System Calls & POSIX

System calls are the gateway between your program and the operating system kernel. Think of them as the reception desk at a secure government building: your program (a visitor) cannot walk into the vault and grab files directly. Instead, you fill out a request form (set up registers), ring a bell (execute the syscall instruction), and a trusted employee (the kernel) fetches what you need and hands it back through the window. Every printf, every file open, every network packet your C program sends eventually passes through this gateway. Understanding system calls is the difference between knowing C syntax and understanding how programs actually interact with hardware.

System call path from user space to kernel

User Space vs Kernel Space

┌─────────────────────────────────────────────────────────────────────────────┐
│                           USER SPACE                                         │
│                                                                              │
│  ┌─────────┐  ┌─────────┐  ┌─────────┐                                      │
│  │  Your   │  │  libc   │  │ Other   │                                      │
│  │ Program │  │ (glibc) │  │ Libraries│                                     │
│  └────┬────┘  └────┬────┘  └─────────┘                                      │
│       │           │                                                          │
│       │   printf()│ → write() wrapper                                       │
│       └───────────┼──────────────────────────────────────────────────────┐  │
│                   │                                                      │  │
├───────────────────┴──────────────────────────────────────────────────────┼──┤
│                          SYSTEM CALL INTERFACE                            │  │
│                                                                          │  │
│         syscall(SYS_write, fd, buf, count)                              │  │
│                              │                                           │  │
├──────────────────────────────┼───────────────────────────────────────────┴──┤
│                              ▼                                              │
│                         KERNEL SPACE                                        │
│                                                                              │
│  ┌──────────────┐  ┌──────────────┐  ┌──────────────┐                      │
│  │   Scheduler  │  │  Filesystem  │  │   Memory     │                      │
│  │              │  │   (VFS)      │  │  Management  │                      │
│  └──────────────┘  └──────────────┘  └──────────────┘                      │
│                                                                              │
│  ┌──────────────────────────────────────────────────────────────────────┐  │
│  │                        Hardware Drivers                               │  │
│  └──────────────────────────────────────────────────────────────────────┘  │
│                                                                              │
└─────────────────────────────────────────────────────────────────────────────┘

The Transition Steps

User Mode (Ring 3): Your program runs with limited privileges. It cannot access hardware directly.
Library Call: You call printf(). The C library (libc) formats the string and calls write().
System Call: The write() wrapper puts arguments in CPU registers (e.g., rax=1 for write) and executes a special instruction (syscall on x86-64).
Mode Switch: The CPU switches to Kernel Mode (Ring 0) and jumps to a predefined kernel entry point.
Kernel Execution: The kernel validates arguments, checks permissions, and performs the operation (e.g., writing to the terminal buffer).
Return: The kernel executes sysret, switching the CPU back to User Mode and returning the result (number of bytes written or error).

Making System Calls

Via libc Wrappers

In practice, you almost never call system calls directly. Instead, you call libc wrapper functions that set up the arguments, execute the syscall instruction, check for errors, and set errno. This is like having an assistant who fills out the government forms for you.

#include <unistd.h>
#include <fcntl.h>

int main(void) {
    // These are libc wrapper functions, not raw syscalls.
    // open() calls the kernel's sys_open, which allocates a file descriptor,
    // checks permissions, and returns an integer handle to your program.
    int fd = open("file.txt", O_RDONLY);
    
    char buffer[1024];
    // read() asks the kernel to copy bytes from the file into your buffer.
    // The kernel does the actual disk I/O (or returns cached data from the page cache).
    ssize_t bytes = read(fd, buffer, sizeof(buffer));
    
    // write() to STDOUT_FILENO (fd 1) asks the kernel to send bytes to the terminal.
    write(STDOUT_FILENO, buffer, bytes);
    
    // close() tells the kernel we are done with this file descriptor.
    // The kernel frees the fd slot and flushes any pending writes.
    close(fd);
    return 0;
}

Direct System Calls

#include <unistd.h>
#include <sys/syscall.h>

int main(void) {
    // Direct syscall (bypasses libc)
    const char *msg = "Hello, kernel!\n";
    
