The interface between an application program and the Operating System is through system calls.
What is a system call?
The operating system is responsible for
- Process Management (starting, running, stopping processes)
- File Management(creating, opening, closing, reading, writing, renaming files)
- Memory Management (allocating, deallocating memory)
- Other stuff (timing, scheduling, network management)
An application program makes a system call to get the operating system to perform a service for it, like reading from a file.
One nice thing about syscalls is that you don’t have to link with a C library, so your executables can be much smaller.
System Call Numbers
For the numbers of the syscalls, look in arch/i386/kernel/entry.S for sys_call_table. The syscall numbers are offsets into that table. Several spots in the table are occupied by the syscall sys_ni_syscall. This is a placeholder that either replaces an obsolete syscall or reserves a spot for future syscalls.
Incidentally, the system calls are called from the function system_call in the same file; in particular, they are called with the assembly instruction ‘call *SYMBOL_NAME(sys_call_table)(,%eax,4)’. The part ‘*SYMBOL_NAME(sys_call_table)’ just gets replaced by a symbol name in sys_call_table. SYMBOL_NAME is a macro defined in include/linux/linkage.h, and it just replaces itself with its argument.
All Linux System Calls:-
| %eax | Name | Source | %ebx | %ecx | %edx | %esx | %edi |
| 1 | sys_exit | kernel/exit.c | int | – | – | – | – |
| 2 | sys_fork | arch/i386/kernel/process.c | struct pt_regs | – | – | – | – |
| 3 | sys_read | fs/read_write.c | unsigned int | char * | size_t | – | – |
| 4 | sys_write | fs/read_write.c | unsigned int | const char * | size_t | – | – |
| 5 | sys_open | fs/open.c | const char * | int | int | – | – |
| 6 | sys_close | fs/open.c | unsigned int | – | – | – | – |
| 7 | sys_waitpid | kernel/exit.c | pid_t | unsigned int * | int | – | – |
| 8 | sys_creat | fs/open.c | const char * | int | – | – | – |
| 9 | sys_link | fs/namei.c | const char * | const char * | – | – | – |
| 10 | sys_unlink | fs/namei.c | const char * | – | – | – | – |
| 11 | sys_execve | arch/i386/kernel/process.c | struct pt_regs | – | – | – | – |
| 12 | sys_chdir | fs/open.c | const char * | – | – | – | – |
| 13 | sys_time | kernel/time.c | int * | – | – | – | – |
| 14 | sys_mknod | fs/namei.c | const char * | int | dev_t | – | – |
| 15 | sys_chmod | fs/open.c | const char * | mode_t | – | – | – |
| 16 | sys_lchown | fs/open.c | const char * | uid_t | gid_t | – | – |
| 18 | sys_stat | fs/stat.c | char * | struct __old_kernel_stat * | – | – | – |
| 19 | sys_lseek | fs/read_write.c | unsigned int | off_t | unsigned int | – | – |
| 20 | sys_getpid | kernel/sched.c | – | – | – | – | – |
| 21 | sys_mount | fs/super.c | char * | char * | char * | – | – |
| 22 | sys_oldumount | fs/super.c | char * | – | – | – | – |
| 23 | sys_setuid | kernel/sys.c | uid_t | – | – | – | – |
| 24 | sys_getuid | kernel/sched.c | – | – | – | – | – |
| 25 | sys_stime | kernel/time.c | int * | – | – | – | – |
| 26 | sys_ptrace | arch/i386/kernel/ptrace.c | long | long | long | long | – |
| 27 | sys_alarm | kernel/sched.c | unsigned int | – | – | – | – |
| 28 | sys_fstat | fs/stat.c | unsigned int | struct __old_kernel_stat * | – | – | – |
| 29 | sys_pause | arch/i386/kernel/sys_i386.c | – | – | – | – | – |
| 30 | sys_utime | fs/open.c | char * | struct utimbuf * | – | – | – |
| 33 | sys_access | fs/open.c | const char * | int | – | – | – |
| 34 | sys_nice | kernel/sched.c | int | – | – | – | – |
| 36 | sys_sync | fs/buffer.c | – | – | – | – | – |
| 37 | sys_kill | kernel/signal.c | int | int | – | – | – |
| 38 | sys_rename | fs/namei.c | const char * | const char * | – | – | – |
