system-calls

问题 I'm trying to run two executables consecutively using this c code: #include <stdio.h> #include <unistd.h> int main (int argc, char *argv[]) { fork(); execv("./prcs1", &argv[1]); // GIVE ADDRESS OF 2nd element as starting point to skip source.txt fork(); execv("./prcs2", argv); printf("EXECV Failed\n"); } The program exits after the first execv() call despite the fork, it never gets to the second execv(). I've tried calling wait() after the first fork but I'm not sure that's what it's missing.

问题 I want to create copies of a process using fork() in C. I cant figure out how to pass arguments to the copies of my process. For example,I want to pass an integer to the process copies. Or I what to do, if I have a loop in which I call fork() and want to pass a unique value to processes (e.g. 0...N) for (int i = 0; i < 4; ++i) { fork(); // pass a unique value to new processes. } 回答1: The nice part about fork() is that each process you spawn automatically gets a copy of everything the parent

问题 I'm writing a program that loads and executes code from file. But i got a problem: "write" syscall does not work. Code successfully loads and executes, but does not display any text on the screen. Program that loads code: #include < stdio.h > #include < stdlib.h > int main(int argc,char* argv[]) { unsigned int f_size = 0; unsigned char* code_buf = NULL; void (*func_call)(void) = NULL; if(argc < 2) { printf("Usage: %s <FILE>\n",argv[0]); return 1; } FILE* fp = fopen(argv[1],"rb"); if(!fp) {

问题 I can understand how one can write a program that uses multiple processes or threads: fork() a new process and use IPC, or create multiple threads and use those sorts of communication mechanisms. I also understand context switching. That is, with only once CPU, the operating system schedules time for each process (and there are tons of scheduling algorithms out there) and thereby we achieve running multiple processes simultaneously. And now that we have multi-core processors (or multi

问题 I was doing an exercise for university where I had to return a value with exit, that value was actually a count of something. This could be above 255 (which exit() can't handle) but the teacher suggested to use test data where the count could never go above that value. After all this, I needed to handle this count value, the exit status, I got this value in the main process by using waitpid(). To my surprise, if the child process returned 1, the "real" value in the main process was 256, 2 was

问题 here is the code(exit.s): .section .data, .section .text, .globl _start _start: movl $1, %eax movl $32, %ebx syscall when I execute " as exit.s -o exit.o && ld exit.o -o exit -e _start && ./exit" the return is "Bus error: 10" and the output of " echo $? " is 138 I also tried the example of the correct answer in this question: Process command line in Linux 64 bit stil get "bus error"... 回答1: First, you are using old 32-bit Linux kernel calling convention on Mac OS X - this absolutely doesn't

问题 On Linux (or Solaris) is there a better way than hand parsing /proc/self/maps repeatedly to figure out whether or not you can read, write or execute whatever is stored at one or more addresses in memory? For instance, in Windows you have VirtualQuery . In Linux, I can mprotect to change those values, but I can't read them back. Furthermore, is there any way to know when those permissions change (e.g. when someone uses mmap on a file behind my back) other than doing something terribly invasive

问题 Suppose I want to completely take over the open() system call, maybe to wrap the actual syscall and perform some logging. One way to do this is to use LD_PRELOAD to load a (user-made) shared object library that takes over the open() entry point. The user-made open() routine then obtains the pointer to the glibc function open() by dlsym() ing it, and calling it. The solution proposed above is a dynamic solution, however. Suppose I want to link my own open() wrapper statically. How would I do

问题 I'm having trouble finding the good documentation for writing 64-bit assembly on MacOS. The 64-bit SysV ABI says the following in section A.2.1 and this SO post quotes it: A system-call is done via the syscall instruction. The kernel destroys registers %rcx and %r11. Returning from the syscall, register %rax contains the result of the system-call. A value in the range between -4095 and -1 indicates an error, it is -errno. Those two sentences are ok on Linux but are wrong on macOS Sierra with

问题 I know what dup / dup2 does, but I have no idea when it would be used. Any practical examples? Thanks. 回答1: One example use would be I/O redirection. For this you fork a child process and close the stdin or stdout file descriptors (0 and 1) and then you do a dup() on another filedescriptor of your choice which will now be mapped to the lowest available file descriptor, which is in this case 0 or 1. Using this you can now exec any child process which is possibly unaware of your application and