UNIX/Linux System Calls

1#include <stdio.h>
2#include <fcntl.h>    // For open() and flags
3#include <unistd.h>   // For close()
4
5int main() {
6    // Open existing file for reading
7    int fd1 = open("data.txt", O_RDONLY);
8    if (fd1 == -1) {
9        perror("Error opening file");
10        return 1;
11    }
12    printf("File opened, fd = %d\n", fd1);
13    close(fd1);
14    
15    // Create new file (or truncate existing)
16    // 0644 = rw-r--r-- (owner read/write, others read)
17    int fd2 = open("output.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
18    if (fd2 == -1) {
19        perror("Error creating file");
20        return 1;
21    }
22    close(fd2);
23    
24    return 0;
25}

1#include <stdio.h>
2#include <fcntl.h>
3#include <unistd.h>
4#include <string.h>
5
6int main() {
7    // Write to stdout using system call
8    char *msg = "Hello from write()!\n";
9    write(1, msg, strlen(msg));  // 1 = stdout
10    
11    // Copy file using read/write
12    int src = open("source.txt", O_RDONLY);
13    int dst = open("copy.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);
14    
15    if (src == -1 || dst == -1) {
16        perror("Error opening files");
17        return 1;
18    }
19    
20    char buffer[1024];
21    ssize_t bytes;
22    
23    // Read and write in chunks
24    while ((bytes = read(src, buffer, sizeof(buffer))) > 0) {
25        write(dst, buffer, bytes);
26    }
27    
28    close(src);
29    close(dst);
30    printf("File copied!\n");
31    
32    return 0;
33}

1#include <stdio.h>
2#include <unistd.h>
3#include <sys/wait.h>
4
5int main() {
6    printf("Parent PID: %d\n", getpid());
7    
8    pid_t pid = fork();  // Create child process
9    
10    if (pid == -1) {
11        perror("fork failed");
12        return 1;
13    }
14    else if (pid == 0) {
15        // Child process
16        printf("Child: My PID = %d, Parent PID = %d\n", 
17               getpid(), getppid());
18    }
19    else {
20        // Parent process
21        printf("Parent: Created child with PID = %d\n", pid);
22        wait(NULL);  // Wait for child to finish
23        printf("Parent: Child finished\n");
24    }
25    
26    return 0;
27}

1#include <stdio.h>
2#include <unistd.h>
3#include <sys/wait.h>
4
5int main() {
6    pid_t pid = fork();
7    
8    if (pid == 0) {
9        // Child: run "ls -la"
10        printf("Child running ls...\n");
11        execlp("ls", "ls", "-la", NULL);  // Search PATH
12        
13        // Only reached if exec fails
14        perror("exec failed");
15        return 1;
16    }
17    else {
18        // Parent waits
19        wait(NULL);
20        printf("Command completed\n");
21    }
22    
23    return 0;
24}

1#include <stdio.h>
2#include <unistd.h>
3#include <sys/stat.h>
4
5int main() {
6    char cwd[256];
7    
8    // Get current directory
9    if (getcwd(cwd, sizeof(cwd)) != NULL) {
10        printf("Current dir: %s\n", cwd);
11    }
12    
13    // Create a new directory
14    if (mkdir("mydir", 0755) == 0) {
15        printf("Created mydir/\n");
16    }
17    
18    // Change to new directory
19    if (chdir("mydir") == 0) {
20        getcwd(cwd, sizeof(cwd));
21        printf("Now in: %s\n", cwd);
22    }
23    
24    // Go back and remove
25    chdir("..");
26    rmdir("mydir");
27    printf("Removed mydir/\n");
28    
29    return 0;
30}

1#include <stdio.h>
2#include <fcntl.h>
3#include <unistd.h>
4#include <errno.h>
5#include <string.h>
6
7int main() {
8    int fd = open("nonexistent.txt", O_RDONLY);
9    
10    if (fd == -1) {
11        // Method 1: perror (prints to stderr)
12        perror("Error opening file");
13        
14        // Method 2: strerror (returns string)
15        printf("Error: %s\n", strerror(errno));
16        
17        // Method 3: Check specific error
18        if (errno == ENOENT) {
19            printf("File not found!\n");
20        } else if (errno == EACCES) {
21            printf("Permission denied!\n");
22        }
23        
24        return 1;
25    }
26    
27    close(fd);
28    return 0;
29}