CSC209H Lecture 5. Dan Zingaro. February 4, 2015

Transcription

1 CSC209H Lecture 5 Dan Zingaro February 4, 2015

2 Why Makefiles? (King 15.4) C programs can contain multiple.c files that can be separately compiled to object code Let s say that our program comprises addone.c, sayit.c, and prog.c One way to compile it: gcc addone.c sayit.c prog.c At least two problems with that Typing: what if there are fifty files? Compilation time: what if you change something only in one of the.c files? Why recompile them all?

3 Separate Compilation You can compile any.c file (even if it doesn t have a main function) into a.o object file using gcc -c gcc -c addone.c gcc -c sayit.c gcc -c prog.c With these object files, you can link them into an executable file: gcc -o prog addone.o sayit.o prog.o

4 Makefiles Makefiles are processed by a program called make Makefiles contain information about relationships between targets and dependencies e.g. this.o file (target) depends on this.c file (dependency) make looks at file timestamps, and only re-compiles a target if the target does not exist or one or more of its dependencies is newer

5 Sample Makefile Call your makefiles makefile Type make to run the first rule, or make x to run rule x FLAGS = -g -Wall prog: addone.o sayit.o prog.o gcc ${FLAGS} -o prog addone.o sayit.o prog.o addone.o: addone.c addone.h gcc ${FLAGS} -c addone.c sayit.o: sayit.c sayit.h gcc ${FLAGS} -c sayit.c prog.o: prog.c addone.h sayit.h gcc ${FLAGS} -c prog.c clean: rm *.o prog

6 Special Variables $^ expands to all dependencies $< expands to the first dependency FLAGS = -g -Wall prog: addone.o sayit.o prog.o gcc ${FLAGS} -o prog $^ addone.o: addone.c addone.h gcc ${FLAGS} -c $< sayit.o: sayit.c sayit.h gcc ${FLAGS} -c $< prog.o: prog.c addone.h sayit.h gcc ${FLAGS} -c $< clean: rm *.o prog

7 Phony Targets Above, we had a target of clean clean is not a special keyword; it s just a filename that make won t find (unless you want to ruin things by making one!) So, since make finds no file named clean, it runs the associated actions It s common to include the clean rule to let you start from scratch in the build process

8 System Calls A System Call is a call into the Unix kernel. Three important categories of syscalls: Process management (starting today) fork, exec, wait, waitpid Signals Simple form of inter-process communication File management open, read, write, close They are called as if regular C function calls, but they are part of Unix (not C)

9 What is a Process? Process: program in execution If I load two copies of emacs, I have two emacs processes running It s also possible to run one program that itself creates more processes (using fork) Processes exist in a hierarchy Processes can have child processes Each process has a parent process Each process has a unique Process IDentifier (PID)

10 fork (Kerrisk 24.2) fork creates a near-clone of the calling process Same values of variables Same point of execution Same open files, same current working directory Both processes run concurrently; we can t predict their order of execution The new process created by fork is the child of the original parent process

11 fork: Example 1 (fork1.c) printf("good afternoon\n"); fork(); printf("good evening\n"); parent printf ("Good afternoon\n"); fork child printf ("Good evening\n"); printf ("Good evening\n");

12 Fork Return Value If fork succeeds, it returns the newly-created child s PID to the parent, and 0 to the child This differing return value allows us to have parent and child do different things after the fork If fork fails, it returns -1 to the parent and sets errno fork can fail if you run into per-user or system-wide process limits PIDs are positive integers; -1 cannot be confused for a PID

13 fork: Example 2 (fork2.c) pid_t child; printf("one program\n"); if ((child = fork()) == -1) { perror("fork"); exit(1); } if (child!= 0) //parent printf("pizza!\n"); else //child printf("chocolate!\n"); parent printf ("One program\n"); fork child printf ("Pizza!\n"); printf ("Chocolate!\n");

14 getpid and getppid (getpid.c, Kerrisk 6.2) getpid returns PID of current process getppid returns PID of parent process #include <unistd.h> #include <stdio.h> int main(void) { pid_t child; printf("i am %d\n", getpid()); child = fork(); if (child) printf("parent is %d\n", getpid()); else { printf("child is %d\n", getpid()); printf("child s parent is %d\n", getppid()); } return 0; }

15 fork: Variables (fork3.c) #include <unistd.h> #include <stdio.h> int main(void) { int i = 5; pid_t child; child = fork(); if (child) { i++; printf("%d\n", i); } else { i++; printf("%d\n", i); } return 0; }

16 fork: Tracing a Loop (fork4.c) #include <unistd.h> #include <stdio.h> int main(void) { int i; pid_t child; for (i = 0; i < 3; i++) { child = fork(); if (child == 0) printf("child %d saying hi\n", getpid()); } return 0; }

17 fork: Tracing a Loop... Here is the resulting process tree. The value of i in each node is the value of i when that process gets created. c1 i=0 c2 i=0 c3 i=1 c4 i=2 c5 i=1 c6 i=2 c7 i=2 c8 i=2