| 39 | sys_mkdir | fs/namei.c | const char * | int | – | – | – |
| 40 | sys_rmdir | fs/namei.c | const char * | – | – | – | – |
| 41 | sys_dup | fs/fcntl.c | unsigned int | – | – | – | – |
| 42 | sys_pipe | arch/i386/kernel/sys_i386.c | unsigned long * | – | – | – | – |
| 43 | sys_times | kernel/sys.c | struct tms * | – | – | – | – |
| 45 | sys_brk | mm/mmap.c | unsigned long | – | – | – | – |
| 46 | sys_setgid | kernel/sys.c | gid_t | – | – | – | – |
| 47 | sys_getgid | kernel/sched.c | – | – | – | – | – |
| 48 | sys_signal | kernel/signal.c | int | __sighandler_t | – | – | – |
| 49 | sys_geteuid | kernel/sched.c | – | – | – | – | – |
| 50 | sys_getegid | kernel/sched.c | – | – | – | – | – |
| 51 | sys_acct | kernel/acct.c | const char * | – | – | – | – |
| 52 | sys_umount | fs/super.c | char * | int | – | – | – |
| 54 | sys_ioctl | fs/ioctl.c | unsigned int | unsigned int | unsigned long | – | – |
| 55 | sys_fcntl | fs/fcntl.c | unsigned int | unsigned int | unsigned long | – | – |
| 57 | sys_setpgid | kernel/sys.c | pid_t | pid_t | – | – | – |
| 59 | sys_olduname | arch/i386/kernel/sys_i386.c | struct oldold_utsname * | – | – | – | – |
| 60 | sys_umask | kernel/sys.c | int | – | – | – | – |
| 61 | sys_chroot | fs/open.c | const char * | – | – | – | – |
| 62 | sys_ustat | fs/super.c | dev_t | struct ustat * | – | – | – |
| 63 | sys_dup2 | fs/fcntl.c | unsigned int | unsigned int | – | – | – |
| 64 | sys_getppid | kernel/sched.c | – | – | – | – | – |
| 65 | sys_getpgrp | kernel/sys.c | – | – | – | – | – |
| 66 | sys_setsid | kernel/sys.c | – | – | – | – | – |
| 67 | sys_sigaction | arch/i386/kernel/signal.c | int | const struct old_sigaction * | struct old_sigaction * | – | – |
| 68 | sys_sgetmask | kernel/signal.c | – | – | – | – | – |
| 69 | sys_ssetmask | kernel/signal.c | int | – | – | – | – |
| 70 | sys_setreuid | kernel/sys.c | uid_t | uid_t | – | – | – |
| 71 | sys_setregid | kernel/sys.c | gid_t | gid_t | – | – | – |
| 72 | sys_sigsuspend | arch/i386/kernel/signal.c | int | int | old_sigset_t | – | – |
| 73 | sys_sigpending | kernel/signal.c | old_sigset_t * | – | – | – | – |
| 74 | sys_sethostname | kernel/sys.c | char * | int | – | – | – |
| 75 | sys_setrlimit | kernel/sys.c | unsigned int | struct rlimit * | – | – | – |
| 76 | sys_getrlimit | kernel/sys.c | unsigned int | struct rlimit * | – | – | – |
| 77 | sys_getrusage | kernel/sys.c | int | struct rusage * | – | – | – |
| 78 | sys_gettimeofday | kernel/time.c | struct timeval * | struct timezone * | – | – | – |
| 79 | sys_settimeofday | kernel/time.c | struct timeval * | struct timezone * | – | – | – |
| 80 | sys_getgroups | kernel/sys.c | int | gid_t * | – | – | – |
| 81 | sys_setgroups | kernel/sys.c | int | gid_t * | – | – | – |
| 82 | old_select | arch/i386/kernel/sys_i386.c | struct sel_arg_struct * | – | – | – | – |
| 83 | sys_symlink | fs/namei.c | const char * | const char * | – | – | – |
| 84 | sys_lstat | fs/stat.c | char * | struct __old_kernel_stat * | – | – | – |
| 85 | sys_readlink | fs/stat.c | const char * | char * | int | – | – |
| 86 | sys_uselib | fs/exec.c | const char * | – | – | – | – |
| 87 | sys_swapon | mm/swapfile.c | const char * | int | – | – | – |
| 88 | sys_reboot | kernel/sys.c | int | int | int | void * | – |
| 89 | old_readdir | fs/readdir.c | unsigned int | void * | unsigned int | – | – |
| 90 | old_mmap | arch/i386/kernel/sys_i386.c | struct mmap_arg_struct * | – | – | – | – |
| 91 | sys_munmap | mm/mmap.c | unsigned long | size_t | – | – | – |
| 92 | sys_truncate | fs/open.c | const char * | unsigned long | – | – | – |
| 93 | sys_ftruncate | fs/open.c | unsigned int | unsigned long | – | – | – |
| 94 | sys_fchmod | fs/open.c | unsigned int | mode_t | – | – | – |
