操作系统调用过程_系统调用的功能

目录

💨什么是系统调用，有何作用？

💨系统调用与库函数的区别

💨系统调用背后的过程

 💨总结

我们将带着以下问题去学习什么是系统调用

什么是系统调用，有何作用？

操作系统作为用户和计算机硬件之间的接口，需要向上提供一些简单易用的服务。主要包括命令接口和程序接口。其中，程序接口由一组系统调用组成。

“系统调用”是操作系统提供给应用程序(程序员/编程人员)使用的接口，可以理解为一种可供应用程序调用的特殊函数，应用程序可以发出系统调用请求来获得操作系统的服务。

问题：操作系统为什么要提供"系统调用"功能？

生活场景：你去学校打印店打印论文，当你按下”打印“之后，打印机开始工作。你的论文打印到一半时，另一位同学按下了”打印“按钮开始打印他自己的论文。最终，打印机还是先打印完你的论文后，再打印另一位同学的论文。

那么如果各个进程可以随意地使用打印机，会发生什么情况？

你的论文打印到一半时，另一位同学按下了”打印按钮“开始打印他自己的论文，结果你的论文就不打印了，开始打印另一位同学的论文...怎么不先来后到了！

解决办法：操作系统提供”系统调用“功能，用户进程想要使用打印机这种共享资源，只能通过系统调用向操作系统发起请求。操作系统会对各个请求进行协调管理。

应用程序通过系统调用请求操作系统的服务。系统中的各种共享资源都由操作系统统一掌握，因此在用户程序中，凡是与资源有关的操作(如存储分配、I/O操作、文件管理等)，都必须通过系统调用的方式向操作系统提出服务请求，由操作系统代为完成。这样可以保证系统的稳定性和安全性，防止用户进行非法操作。

系统调用相关处理涉及到对系统资源的管理、对进程的控制，这些功能需要执行一些特权指令才能完成，因此系统调用的相关处理需要在核心态下进行。

系统调用与库函数的区别

不涉及系统调用的库函数：如"取绝对值"的函数

涉及系统调用的库函数：如最常见的打印"print"函数

系统调用背后的过程

 int 指令的参数 x 指明了系统调用号。此处的 int 不是整型的意思，其实是interrupt的缩写。

在Linux系统中可以通过  # cat /usr/include/asm/unistd_32.h 或者 # cat /usr/include/asm/unistd_64.h 命令查看系统调用号

