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50

It's due to the technical constraints of the time. The POSIX standard was created in the 1980s and referred to UNIX, which was born in the 1970. Several C compilers at that time were limited to identifiers that were 6 or 8 characters long, so that settled the standard for the length of variable and function names.


22

dr01 is right, but there's also another reason - usability. Back in the day, you didn't have something as comfortable as a keyboard to type on. If you were lucky, you had something akin to an old-school typewriter. If you were unlucky, you had to deal with systems that required actual physical work to operate (as in, it took a lot of force to press the ...


17

In addition to the other answers, I would like to point out that Unix was developed as a reaction to Multics, CTSS, and other contemporary operating systems, which were significantly more verbose about their naming conventions. You can get a feel for these OSes at http://www.multicians.org/devdoc.html. For example, ...


10

The kernel does job scheduling and provides system calls. When a process is running, the kernel schedules its runtime - especially it assigns a PID to it - such information is stored inside the kernel address space, in data structures (e.g. inside a task struct). Thus, when a process calls the getpid() system call, the kernel just has to look in the task ...


9

Check this Wikipedia link on Exec function and this link on Starting a process with the exec() calls e – An array of pointers to environment variables is explicitly passed to the new process image. The "e" suffix versions pass an environment to the program. An environment is just that—a kind of "context" for the program to operate in. For example, ...


7

Dennis Ritchie set himself a constraint with C that it wouldn't rely on any linker features that weren't also required by Fortran. Hence the 6 character limit on external names.


6

Because of how waitpid works. On a POSIX system, a signal (SIGCHLD) is delivered to a parent process when one of its child processes dies. At a high level, all waitpid is doing is blocking until a SIGCHLD signal is delivered for the process (or one of the processes) specified. You can't wait on arbitrary processes, because the SIGCHLD signal would never ...


6

The file /proc/kallsyms lists all the symbols of the running kernel. By convention, system calls have a name that begin with sys_. On a 64-bit system, system calls for 32-bit programs have a name that begin with sys32_. Strictly speaking, this lists internal kernel functions, not system call, but I think that the correspondence does work (every system call ...


6

When a system call is executed, there is a privilege switch, i.e. the executed code is allowed to execute more instructions and access data forbidden to userland code. There is however no process context switch so the kernel code is still running in the calling process context. That means the kernel does not need to search which process is calling it, it ...


5

As it so happens, there is another significant interface with the kernel: the /proc and /sys virtual filesystems. While they do not hold regular files, their contents are direct gateways to the kernel: to act on them is to act directly on kernel-allocated memory. For instance, if you want to drop all memory caches, you may use... echo 3 > ...


5

This is all perfectly normal. You aren't supposed to prevent the failing library lookups from happening. execve("./hello", ["./hello"], [/* 62 vars */]) = 0 This is your program starting. Since it is dynamically linked, the first code to execute is from the dynamic loader. brk(0) = 0x85a5000 mmap2(NULL, 4096, ...


5

I would say that it is misleading to call getpid() a "linux system call". That gives the impression that it is a Linux-specific system call, which it isn't. Actually, getpid() and many other system calls are specified by POSIX, and you will find it implemented on both Linux and MacOS and on many other systems, with identical behaviour. The majority of ...


4

I will try to answer questions as briefly as possible. The questions you are asking are usually addressed in introductory operating systems courses at universities but I will assume you have not taken such a course. Memory isolation for userspace processes is very desirable - not only to protect the kernel from malicious userspace programs, but also to ...


4

In a single CPU system, there is a global variable that points to the proc structure of the running process or the current thread. The proc structure contains the process id. In a multi CPU system, there is either a similar pointer for every CPU or the MMU context is used to set up such a global variable for the syscall. int64_t getpid(void) { ...


3

Mysql doesn't have a kernel module, therefore it runs in user mode. Perhaps what you are seeing is that mysql is using memory-mapped files instead of calling read/write. So, accessing a page of memory causes a read/write without using a syscall. Or, perhaps you called strace without "-f" to follow the child processes?


