name: c-systems-programming description: Use when writing low-level system software in C requiring file I/O, process management, signals, and system calls. allowed-tools:

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C Systems Programming

Master C systems programming including file I/O, process management, inter-process communication, signals, and system calls for writing robust low-level system software.

File I/O Operations

File Descriptors

File descriptors are integers that represent open files in Unix-like systems. Standard file descriptors:

0 - Standard input (STDIN_FILENO)
1 - Standard output (STDOUT_FILENO)
2 - Standard error (STDERR_FILENO)

Basic File Operations

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

int main(void) {
    int fd;
    char buffer[1024];
    ssize_t bytes_read, bytes_written;

    // Open file for reading
    fd = open("input.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    // Read from file
    bytes_read = read(fd, buffer, sizeof(buffer) - 1);
    if (bytes_read == -1) {
        perror("read");
        close(fd);
        return 1;
    }
    buffer[bytes_read] = '\0';

    // Close file
    if (close(fd) == -1) {
        perror("close");
        return 1;
    }

    printf("Read %zd bytes: %s\n", bytes_read, buffer);
    return 0;
}

Writing to Files

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

int write_file(const char *filename, const char *data) {
    int fd;
    ssize_t bytes_written;
    size_t len = strlen(data);

    // Open file for writing, create if doesn't exist, truncate if exists
    fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (fd == -1) {
        perror("open");
        return -1;
    }

    // Write data
    bytes_written = write(fd, data, len);
    if (bytes_written == -1) {
        perror("write");
        close(fd);
        return -1;
    }

    if ((size_t)bytes_written != len) {
        fprintf(stderr, "Partial write: %zd of %zu bytes\n",
                bytes_written, len);
        close(fd);
        return -1;
    }

    if (close(fd) == -1) {
        perror("close");
        return -1;
    }

    return 0;
}

File Positioning

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

int main(void) {
    int fd;
    char buffer[10];
    off_t offset;

    fd = open("data.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
        return 1;
    }

    // Seek to byte 10 from start
    offset = lseek(fd, 10, SEEK_SET);
    if (offset == -1) {
        perror("lseek");
        close(fd);
        return 1;
    }

    // Read 10 bytes
    if (read(fd, buffer, sizeof(buffer)) == -1) {
        perror("read");
        close(fd);
        return 1;
    }

    // Seek to end of file
    offset = lseek(fd, 0, SEEK_END);
    printf("File size: %lld bytes\n", (long long)offset);

    close(fd);
    return 0;
}

Process Management

Creating Processes with fork()

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

int main(void) {
    pid_t pid;
    int status;

    printf("Parent process (PID: %d)\n", getpid());

    pid = fork();
    if (pid == -1) {
        perror("fork");
        return 1;
    }

    if (pid == 0) {
        // Child process
        printf("Child process (PID: %d, Parent: %d)\n",
               getpid(), getppid());
        sleep(2);
        printf("Child exiting\n");
        return 42;
    } else {
        // Parent process
        printf("Parent created child (PID: %d)\n", pid);

        // Wait for child to exit
        if (waitpid(pid, &status, 0) == -1) {
            perror("waitpid");
            return 1;
        }

        if (WIFEXITED(status)) {
            printf("Child exited with status: %d\n",
                   WEXITSTATUS(status));
        }
    }

    return 0;
}

Executing Programs with exec()

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

int main(void) {
    pid_t pid;

    pid = fork();
    if (pid == -1) {
        perror("fork");
        return 1;
    }

    if (pid == 0) {
        // Child process - execute ls command
        char *args[] = {"ls", "-l", "/tmp", NULL};
        char *envp[] = {NULL};

        execve("/bin/ls", args, envp);

        // If execve returns, an error occurred
        perror("execve");
        return 1;
    } else {
        // Parent process - wait for child
        int status;
        waitpid(pid, &status, 0);
        printf("Child completed\n");
    }

    return 0;
}

Process Information

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

void print_process_info(void) {
    printf("Process ID (PID): %d\n", getpid());
    printf("Parent Process ID (PPID): %d\n", getppid());
    printf("Process Group ID (PGID): %d\n", getpgrp());
    printf("User ID (UID): %d\n", getuid());
    printf("Effective User ID (EUID): %d\n", geteuid());
    printf("Group ID (GID): %d\n", getgid());
    printf("Effective Group ID (EGID): %d\n", getegid());
}

int main(void) {
    print_process_info();
    return 0;
}

Inter-Process Communication

Pipes

#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/wait.h>

int main(void) {
    int pipefd[2];
    pid_t pid;
    char buffer[100];

