system-calls

What is the difference between a system call and a function call? Is fopen() a system call or a function call? A system call is a call into kernel code, typically performed by executing an interrupt. The interrupt causes the kernel to take over and perform the requested action, then hands control back to the application. This mode switching is the reason that system calls are slower to execute than an equivalent application-level function. fopen is a function from the C library that, internally, performs one or more system calls. Generally, as a C programmer, you rarely need to use system

I can understand how one can write a program that uses multiple processes or threads: fork() a new process and use IPC, or create multiple threads and use those sorts of communication mechanisms. I also understand context switching. That is, with only once CPU, the operating system schedules time for each process (and there are tons of scheduling algorithms out there) and thereby we achieve running multiple processes simultaneously. And now that we have multi-core processors (or multi-processor computers), we could have two processes running simultaneously on two separate cores. My question is

I'm trying to understand the use of dup2 and dup . From the man page : DESCRIPTION dup and dup2 create a copy of the file descriptor oldfd. After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other. The two descriptors do not share the close-on-exec flag, however. dup uses the lowest-numbered unused descriptor for the new descriptor. dup2 makes newfd be the copy of oldfd,

I'm having trouble finding the good documentation for writing 64-bit assembly on MacOS. The 64-bit SysV ABI says the following in section A.2.1 and this SO post quotes it: A system-call is done via the syscall instruction. The kernel destroys registers %rcx and %r11. Returning from the syscall, register %rax contains the result of the system-call. A value in the range between -4095 and -1 indicates an error, it is -errno. Those two sentences are ok on Linux but are wrong on macOS Sierra with the following code: global _start extern _exit section .text _start: ; Align stack to 16 bytes for libc