Pipe-Related System Calls

1. COS 431 University of Maine Pipe-Related System Calls

2. Process Management System Calls • Fork system call int pid = fork(); • Only to create new process • Creates exact duplicate of current process except... • ...different return values • Exec system call family int s = exec(file,argv,envp); • Replaces the current core image with new program • No return value -- why not? • Main program in C: main(int argc, char *argv[], char *envp[]);

3. Process Management System Calls • Fork and exec work together to start new programs: ... if (fork() == 0) { /* child process: execute new program */ execve(command, parameters, 0); } else { /* parent continues */ ... } • Different forms of exec: execl(const char *path, const char *arg0, ..., const char *argn, char * /* NULL*/); execv(const char *path, char *const argv[]); execle(const char *path, const char *arg0,...,const char *argn, char *const envp[]); execve(const char *path, char *const argv[], char *const envp[]); execlp(const char *file, const char *arg0,...,const char *argn, char * /* NULL */); execvp (const char *file, char *const argv[]);

4. 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);

5. printf(“this goes to stdout\n”); fd = dup(1); /* duplicate standard out */ close(1); /* close stdout */ fdnew = open(“file”,O_WRONLY); /* fdnew = 1! */ … // write stuff here to "stdout" (fd 1) => file close(fdnew); dup(fd); /* restore 1 (stdout) to old dest. */ File Management System Calls • fd = dup(fd); • Duplicates a file descriptor -- “aliased” • Useful for situation such as: • want to write some output to standard out, then to a file, then back to standard out -- all using printf • if close stdout, then it’s gone -- can’t get it back! • instead:

6. printf(“this goes to stdout\n”); dup2(1,3); // make 3 alias for 1 fdnew = open(“file”,O_WRONLY); // fdnew = 4 close(1); // close old standard out dup2(fdnew,1); // make 1 point to file … // write stuff here to "stdout" (fd 1) => file close(fdnew); dup2(3,1); // restore STDOUT to 1 Close(3); File Management System Calls • int s = dup2(oldfd,newfd);

7. File Management System Calls • s = pipe(&fd[0]); • Create a pipe • “fd” is a two-element array of int – holds two file descriptors: • fd[0] - for reading • fd[1] - for writing • Usually idiom is something like: • Parent process creates a pipe • Parent forks a new process • Parent closes read • Child closes write • In Solaris (and possibly others): bidirectional -- data written to fd[0] is read from fd[1] and vice versa

8. Aside: Creating Simple Two-Process Pipeline #define STD_INPUT 0 #define STD_OUTPUT 1 #include <unistd.h> /* for Solaris */ pipeline(process1, process2) char *process1, *process2; /* program names */ { int fd[2]; pipe(&fd[0]); /*create pipe*/ if (fork() != 0) { /* parent */ close(fd[0]); close(STD_OUTPUT); dup(fd[1]); /* sets stdout to this pipe end */ close(fd[1]); /* don’t need fd anymore */ execl(process1,process1,0); } else { /* child */ close(fd[1]); close(STD_INPUT); dup(fd[0]); /* replace stdin */ close(fd[0]); execl(proces2,process2,0); } }