    // Create pipe
    if (pipe(pipefd) == -1) {
        perror("pipe");
        return 1;
    }

    pid = fork();
    if (pid == -1) {
        perror("fork");
        return 1;
    }

    if (pid == 0) {
        // Child process - writes to pipe
        close(pipefd[0]);  // Close read end

        const char *msg = "Hello from child process!";
        write(pipefd[1], msg, strlen(msg) + 1);
        close(pipefd[1]);

        return 0;
    } else {
        // Parent process - reads from pipe
        close(pipefd[1]);  // Close write end

        read(pipefd[0], buffer, sizeof(buffer));
        printf("Parent received: %s\n", buffer);
        close(pipefd[0]);

        wait(NULL);
    }

    return 0;
}

Named Pipes (FIFOs)

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

// Writer process
void fifo_writer(void) {
    const char *fifo_path = "/tmp/myfifo";
    int fd;
    const char *msg = "Message through FIFO";

    // Create FIFO if it doesn't exist
    if (mkfifo(fifo_path, 0666) == -1) {
        perror("mkfifo");
        // Continue if already exists
    }

    fd = open(fifo_path, O_WRONLY);
    if (fd == -1) {
        perror("open");
        return;
    }

    write(fd, msg, strlen(msg) + 1);
    close(fd);
}

// Reader process
void fifo_reader(void) {
    const char *fifo_path = "/tmp/myfifo";
    int fd;
    char buffer[100];

    fd = open(fifo_path, O_RDONLY);
    if (fd == -1) {
        perror("open");
        return;
    }

    read(fd, buffer, sizeof(buffer));
    printf("Received: %s\n", buffer);
    close(fd);

    unlink(fifo_path);
}

Signal Handling

Basic Signal Handling

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

volatile sig_atomic_t keep_running = 1;

void signal_handler(int signum) {
    if (signum == SIGINT) {
        printf("\nReceived SIGINT (Ctrl+C)\n");
        keep_running = 0;
    } else if (signum == SIGTERM) {
        printf("Received SIGTERM\n");
        keep_running = 0;
    }
}

int main(void) {
    struct sigaction sa;

    // Setup signal handler
    sa.sa_handler = signal_handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;

    if (sigaction(SIGINT, &sa, NULL) == -1) {
        perror("sigaction SIGINT");
        return 1;
    }

    if (sigaction(SIGTERM, &sa, NULL) == -1) {
        perror("sigaction SIGTERM");
        return 1;
    }

    printf("Running... Press Ctrl+C to stop\n");
    while (keep_running) {
        printf("Working...\n");
        sleep(1);
    }

    printf("Cleaning up and exiting\n");
    return 0;
}

Sending Signals

#include <signal.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>

int main(void) {
    pid_t pid;

    pid = fork();
    if (pid == -1) {
        perror("fork");
        return 1;
    }

    if (pid == 0) {
        // Child process - pause until signal received
        printf("Child waiting for signal...\n");
        pause();
        printf("Child received signal\n");
        return 0;
    } else {
        // Parent process - send signal after delay
        printf("Parent sleeping...\n");
        sleep(2);

        printf("Parent sending SIGUSR1 to child\n");
        if (kill(pid, SIGUSR1) == -1) {
            perror("kill");
            return 1;
        }

        wait(NULL);
        printf("Child process completed\n");
    }

    return 0;
}

Signal Masking

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

int main(void) {
    sigset_t set, oldset;

    // Initialize signal set
    sigemptyset(&set);
    sigaddset(&set, SIGINT);

    // Block SIGINT
    if (sigprocmask(SIG_BLOCK, &set, &oldset) == -1) {
        perror("sigprocmask");
        return 1;
    }

    printf("SIGINT blocked for 5 seconds (Ctrl+C won't work)\n");
    sleep(5);

    // Unblock SIGINT
    if (sigprocmask(SIG_SETMASK, &oldset, NULL) == -1) {
        perror("sigprocmask");
        return 1;
    }

    printf("SIGINT unblocked (Ctrl+C will work now)\n");
    sleep(5);

    return 0;
}

System Calls

Common System Calls

#include <unistd.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <stdio.h>
#include <time.h>

void demonstrate_system_calls(void) {
    struct stat file_stat;
    char cwd[1024];
    time_t current_time;

