unistd.h

Learn basic functions in unistd.h.

The unistd.h header

unistd.h is the core POSIX header. It declares functions for file descriptors, processes and low-level I/O. The most common ones are read, write, close, fork and getpid.

These differ from fread/fwrite in stdio.h: they do not buffer, they work directly with descriptors. Each call returns ssize_t, the number of bytes read or written. On error it returns -1.

ssize_t and errno

POSIX I/O functions return a value of type ssize_t. This is a signed integer: positive on success, 0 at end of file, -1 on error.

On error the global errno variable is set. You include <errno.h> to read it. For example, when read fails, errno may be EAGAIN or EBADF. Always check the return value; if -1, inspect errno.

Exercise Output with write

Define a message with const char *msg = "Merhaba\n";. Write it to stdout with write(1, msg, 8). The 8 is the byte count (Merhaba plus newline).

Show example solution

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

int main(void) {
    const char *msg = "Merhaba\n";
    write(1, msg, 8);
    return 0;
}

Test yourself

Match functions to purposes

read

Reads from file
write

Writes to file
close

Closes descriptor
getpid

Returns process ID
In write(1, buf, n), what does 1 specify?
- The file name
- The file descriptor
- The byte count
- The buffer size
Show answer

The file descriptor — 1 is the stdout descriptor.

write call

ssize_t n = ____(1, msg, 8);

Show answer

ssize_t n = write(1, msg, 8);

Next lesson: File descriptors

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