glibc, and this may help understand this:</p> <pre><code> .text ENTRY (syscall) movq %rdi, %rax /* Syscall number -> rax. */ movq %rsi, %rdi /* shift arg1 - arg5. */ movq %rdx, %rsi movq %rcx, %rdx movq %r8, %r10 movq %r9, %r8 movq 8(%rsp),%r9 /* arg6 is on the stack. */ syscall /* Do the system call. */ cmpq $-4095, %rax /* Check %rax for error. */ jae SYSCALL_ERROR_LABEL /* Jump to error handler if error. */ L(pseudo_end): ret /* Return to caller. */ PSEUDO_END (syscall) File: "sysdeps/unix/sysv/linux/x86_64/syscall.S" </code></pre> <p><strong>Linus said he will make sure the no syscall returns a value in -1 .. -4095 as a valid result so we can savely test with -4095.</strong> </p> <p>Update: so I <em>guess</em> it's your c library that converts the return value. You platform ABI may define that syscall returning {-1, -256} on failure so the C wrapper sets the return value to -1 in such cases and set the errno accordingly. </p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

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