    // SYS_write = 1 on x86-64 Linux
    syscall(SYS_write, STDOUT_FILENO, msg, 15);
    
    // Inline assembly (x86-64)
    // Register convention: rax=syscall#, rdi=arg1, rsi=arg2, rdx=arg3
    long ret;
    __asm__ volatile (
        "syscall"
        : "=a" (ret)
        : "a" (SYS_write), "D" (STDOUT_FILENO), "S" (msg), "d" (15)
        : "rcx", "r11", "memory"
    );
    
    return 0;
}

Error Handling

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>

int main(void) {
    int fd = open("/nonexistent", O_RDONLY);
    
    if (fd == -1) {
        // errno is set by the system call
        fprintf(stderr, "Error code: %d\n", errno);
        fprintf(stderr, "Error message: %s\n", strerror(errno));
        perror("open");  // Prints: "open: No such file or directory"
        
        // Common errno values
        switch (errno) {
            case ENOENT: printf("File not found\n"); break;
            case EACCES: printf("Permission denied\n"); break;
            case EEXIST: printf("File exists\n"); break;
            case EINTR:  printf("Interrupted\n"); break;
            case EINVAL: printf("Invalid argument\n"); break;
            case ENOMEM: printf("Out of memory\n"); break;
            case ENOSPC: printf("No space left\n"); break;
        }
        
        return 1;
    }
    
    close(fd);
    return 0;
}

// Robust error-handling wrapper
// Why EINTR handling matters: if a signal arrives while your program is blocked
// in a system call, the kernel interrupts the call and returns -1 with errno=EINTR.
// This is not an error -- it just means "try again." Without this retry loop,
// your program would falsely report errors whenever it receives a signal (which
// happens more often than you think: SIGCHLD when a child process exits,
// SIGALRM from timers, etc.).
int safe_open(const char *path, int flags) {
    int fd;
    
    do {
        fd = open(path, flags);
    } while (fd == -1 && errno == EINTR);  // Retry on signal interruption
    
    return fd;
}

// Robust read (handles partial reads and interrupts)
// A common beginner mistake: assuming read() always returns the full count.
// In reality, read() can return fewer bytes than requested for many reasons:
// - Signal interrupted the call
// - Reading from a pipe or socket (data arrives in chunks)
// - Reading near end of file
// - Kernel decided to return a partial buffer
// Production code MUST loop until it gets all requested bytes or hits EOF/error.
ssize_t safe_read(int fd, void *buf, size_t count) {
    ssize_t total = 0;
    char *ptr = buf;
    
    while (count > 0) {
        ssize_t n = read(fd, ptr, count);
        
        if (n == -1) {
            if (errno == EINTR) continue;  // Signal interrupted us, just retry
            return -1;  // Actual error (EBADF, EIO, etc.)
        }
        
        if (n == 0) break;  // EOF -- no more data available
        
        total += n;
        ptr += n;
        count -= n;
    }
    
    return total;
}

File Descriptors

File descriptors are the kernel’s universal handle for “anything you can read from or write to.” Files, pipes, sockets, terminals, even special devices like /dev/null — they all look the same to your program: just an integer you pass to read() and write(). This uniform interface is one of Unix’s most powerful design decisions, and it is why shell pipes like cat file.txt | grep pattern | wc -l just work.

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/stat.h>

int main(void) {
    // Standard file descriptors -- every process starts with these three open:
    // 0 = stdin  (where input comes from, usually the keyboard)
    // 1 = stdout (where output goes, usually the terminal)
    // 2 = stderr (where error messages go, also the terminal but separate)
    
    // Open returns the lowest available fd number.
    // Since 0, 1, 2 are already taken, the first open() usually returns 3.
    int fd = open("file.txt", O_RDWR | O_CREAT, 0644);
    printf("Opened fd: %d\n", fd);  // Usually 3
    