| 95 | sys_fchown | fs/open.c | unsigned int | uid_t | gid_t | – | – |
| 96 | sys_getpriority | kernel/sys.c | int | int | – | – | – |
| 97 | sys_setpriority | kernel/sys.c | int | int | int | – | – |
| 99 | sys_statfs | fs/open.c | const char * | struct statfs * | – | – | – |
| 100 | sys_fstatfs | fs/open.c | unsigned int | struct statfs * | – | – | – |
| 101 | sys_ioperm | arch/i386/kernel/ioport.c | unsigned long | unsigned long | int | – | – |
| 102 | sys_socketcall | net/socket.c | int | unsigned long * | – | – | – |
| 103 | sys_syslog | kernel/printk.c | int | char * | int | – | – |
| 104 | sys_setitimer | kernel/itimer.c | int | struct itimerval * | struct itimerval * | – | – |
| 105 | sys_getitimer | kernel/itimer.c | int | struct itimerval * | – | – | – |
| 106 | sys_newstat | fs/stat.c | char * | struct stat * | – | – | – |
| 107 | sys_newlstat | fs/stat.c | char * | struct stat * | – | – | – |
| 108 | sys_newfstat | fs/stat.c | unsigned int | struct stat * | – | – | – |
| 109 | sys_uname | arch/i386/kernel/sys_i386.c | struct old_utsname * | – | – | – | – |
| 110 | sys_iopl | arch/i386/kernel/ioport.c | unsigned long | – | – | – | – |
| 111 | sys_vhangup | fs/open.c | – | – | – | – | – |
| 112 | sys_idle | arch/i386/kernel/process.c | – | – | – | – | – |
| 113 | sys_vm86old | arch/i386/kernel/vm86.c | unsigned long | struct vm86plus_struct * | – | – | – |
| 114 | sys_wait4 | kernel/exit.c | pid_t | unsigned long * | int options | struct rusage * | – |
| 115 | sys_swapoff | mm/swapfile.c | const char * | – | – | – | – |
| 116 | sys_sysinfo | kernel/info.c | struct sysinfo * | – | – | – | – |
| 117 | sys_ipc | arch/i386/kernel/sys_i386.c | uint | int | int | int | void * |
| 118 | sys_fsync | fs/buffer.c | unsigned int | – | – | – | – |
| 119 | sys_sigreturn | arch/i386/kernel/signal.c | unsigned long | – | – | – | – |
| 120 | sys_clone | arch/i386/kernel/process.c | struct pt_regs | – | – | – | – |
| 121 | sys_setdomainname | kernel/sys.c | char * | int | – | – | – |
| 122 | sys_newuname | kernel/sys.c | struct new_utsname * | – | – | – | – |
| 123 | sys_modify_ldt | arch/i386/kernel/ldt.c | int | void * | unsigned long | – | – |
| 124 | sys_adjtimex | kernel/time.c | struct timex * | – | – | – | – |
| 125 | sys_mprotect | mm/mprotect.c | unsigned long | size_t | unsigned long | – | – |
| 126 | sys_sigprocmask | kernel/signal.c | int | old_sigset_t * | old_sigset_t * | – | – |
| 127 | sys_create_module | kernel/module.c | const char * | size_t | – | – | – |
| 128 | sys_init_module | kernel/module.c | const char * | struct module * | – | – | – |
| 129 | sys_delete_module | kernel/module.c | const char * | – | – | – | – |
| 130 | sys_get_kernel_syms | kernel/module.c | struct kernel_sym * | – | – | – | – |
| 131 | sys_quotactl | fs/dquot.c | int | const char * | int | caddr_t | – |
| 132 | sys_getpgid | kernel/sys.c | pid_t | – | – | – | – |
| 133 | sys_fchdir | fs/open.c | unsigned int | – | – | – | – |
| 134 | sys_bdflush | fs/buffer.c | int | long | – | – | – |
| 135 | sys_sysfs | fs/super.c | int | unsigned long | unsigned long | – | – |
| 136 | sys_personality | kernel/exec_domain.c | unsigned long | – | – | – | – |
| 138 | sys_setfsuid | kernel/sys.c | uid_t | – | – | – | – |
| 139 | sys_setfsgid | kernel/sys.c | gid_t | – | – | – | – |
| 140 | sys_llseek | fs/read_write.c | unsigned int | unsigned long | unsigned long | loff_t * | unsigned int |
| 141 | sys_getdents | fs/readdir.c | unsigned int | void * | unsigned int | – | – |
| 142 | sys_select | fs/select.c | int | fd_set * | fd_set * | fd_set * | struct timeval * |
| 143 | sys_flock | fs/locks.c | unsigned int | unsigned int | – | – | – |
| 144 | sys_msync | mm/filemap.c | unsigned long | size_t | int | – | – |