    // Get current working directory
    if (getcwd(cwd, sizeof(cwd)) != NULL) {
        printf("Current directory: %s\n", cwd);
    }

    // Get file information
    if (stat("/etc/passwd", &file_stat) == 0) {
        printf("File size: %lld bytes\n",
               (long long)file_stat.st_size);
        printf("Permissions: %o\n", file_stat.st_mode & 0777);
        printf("Last modified: %s", ctime(&file_stat.st_mtime));
    }

    // Get current time
    current_time = time(NULL);
    printf("Current time: %s", ctime(&current_time));

    // Get process times
    printf("Process times:\n");
    printf("  Ticks per second: %ld\n", sysconf(_SC_CLK_TCK));
}

Directory Operations

#include <sys/types.h>
#include <dirent.h>
#include <stdio.h>
#include <errno.h>

int list_directory(const char *path) {
    DIR *dir;
    struct dirent *entry;

    dir = opendir(path);
    if (dir == NULL) {
        perror("opendir");
        return -1;
    }

    printf("Contents of %s:\n", path);
    errno = 0;
    while ((entry = readdir(dir)) != NULL) {
        printf("  %s (type: %d)\n", entry->d_name, entry->d_type);
    }

    if (errno != 0) {
        perror("readdir");
        closedir(dir);
        return -1;
    }

    if (closedir(dir) == -1) {
        perror("closedir");
        return -1;
    }

    return 0;
}

Error Handling

Using errno

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

void demonstrate_error_handling(void) {
    int fd;

    // Attempt to open non-existent file
    fd = open("/nonexistent/file.txt", O_RDONLY);
    if (fd == -1) {
        int saved_errno = errno;  // Save errno immediately

        printf("Error code: %d\n", saved_errno);
        printf("Error message (strerror): %s\n", strerror(saved_errno));

        // Alternative using perror
        errno = saved_errno;
        perror("open");

        // Check specific error
        if (saved_errno == ENOENT) {
            fprintf(stderr, "File does not exist\n");
        } else if (saved_errno == EACCES) {
            fprintf(stderr, "Permission denied\n");
        }
    }
}

Robust Error Handling Pattern

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

int copy_file(const char *src, const char *dst) {
    int src_fd = -1, dst_fd = -1;
    char buffer[4096];
    ssize_t bytes_read, bytes_written;
    int ret = -1;

    // Open source file
    src_fd = open(src, O_RDONLY);
    if (src_fd == -1) {
        fprintf(stderr, "Cannot open source file %s: %s\n",
                src, strerror(errno));
        goto cleanup;
    }

    // Open destination file
    dst_fd = open(dst, O_WRONLY | O_CREAT | O_TRUNC, 0644);
    if (dst_fd == -1) {
        fprintf(stderr, "Cannot open destination file %s: %s\n",
                dst, strerror(errno));
        goto cleanup;
    }

    // Copy data
    while ((bytes_read = read(src_fd, buffer, sizeof(buffer))) > 0) {
        bytes_written = write(dst_fd, buffer, bytes_read);
        if (bytes_written != bytes_read) {
            fprintf(stderr, "Write error: %s\n", strerror(errno));
            goto cleanup;
        }
    }

    if (bytes_read == -1) {
        fprintf(stderr, "Read error: %s\n", strerror(errno));
        goto cleanup;
    }

    ret = 0;  // Success

cleanup:
    if (src_fd != -1) close(src_fd);
    if (dst_fd != -1) close(dst_fd);
    return ret;
}

POSIX APIs

POSIX Threads Basics

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

void *thread_function(void *arg) {
    int *value = (int *)arg;
    printf("Thread running with value: %d\n", *value);
    sleep(1);
    *value = 100;
    return value;
}

int main(void) {
    pthread_t thread;
    int value = 42;
    void *result;

    if (pthread_create(&thread, NULL, thread_function, &value) != 0) {
        perror("pthread_create");
        return 1;
    }

    printf("Main thread waiting...\n");

    if (pthread_join(thread, &result) != 0) {
        perror("pthread_join");
        return 1;
    }

    printf("Thread returned: %d\n", *(int *)result);
    return 0;
}

POSIX Shared Memory

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

int main(void) {
    const char *name = "/my_shm";
    const size_t size = 4096;
    int shm_fd;
    void *ptr;