    // Duplicate file descriptor
    int fd2 = dup(fd);  // fd2 points to same file
    
    // Duplicate to specific number
    int fd3 = dup2(fd, 10);  // fd 10 now points to same file
    
    // Get/set file descriptor flags
    int flags = fcntl(fd, F_GETFL);
    fcntl(fd, F_SETFL, flags | O_NONBLOCK);
    
    // Get file info
    struct stat st;
    fstat(fd, &st);
    printf("Size: %ld bytes\n", st.st_size);
    printf("Mode: %o\n", st.st_mode);
    printf("Is regular file: %d\n", S_ISREG(st.st_mode));
    printf("Is directory: %d\n", S_ISDIR(st.st_mode));
    
    close(fd);
    close(fd2);
    close(fd3);
    
    return 0;
}

Process Information

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <pwd.h>

int main(void) {
    printf("PID:  %d\n", getpid());
    printf("PPID: %d\n", getppid());
    printf("UID:  %d\n", getuid());
    printf("EUID: %d\n", geteuid());
    printf("GID:  %d\n", getgid());
    
    // Get username
    struct passwd *pw = getpwuid(getuid());
    printf("User: %s\n", pw->pw_name);
    printf("Home: %s\n", pw->pw_dir);
    
    // Current working directory
    char cwd[1024];
    if (getcwd(cwd, sizeof(cwd))) {
        printf("CWD:  %s\n", cwd);
    }
    
    // Hostname
    char hostname[256];
    gethostname(hostname, sizeof(hostname));
    printf("Host: %s\n", hostname);
    
    return 0;
}

Environment Variables

#include <stdio.h>
#include <stdlib.h>

extern char **environ;  // Global environment

int main(int argc, char *argv[], char *envp[]) {
    // Get environment variable
    const char *path = getenv("PATH");
    if (path) {
        printf("PATH: %s\n", path);
    }
    
    // Set environment variable
    setenv("MY_VAR", "my_value", 1);  // 1 = overwrite
    putenv("ANOTHER_VAR=another_value");  // Alternative
    
    // Remove environment variable
    unsetenv("MY_VAR");
    
    // Iterate all environment variables
    printf("\n=== All Environment Variables ===\n");
    for (char **env = environ; *env; env++) {
        printf("%s\n", *env);
    }
    
    // Or using the envp parameter
    // for (int i = 0; envp[i]; i++) { ... }
    
    return 0;
}

Time and Date

#include <stdio.h>
#include <time.h>
#include <sys/time.h>

int main(void) {
    // Current time (seconds since epoch)
    time_t now = time(NULL);
    printf("Epoch time: %ld\n", now);
    
    // Human-readable
    struct tm *local = localtime(&now);
    printf("Local: %04d-%02d-%02d %02d:%02d:%02d\n",
           local->tm_year + 1900, local->tm_mon + 1, local->tm_mday,
           local->tm_hour, local->tm_min, local->tm_sec);
    
    // UTC
    struct tm *utc = gmtime(&now);
    printf("UTC:   %04d-%02d-%02d %02d:%02d:%02d\n",
           utc->tm_year + 1900, utc->tm_mon + 1, utc->tm_mday,
           utc->tm_hour, utc->tm_min, utc->tm_sec);
    
    // Formatted string
    char buf[100];
    strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S %Z", local);
    printf("Formatted: %s\n", buf);
    
    // High-resolution time
    struct timeval tv;
    gettimeofday(&tv, NULL);
    printf("Microseconds: %ld.%06ld\n", tv.tv_sec, tv.tv_usec);
    
    // Monotonic clock (for measuring intervals)
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    printf("Monotonic: %ld.%09ld\n", ts.tv_sec, ts.tv_nsec);
    
    // Sleep
    sleep(1);           // Seconds
    usleep(100000);     // Microseconds
    nanosleep(&(struct timespec){0, 100000000}, NULL);  // 100ms
    
    return 0;
}

// Measuring execution time
void measure_time(void (*func)(void)) {
    struct timespec start, end;
    
    clock_gettime(CLOCK_MONOTONIC, &start);
    func();
    clock_gettime(CLOCK_MONOTONIC, &end);
    
    double elapsed = (end.tv_sec - start.tv_sec) +
                     (end.tv_nsec - start.tv_nsec) / 1e9;
    printf("Elapsed: %.6f seconds\n", elapsed);
}

