What is POSIX?

Learn the POSIX standard and its relation to C.

What is POSIX?

POSIX (Portable Operating System Interface) is a standard defined by IEEE that specifies a common interface for UNIX-like systems. When you do systems programming in C, you use POSIX functions.

POSIX defines low-level APIs such as file operations (open, read, write), process management (fork, exec) and sockets. Its header is <unistd.h>. Linux, macOS and BSD systems are POSIX compliant.

Why POSIX?

With POSIX, you can compile and run the same C code on Linux, macOS and BSD without changes. The standard C library (stdio.h) provides high-level I/O, while POSIX provides low-level I/O.

For example, fopen opens a file with a FILE structure; open returns a file descriptor. Descriptors are faster and more controllable: with read and write you work directly at the byte level. In this lesson you will see the core POSIX concepts.

Exercise Print signal names

Use printf to print three POSIX signal names, one per line: SIGINT, SIGTERM and SIGKILL. End each line with \n.

Show example solution

#include <stdio.h>
#include <signal.h>

int main(void) {
    printf("SIGINT\n");
    printf("SIGTERM\n");
    printf("SIGKILL\n");
    return 0;
}

Test yourself

What does POSIX stand for?
- Portable Operating System Interface
- Portable Operating System Internal
- Process Oriented System Interface
- Parallel Operating System Interface
Show answer

Portable Operating System Interface — POSIX stands for Portable Operating System Interface.

POSIX header file

#include <____.h>
#include <stdio.h>

Show answer

#include <unistd.h>
#include <stdio.h>

Match the POSIX concepts

POSIX

Portable interface
unistd.h

POSIX header
fcntl.h

File control
sys/types.h

Type definitions

Next lesson: unistd.h

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