Tag: #posix

Creating and using an SSH certificate authority, an alternative to the default “trust on first use” model. 2018-03-16

Reverse DNS lookup maps an IP address to a domain name by querying PTR records. The mapping is not always consistent with the forward DNS lookup. 2018-02-09

nscd, a local DNS resolver within glibc, is non-standard. Instead, use a local DNS server like named or dnscache. 2018-02-05

getaddrinfo ostensibly does DNS lookups. Sounds simple, but it uses more than 100 system calls! Let’s trace the crazy path of address lookup on Linux. 2018-02-03

A TCP server that uses pthread to serve multiple clients concurrently, with an “echo” server for each connection. 2017-02-28

The domain and type arguments to socket() describe the protocol family and socket type, respectively. The protocol argument specifies the actual protocol to use, which may be inferred from the domain and type. 2017-02-27

A TCP server that uses the fork system call to create a new child process for each accepted connection, allowing it to handle multiple clients concurrently. 2017-02-25

mode_t is a bitfield type in C that represents file permissions, including read, write, and execute permissions for the owner, group, and other classes. It provides symbolic constants to set and check these permissions. 2017-02-24

ssize_t is a signed version of size_t, used to represent return values from system calls that may need to indicate error with -1. 2017-02-22

A FIFO is a special file that allows inter-process communication. The mkfifo system call creates a FIFO, enabling processes to read from and write to it. 2017-02-21

lsof lists open system resources, including pipes, sockets, and yes, files. It shows their type, owner, and location. 2017-02-20

A C function to create a new process, set up its standard input/output/error pipes, and return a struct containing the process ID and pipe file descriptors. 2017-02-17

To close a file descriptor in C, use the close system call. Multiple descriptors can reference the same underlying file or pipe. The pipe is only closed when all references are closed. 2017-02-16

Use the dup system call to duplicate a file descriptor in C, allowing two references to the same underlying pipe. 2017-02-15

UNIX represents various system components, like processes, file descriptors, and memory, using in-memory data structures that can be modeled as relational database tables. 2017-02-14

To call a program from C, use `fork` then `execve`. There is no more direct way! 2017-02-07

fork duplicates the current process. It returns 0 in the child process. In the parent process, it returns the child’s new process id. 2017-02-06

execve replaces the current process with a new one. It takes a path, an argument array, and an environment array. The process never returns unless execve fails. 2017-02-05

Access environment variables in C by using the global environ variable, which points to an array of string pointers representing the environment. 2017-02-02

All system calls on macOS 10.12, categorized by the functionality they provide, such as process management, file operations, memory management, etc. 2017-01-31

The man command is used to access the UNIX manual pages, which are organized into numbered sections. man searches a predefined path to find the relevant manual page. 2017-01-30

Processes can communicate via files, pipes, signals, sockets, message queues, semaphores, and shared memory. 2017-01-29

The syscalls needed for a simple UDP echo server are socket, bind, recvfrom, sendto, and close. 2016-12-19

Kqueue is a more efficient alternative to select for managing multiple TCP connections, providing a publish-subscribe model for tracking events in the kernel. 2016-12-18

The select syscall allows a process to sleep and wake up when a file descriptor is ready for reading, writing, or has an exceptional condition. This enables a TCP server to handle multiple clients concurrently. 2016-12-16

File descriptors should be called “resource descriptors”. They represent various system resources, not just regular files. The operations you can perform depend on the specific resource. 2016-12-15

A minimal TCP server in C uses the socket, bind, listen, accept, recv, send, and close syscalls to manage connections. 2016-12-14