Resource Limits

#include <stdio.h>
#include <sys/resource.h>

int main(void) {
    struct rlimit rl;
    
    // Get limits
    getrlimit(RLIMIT_NOFILE, &rl);
    printf("Max open files: %ld (soft), %ld (hard)\n",
           rl.rlim_cur, rl.rlim_max);
    
    getrlimit(RLIMIT_STACK, &rl);
    printf("Stack size: %ld KB (soft), %ld KB (hard)\n",
           rl.rlim_cur / 1024, rl.rlim_max / 1024);
    
    getrlimit(RLIMIT_AS, &rl);
    printf("Address space: %ld MB (soft)\n", rl.rlim_cur / (1024*1024));
    
    // Set limits
    rl.rlim_cur = 1024 * 1024 * 100;  // 100 MB
    setrlimit(RLIMIT_AS, &rl);
    
    // Resource usage
    struct rusage usage;
    getrusage(RUSAGE_SELF, &usage);
    printf("User time: %ld.%06ld s\n",
           usage.ru_utime.tv_sec, usage.ru_utime.tv_usec);
    printf("System time: %ld.%06ld s\n",
           usage.ru_stime.tv_sec, usage.ru_stime.tv_usec);
    printf("Max RSS: %ld KB\n", usage.ru_maxrss);
    
    return 0;
}

POSIX Portability

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

int main(void) {
    // Check POSIX features
    #ifdef _POSIX_VERSION
        printf("POSIX version: %ld\n", _POSIX_VERSION);
    #endif
    
    // Runtime capability checks
    long val;
    
    val = sysconf(_SC_PAGESIZE);
    printf("Page size: %ld\n", val);
    
    val = sysconf(_SC_NPROCESSORS_ONLN);
    printf("CPUs online: %ld\n", val);
    
    val = sysconf(_SC_OPEN_MAX);
    printf("Max open files: %ld\n", val);
    
    val = sysconf(_SC_CLK_TCK);
    printf("Clock ticks/sec: %ld\n", val);
    
    // Path configuration
    val = pathconf("/", _PC_NAME_MAX);
    printf("Max filename length: %ld\n", val);
    
    val = pathconf("/", _PC_PATH_MAX);
    printf("Max path length: %ld\n", val);
    
    return 0;
}

Common System Call Reference

Category	System Calls
File I/O	open, close, read, write, lseek, pread, pwrite
File Info	stat, fstat, lstat, access, chmod, chown
Directories	mkdir, rmdir, chdir, getcwd, opendir, readdir
Processes	fork, exec*, wait, waitpid, exit, _exit
Signals	kill, sigaction, sigprocmask, pause, sigsuspend
Memory	mmap, munmap, mprotect, brk, sbrk
IPC	pipe, socketpair, shmget, semget, msgget
Network	socket, bind, listen, accept, connect, send, recv
Time	time, gettimeofday, clock_gettime, nanosleep
Misc	ioctl, fcntl, dup, dup2, select, poll, epoll

Exercises

System Info Tool

Build a tool that prints comprehensive system information (CPU, memory, disk, network).

Safe Wrapper Library

Create a library of safe wrappers for common system calls with proper error handling and EINTR retry.

Syscall Tracer

Use ptrace to build a simple strace-like tool.

Resource Monitor

Build a tool that monitors a process’s resource usage over time.

Next Up

Concurrency

Process and thread programming

Undefined Behavior Concurrency

​System Calls & POSIX

​User Space vs Kernel Space

​The Transition Steps

​Making System Calls

​Via libc Wrappers

​Direct System Calls

​Error Handling

​File Descriptors

​Process Information

​Environment Variables

​Time and Date

​Resource Limits

​POSIX Portability

​Common System Call Reference

​Exercises

​Next Up