#ifndef _ASM_X86_UNISTD_32_H #define _ASM_X86_UNISTD_32_H 1 #define __NR_restart_syscall 0 #define __NR_exit 1 #define __NR_fork 2 #define __NR_read 3 #define __NR_write 4 #define __NR_open 5 #define __NR_close 6 #define __NR_waitpid 7 #define __NR_creat 8 #define __NR_link 9 #define __NR_unlink 10 #define __NR_execve 11 #define __NR_chdir 12 #define __NR_time 13 #define __NR_mknod 14 #define __NR_chmod 15 #define __NR_lchown 16 #define __NR_break 17 #define __NR_oldstat 18 #define __NR_lseek 19 #define __NR_getpid 20 #define __NR_mount 21 #define __NR_umount 22 #define __NR_setuid 23 #define __NR_getuid 24 #define __NR_stime 25 #define __NR_ptrace 26 #define __NR_alarm 27 #define __NR_oldfstat 28 #define __NR_pause 29 #define __NR_utime 30 #define __NR_stty 31 #define __NR_gtty 32 #define __NR_access 33 #define __NR_nice 34 #define __NR_ftime 35 #define __NR_sync 36 #define __NR_kill 37 #define __NR_rename 38 #define __NR_mkdir 39 #define __NR_rmdir 40 #define __NR_dup 41 #define __NR_pipe 42 #define __NR_times 43 #define __NR_prof 44 #define __NR_brk 45 #define __NR_setgid 46 #define __NR_getgid 47 #define __NR_signal 48 #define __NR_geteuid 49 #define __NR_getegid 50 #define __NR_acct 51 #define __NR_umount2 52 #define __NR_lock 53 #define __NR_ioctl 54 #define __NR_fcntl 55 #define __NR_mpx 56 #define __NR_setpgid 57 #define __NR_ulimit 58 #define __NR_oldolduname 59 #define __NR_umask 60 #define __NR_chroot 61 #define __NR_ustat 62 #define __NR_dup2 63 #define __NR_getppid 64 #define __NR_getpgrp 65 #define __NR_setsid 66 #define __NR_sigaction 67 #define __NR_sgetmask 68 #define __NR_ssetmask 69 #define __NR_setreuid 70 #define __NR_setregid 71 #define __NR_sigsuspend 72 #define __NR_sigpending 73 #define __NR_sethostname 74 #define __NR_setrlimit 75 #define __NR_getrlimit 76 #define __NR_getrusage 77 #define __NR_gettimeofday 78 #define __NR_settimeofday 79 #define __NR_getgroups 80 #define __NR_setgroups 81 #define __NR_select 82 #define __NR_symlink 83 #define __NR_oldlstat 84 #define __NR_readlink 85 #define __NR_uselib 86 #define __NR_swapon 87 #define __NR_reboot 88 #define __NR_readdir 89 #define __NR_mmap 90 #define __NR_munmap 91 #define __NR_truncate 92 #define __NR_ftruncate 93 #define __NR_fchmod 94 #define __NR_fchown 95 #define __NR_getpriority 96 #define __NR_setpriority 97 #define __NR_profil 98 #define __NR_statfs 99 #define __NR_fstatfs 100 #define __NR_ioperm 101 #define __NR_socketcall 102 #define __NR_syslog 103 #define __NR_setitimer 104 #define __NR_getitimer 105 #define __NR_stat 106 #define __NR_lstat 107 #define __NR_fstat 108 #define __NR_olduname 109 #define __NR_iopl 110 #define __NR_vhangup 111 #define __NR_idle 112 #define __NR_vm86old 113 #define __NR_wait4 114 #define __NR_swapoff 115 #define __NR_sysinfo 116 #define __NR_ipc 117 #define __NR_fsync 118 #define __NR_sigreturn 119 #define __NR_clone 120 #define __NR_setdomainname 121 #define __NR_uname 122 #define __NR_modify_ldt 123 #define __NR_adjtimex 124 #define __NR_mprotect 125 #define __NR_sigprocmask 126 #define __NR_create_module 127 #define __NR_init_module 128 #define __NR_delete_module 129 #define __NR_get_kernel_syms 130 #define __NR_quotactl 131 #define __NR_getpgid 132 #define __NR_fchdir 133 #define __NR_bdflush 134 #define __NR_sysfs 135 #define __NR_personality 136 #define __NR_afs_syscall 137 #define __NR_setfsuid 138 #define __NR_setfsgid 139 #define __NR__llseek 140 #define __NR_getdents 141 #define __NR__newselect 142 #define __NR_flock 143 #define __NR_msync 144 #define __NR_readv 145 #define __NR_writev 146 #define __NR_getsid 147 #define __NR_fdatasync 148 #define __NR__sysctl 149 #define __NR_mlock 150 #define __NR_munlock 151 #define __NR_mlockall 152 #define __NR_munlockall 153 #define __NR_sched_setparam 154 #define __NR_sched_getparam 155 #define __NR_sched_setscheduler 156 #define __NR_sched_getscheduler 157 #define __NR_sched_yield 158 #define __NR_sched_get_priority_max 159 #define __NR_sched_get_priority_min 160 #define __NR_sched_rr_get_interval 161 #define __NR_nanosleep 162 #define __NR_mremap 163 #define __NR_setresuid 164 #define __NR_getresuid 165 #define __NR_vm86 166 #define __NR_query_module 167 #define __NR_poll 168 #define __NR_nfsservctl 169 #define __NR_setresgid 170 #define __NR_getresgid 171 #define __NR_prctl 172 #define __NR_rt_sigreturn 173 #define __NR_rt_sigaction 174 #define __NR_rt_sigprocmask 175 #define __NR_rt_sigpending 176 #define __NR_rt_sigtimedwait 177 #define __NR_rt_sigqueueinfo 178 #define __NR_rt_sigsuspend 179 #define __NR_pread64 180 #define __NR_pwrite64 181 #define __NR_chown 182 #define __NR_getcwd 183 #define __NR_capget 184 #define __NR_capset 185 #define __NR_sigaltstack 186 #define __NR_sendfile 187 #define __NR_getpmsg 188 #define __NR_putpmsg 189 #define __NR_vfork 190 #define __NR_ugetrlimit 191 #define __NR_mmap2 192 #define __NR_truncate64 193 #define __NR_ftruncate64 194 #define __NR_stat64 195 #define __NR_lstat64 196 #define __NR_fstat64 197 #define __NR_lchown32 198 #define __NR_getuid32 199 #define __NR_getgid32 200 #define __NR_geteuid32 201 #define __NR_getegid32 202 #define __NR_setreuid32 203 #define __NR_setregid32 204 #define __NR_getgroups32 205 #define __NR_setgroups32 206 #define __NR_fchown32 207 #define __NR_setresuid32 208 #define __NR_getresuid32 209 #define __NR_setresgid32 210 #define __NR_getresgid32 211 #define __NR_chown32 212 #define __NR_setuid32 213 #define __NR_setgid32 214 #define __NR_setfsuid32 215 #define __NR_setfsgid32 216 #define __NR_pivot_root 217 #define __NR_mincore 218 #define __NR_madvise 219 #define __NR_getdents64 220 #define __NR_fcntl64 221 #define __NR_gettid 224 #define __NR_readahead 225 #define __NR_setxattr 226 #define __NR_lsetxattr 227 #define __NR_fsetxattr 228 #define __NR_getxattr 229 #define __NR_lgetxattr 230 #define __NR_fgetxattr 231 #define __NR_listxattr 232 #define __NR_llistxattr 233 #define __NR_flistxattr 234 #define __NR_removexattr 235 #define __NR_lremovexattr 236 #define __NR_fremovexattr 237 #define __NR_tkill 238 #define __NR_sendfile64 239 #define __NR_futex 240 #define __NR_sched_setaffinity 241 #define __NR_sched_getaffinity 242 #define __NR_set_thread_area 243 #define __NR_get_thread_area 244 #define __NR_io_setup 245 #define __NR_io_destroy 246 #define __NR_io_getevents 247 #define __NR_io_submit 248 #define __NR_io_cancel 249 #define __NR_fadvise64 250 #define __NR_exit_group 252 #define __NR_lookup_dcookie 253 #define __NR_epoll_create 254 #define __NR_epoll_ctl 255 #define __NR_epoll_wait 256 #define __NR_remap_file_pages 257 #define __NR_set_tid_address 258 #define __NR_timer_create 259 #define __NR_timer_settime 260 #define __NR_timer_gettime 261 #define __NR_timer_getoverrun 262 #define __NR_timer_delete 263 #define __NR_clock_settime 264 #define __NR_clock_gettime 265 #define __NR_clock_getres 266 #define __NR_clock_nanosleep 267 #define __NR_statfs64 268 #define __NR_fstatfs64 269 #define __NR_tgkill 270 #define __NR_utimes 271 #define __NR_fadvise64_64 272 #define __NR_vserver 273 #define __NR_mbind 274 #define __NR_get_mempolicy 275 #define __NR_set_mempolicy 276 #define __NR_mq_open 277 #define __NR_mq_unlink 278 #define __NR_mq_timedsend 279 #define __NR_mq_timedreceive 280 #define __NR_mq_notify 281 #define __NR_mq_getsetattr 282 #define __NR_kexec_load 283 #define __NR_waitid 284 #define __NR_add_key 286 #define __NR_request_key 287 #define __NR_keyctl 288 #define __NR_ioprio_set 289 #define __NR_ioprio_get 290 #define __NR_inotify_init 291 #define __NR_inotify_add_watch 292 #define __NR_inotify_rm_watch 293 #define __NR_migrate_pages 294 #define __NR_openat 295 #define __NR_mkdirat 296 #define __NR_mknodat 297 #define __NR_fchownat 298 #define __NR_futimesat 299 #define __NR_fstatat64 300 #define __NR_unlinkat 301 #define __NR_renameat 302 #define __NR_linkat 303 #define __NR_symlinkat 304 #define __NR_readlinkat 305 #define __NR_fchmodat 306 #define __NR_faccessat 307 #define __NR_pselect6 308 #define __NR_ppoll 309 #define __NR_unshare 310 #define __NR_set_robust_list 311 #define __NR_get_robust_list 312 #define __NR_splice 313 #define __NR_sync_file_range 314 #define __NR_tee 315 #define __NR_vmsplice 316 #define __NR_move_pages 317 #define __NR_getcpu 318 #define __NR_epoll_pwait 319 #define __NR_utimensat 320 #define __NR_signalfd 321 #define __NR_timerfd_create 322 #define __NR_eventfd 323 #define __NR_fallocate 324 #define __NR_timerfd_settime 325 #define __NR_timerfd_gettime 326 #define __NR_signalfd4 327 #define __NR_eventfd2 328 #define __NR_epoll_create1 329 #define __NR_dup3 330 #define __NR_pipe2 331 #define __NR_inotify_init1 332 #define __NR_preadv 333 #define __NR_pwritev 334 #define __NR_rt_tgsigqueueinfo 335 #define __NR_perf_event_open 336 #define __NR_recvmmsg 337 #define __NR_fanotify_init 338 #define __NR_fanotify_mark 339 #define __NR_prlimit64 340 #define __NR_name_to_handle_at 341 #define __NR_open_by_handle_at 342 #define __NR_clock_adjtime 343 #define __NR_syncfs 344 #define __NR_sendmmsg 345 #define __NR_setns 346 #define __NR_process_vm_readv 347 #define __NR_process_vm_writev 348 #define __NR_kcmp 349 #define __NR_finit_module 350 #define __NR_sched_setattr 351 #define __NR_sched_getattr 352 #define __NR_memfd_create 356 #define __NR_userfaultfd 374 #endif /* _ASM_X86_UNISTD_32_H */

系统调用的大致过程：

传递系统调用参数 --》 执行陷入指令(用户态) --》执行系统调用相应服务程序(核心态)--》返回用户程序

注意事项

1.陷入指令是在用户态执行的，执行陷入指令之后立即引发一个内中断，从而CPU进入核心态

2.发出系统调用请求是在用户态，而对系统调用的相应处理在核心态下进行

3.陷入指令是唯一一个只能在用户态执行，而不可在核心态执行的指令

 总结

系统调用会使处理器从用户态进入核心态(非常重要)。