| 145 | sys_readv | fs/read_write.c | unsigned long | const struct iovec * | unsigned long | – | – |
| 146 | sys_writev | fs/read_write.c | unsigned long | const struct iovec * | unsigned long | – | – |
| 147 | sys_getsid | kernel/sys.c | pid_t | – | – | – | – |
| 148 | sys_fdatasync | fs/buffer.c | unsigned int | – | – | – | – |
| 149 | sys_sysctl | kernel/sysctl.c | struct __sysctl_args * | – | – | – | – |
| 150 | sys_mlock | mm/mlock.c | unsigned long | size_t | – | – | – |
| 151 | sys_munlock | mm/mlock.c | unsigned long | size_t | – | – | – |
| 152 | sys_mlockall | mm/mlock.c | int | – | – | – | – |
| 153 | sys_munlockall | mm/mlock.c | – | – | – | – | – |
| 154 | sys_sched_setparam | kernel/sched.c | pid_t | struct sched_param * | – | – | – |
| 155 | sys_sched_getparam | kernel/sched.c | pid_t | struct sched_param * | – | – | – |
| 156 | sys_sched_setscheduler | kernel/sched.c | pid_t | int | struct sched_param * | – | – |
| 157 | sys_sched_getscheduler | kernel/sched.c | pid_t | – | – | – | – |
| 158 | sys_sched_yield | kernel/sched.c | – | – | – | – | – |
| 159 | sys_sched_get_priority_max | kernel/sched.c | int | – | – | – | – |
| 160 | sys_sched_get_priority_min | kernel/sched.c | int | – | – | – | – |
| 161 | sys_sched_rr_get_interval | kernel/sched.c | pid_t | struct timespec * | – | – | – |
| 162 | sys_nanosleep | kernel/sched.c | struct timespec * | struct timespec * | – | – | – |
| 163 | sys_mremap | mm/mremap.c | unsigned long | unsigned long | unsigned long | unsigned long | – |
| 164 | sys_setresuid | kernel/sys.c | uid_t | uid_t | uid_t | – | – |
| 165 | sys_getresuid | kernel/sys.c | uid_t * | uid_t * | uid_t * | – | – |
| 166 | sys_vm86 | arch/i386/kernel/vm86.c | struct vm86_struct * | – | – | – | – |
| 167 | sys_query_module | kernel/module.c | const char * | int | char * | size_t | size_t * |
| 168 | sys_poll | fs/select.c | struct pollfd * | unsigned int | long | – | – |
| 169 | sys_nfsservctl | fs/filesystems.c | int | void * | void * | – | – |
| 170 | sys_setresgid | kernel/sys.c | gid_t | gid_t | gid_t | – | – |
| 171 | sys_getresgid | kernel/sys.c | gid_t * | gid_t * | gid_t * | – | – |
| 172 | sys_prctl | kernel/sys.c | int | unsigned long | unsigned long | unsigned long | unsigned long |
| 173 | sys_rt_sigreturn | arch/i386/kernel/signal.c | unsigned long | – | – | – | – |
| 174 | sys_rt_sigaction | kernel/signal.c | int | const struct sigaction * | struct sigaction * | size_t | – |
| 175 | sys_rt_sigprocmask | kernel/signal.c | int | sigset_t * | sigset_t * | size_t | – |
| 176 | sys_rt_sigpending | kernel/signal.c | sigset_t * | size_t | – | – | – |
| 177 | sys_rt_sigtimedwait | kernel/signal.c | const sigset_t * | siginfo_t * | const struct timespec * | size_t | – |
| 178 | sys_rt_sigqueueinfo | kernel/signal.c | int | int | siginfo_t * | – | – |
| 179 | sys_rt_sigsuspend | arch/i386/kernel/signal.c | sigset_t * | size_t | – | – | – |
| 180 | sys_pread | fs/read_write.c | unsigned int | char * | size_t | loff_t | – |
| 181 | sys_pwrite | fs/read_write.c | unsigned int | const char * | size_t | loff_t | – |
| 182 | sys_chown | fs/open.c | const char * | uid_t | gid_t | – | – |
| 183 | sys_getcwd | fs/dcache.c | char * | unsigned long | – | – | – |
| 184 | sys_capget | kernel/capability.c | cap_user_header_t | cap_user_data_t | – | – | – |
| 185 | sys_capset | kernel/capability.c | cap_user_header_t | const cap_user_data_t | – | – | – |
| 186 | sys_sigaltstack | arch/i386/kernel/signal.c | const stack_t * | stack_t * | – | – | – |
| 187 | sys_sendfile | mm/filemap.c | int | int | off_t * | size_t | – |
| 190 | sys_vfork | arch/i386/kernel/process.c | struct pt_regs | – | – | – | – |
Technical Interview Q&A 