3

These numbers disambiguate manual pages that have the same name. They represent the manual section that the page should be retrieved from. As an excerpt from Wikipedia states: The manual is generally split into eight numbered sections, organized as follows [...]


2

In theory, if the application obeys certain constraints, yes. In general, no. The most common way to invoke system calls is via the standard-library wrappers for the same. So for example, read(fd,buf,BUFLEN) compiles to assembly call read, and in amd64 object code that would be e8 00 00 00 00 (where the zeroes are covered by a symbol-table entry for read). ...


2

TL;DR I kept finding new alternatives when writing this answer, so I just wrote a bit of details about each of them, and made some stats. Basically, you can either: Read Gilles' answer, which provides a clean and fast way to do it (relies on /proc). Use the documentation resources. Use your system's C header files. Use the kernel source code itself. Use ...


2

In order to read /proc/[pid]/mem, a process must now PTRACE_ATTACH to it. A commonly available utility that does this is gdb Pick a running process (in my case I just opened cat in another window), then attach gdb to that process: [root@qemu ~]# gdb --pid 423 #MORE OUTPUT 0xb771dbac in __kernel_vsyscall () As part of its output while loading symbols, gdb ...


2

It doesn't as much "get" CPU, as it just runs on it. The kernel decides on which core and when and for how long the process runs. It schedules the tasks so that each process gets its time slices on the CPU: it runs for a while, then either after the time slice expires, or a system call occurs, the context is switched to another process. The state of the ...


2

Most of them used to be implemented at some point in Linux kernel history time, but some like at least vserver are still implemented in specific kernels. The majority of these calls is now essentially obsolete but their slot remains and contains a stub which role is not to break old code and allow a re-implementation in a specialized or new kernel should it ...


2

Does a system call function often but not always have a command or utility in shell? Sometimes. System calls that are useful to expose, are usually exposed. A shell that couldn't change directories would not be a very useful shell. Some of them, like chdir, are implemented as shell builtins, because they can only affect the process that called them. ...


2

Read more carefully close(2) and Advanced Linux Programming Your formulation is wrong: close is a system call (listed in syscalls(2) for Linux) by which the application tells the kernel to release a resource (not the other way round). You could use strace(1) to understand the system calls executed by some command or process. See also pthreads(7), ...


2

According to _syscall(2) man page the _syscall0 macro may be obsolete and requires #include <linux/unistd.h>; indeed Linux 4.x don't have it However, you might install musl-libc and use its _syscall function. And you could simply use the indirect syscall(2) in your user code. So your testing program would be #define _GNU_SOURCE /* See ...


2

OS X is a certified UNIX operating system, guaranteeing it implements the POSIX standard. Linux, while not a certified UNIX, also implements the POSIX standard. If you limit your API calls to things that are part of POSIX you should have consistent behavior between OS X and Linux. Aside from POSIX, the C standard library is also standardized and you ...


1

According to http://www.di.uevora.pt/~lmr/syscalls.html, the suffixes indicate the type of arguments: l argn is specified as a list of arguments. v argv is specified as a vector (array of character pointers). e environment is specified as an array of character pointers. p user's PATH is searched for command, and command can be a shell ...


1

There's a fantastic pair of articles on LWN that describe how syscalls work on Linux: "Anatomy of a system call", part 1 and part 2.


1

The numbers are the sections in the manual. (1), for example, are commands.


1

It's in kernel space. This article from Linux Device Drivers is a bit dated but still should generally apply: https://lwn.net/images/pdf/LDD3/ch18.pdf However, there is some effort recently towards replacing the kernel driver with a userspace console called "KMSCON" -- see that project's site for more: http://cgit.freedesktop.org/~dvdhrm/kmscon/tree/README


1

exec here could be a system call or a bash built-in or something else from this . And respective man pages related to system call or bash built-in refer to the exec's man page with numbers in the brackets. So if I want to refer to manpage of bash built-in, I would say exec(1) and if I want to refer to manpage of system call exec() i would say exec(2) The ...



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