Comprehensive Guide to Using System Calls in Unix with C/C++ Programming

1. Using System Calls (Unix) • Have to tell compiler (if C/C++) where to find the headers, etc. – i.e., the “include” files • May have to tell compiler where the library itself lives • Use Unix “man” pages to tell you about this for each system call • Other resources

2. Recall Parameters to main in C. main(int argc, char *argv[]) //also char *envp[] { ………………………………..} argc: Number of command line parameters (always at least 1). argv: Array of pointers to command line parameters. Each parameter is a null terminated string.

3. Example: % ./a.out 1 4 main(int argc, char *argv[]) { ………………………………..} argc = 3 argv points to an array with three elements, each of which is a pointer to a null terminated string.

4. argv “a.out” “1” “4” Example: % ./a.out 1 4

5. The name of the program is always contained in element 0 of argv. Ex: Want to print out name of program (inefficiently): char *ptr ; ptr = argv[0] ; while (*ptr != ‘\0’) printf(“%c”, *ptr++) ;

6. Exec Family of System Calls • Used to begin a processes execution. • Overwrites process (core) image of the calling process with that of called program. • Several flavors, use the one most suited to needs. • int execv( char *path, char *argvec[]) ; • int execl(const char *path, const char *arg0, ..., const char *argn, char * /* NULL*/);

7. exec Function calls int execv( char *path, char *argvec[]) ; pathname: Can be an executable program in your directory (application code) or a system program such as ls, cd, date, ………….. argvec: Array of pointers to NULL terminated strings. First element should always be the name of the program, last element should always be NULL.

8. main (int argc, *argv[]) { int my_pid ; char *args[3] ; my_pid = fork() ; if (my_pid ==0) { args[0] = “./a.out” ; All executed by args[1] = “2” ; child process args[2] = NULL; execv(“./a.out”, args) ; printf(“OOOpppssss.\n”) ; } else printf(“Not a problem!\n”) ; Executed by parent }

9. Program a.out will begin to execute and will have the following parameters to main: argc = 3. argv[0] = ./a.out argv[1] = “2”

10. Process Management System Calls • wait() Blocks parent process until all child processes have terminated • wait(pid) Blocks until a particular child process terminates. • Exit() Terminate the process and de-allocate its resources.

11. Very Simple Shell while(1) { type_prompt() ; read_command(command, parameters) ; if (fork() != 0) //I am the parent wait() ; else { execv(command, parameters) ; printf(“OOOPPPSSS\”) ; } }

12. File Management System Calls • int fd = open(name, how, mode); • “name” is pathname • Mode is file permissions. • “how” is an integer whose bits tell the system call how to open the file: O_RDONLY: read only (0) O_WRONLY: write only (1) O_RDWR: read and write (2) • int s = close(fd);

13. File Management System Calls • int n = read(int fd, void *buffer, int nbytes); • fd: file descriptor • buffer: destination • nbytes: how many bytes to read • n: number read, or 0 on EOF • file position is incremented • int n = write(int fd, void *buffer, int nbytes);

14. File Management System Calls • pos = lseek(fd,offset,whence); • “offset” and “pos” are of type off_t (on Solaris), which is really a long integer • “whence” determines what offset means: • SEEK_SET: file position is set to “offset” bytes • SEEK_CUR: file position is set to current position + “offset” • SEEK_END: file position is set to end of file + “offset” • pos: new file position

15. Reading from stdin • scanf (char *, &var) reads from the terminal according to format string (spaces ignored). Example: int j, k ; scanf(“%d”, &j) ; read first arg, store as int. scanf(“%d”, &k) ;// read second arg, store as int.

16. char x, *buf, *buf1 ; buf = (char *) malloc(1024) ; buf1 = (char *) malloc(1024) ; scanf(“%s”, buf) ; scanf(“%s”, buf1) ;