    // Create shared memory object
    shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666);
    if (shm_fd == -1) {
        perror("shm_open");
        return 1;
    }

    // Set size
    if (ftruncate(shm_fd, size) == -1) {
        perror("ftruncate");
        return 1;
    }

    // Map shared memory
    ptr = mmap(0, size, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd, 0);
    if (ptr == MAP_FAILED) {
        perror("mmap");
        return 1;
    }

    // Write to shared memory
    strcpy((char *)ptr, "Hello, shared memory!");

    // Cleanup
    munmap(ptr, size);
    close(shm_fd);
    shm_unlink(name);

    return 0;
}

Best Practices

Always Check Return Values: Every system call can fail. Check return values and handle errors appropriately using errno, perror, or strerror.
Use Signal-Safe Functions: In signal handlers, only use async-signal-safe functions. Avoid printf, malloc, and other non-reentrant functions.
Close File Descriptors: Always close file descriptors when done to prevent resource leaks. Use cleanup patterns with goto for complex error handling.
Handle Partial I/O: read() and write() may transfer fewer bytes than requested. Always check return values and loop when necessary.
Use O_CLOEXEC: When opening files, use O_CLOEXEC flag to prevent file descriptor leaks across exec() calls.
Proper Process Cleanup: Always wait for child processes using wait() or waitpid() to prevent zombie processes.
Use sigaction Over signal: The sigaction() interface is more portable and reliable than the older signal() function.
Avoid Race Conditions: Be careful with signals and file operations. Use proper synchronization mechanisms like mutexes or semaphores.
Set Signal Masks Carefully: Block signals during critical sections to prevent race conditions, but restore the original mask afterward.
Use POSIX Standards: Prefer POSIX-compliant functions over system-specific extensions for better portability across Unix-like systems.

Common Pitfalls

Ignoring errno: Not checking errno after system call failures or checking it when no error occurred can lead to misleading error messages.
File Descriptor Leaks: Forgetting to close file descriptors in error paths causes resource exhaustion over time.
Zombie Processes: Not waiting for child processes leaves zombie processes that consume system resources until parent exits.
Signal Handler Complexity: Using non-async-signal-safe functions in signal handlers can cause deadlocks or crashes.
Assuming Complete I/O: Assuming read() or write() transfers all requested bytes can lead to data corruption or loss.
Race Conditions with fork: File descriptors and signals can cause race conditions when forking. Use careful synchronization.
Incorrect exec() Usage: Forgetting the NULL terminator in argument arrays for exec() family functions causes undefined behavior.
Buffer Overflows: Not checking sizes when reading into buffers can cause security vulnerabilities and crashes.
Mixing I/O Methods: Mixing low-level I/O (open, read, write) with stdio (fopen, fread, fwrite) on the same file can cause buffering issues.
Hardcoded Paths: Using hardcoded paths instead of environment variables or configuration files reduces portability and flexibility.

When to Use This Skill

Use C systems programming when you need to:

Develop operating system components or kernel modules
Create system utilities and command-line tools
Write device drivers or low-level hardware interfaces
Build high-performance servers requiring direct system control
Implement process managers or job schedulers
Create inter-process communication mechanisms
Develop embedded systems with limited resources
Build tools requiring precise control over processes and signals
Implement custom file systems or storage solutions
Work with real-time systems requiring deterministic behavior

This skill is essential for systems programmers, embedded developers, and anyone working close to the operating system level.

Resources

Documentation

The Linux Programming Interface by Michael Kerrisk
Advanced Programming in the UNIX Environment by W. Richard Stevens
POSIX.1-2017 specification
Linux man-pages project: https://man7.org/linux/man-pages/

Online Resources

Linux System Call Reference: https://syscalls.w3challs.com/
GNU C Library Manual: https://www.gnu.org/software/libc/manual/
The Open Group Base Specifications: https://pubs.opengroup.org/onlinepubs/9699919799/
Linux kernel documentation: https://www.kernel.org/doc/html/latest/

Tools

strace: Trace system calls and signals
ltrace: Library call tracer
gdb: GNU debugger for debugging system programs
valgrind: Memory debugging and profiling
perf: Linux profiling tool for performance analysis

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