06-system-calls-from-user-mode

System Calls from user Mode

System calls give up control from user mode code to kernel code in order to run privileged instructions.

In xv6, there are 21 syscalls, as well as some library functions for user code, in ulib.c.

System Calls

System Call	Purpose
fork	Creates a new process (child process).
exit	Terminates the current process.
wait	Waits for a child process to finish.
pipe	Creates a unidirectional pipe for interprocess communication.
read	Reads data from a file descriptor.
write	Writes data to a file descriptor.
close	Closes an open file descriptor.
kill	Sends a signal to terminate a process.
exec	Replaces the current process with a new program.
open	Opens a file and returns a file descriptor.
mknod	Creates a special file (device file).
unlink	Deletes a file.
fstat	Retrieves file metadata from a file descriptor.
link	Creates a new directory entry (hard link) to an existing file.
mkdir	Creates a new directory.
chdir	Changes the current working directory.
dup	Duplicates an existing file descriptor.
getpid	Returns the process ID of the calling process.
sbrk	Increases or decreases the size of the process’s heap.
sleep	Puts the calling process to sleep for a specified time.
uptime	Returns the system uptime (ticks since boot).

ulib.c functions

Function	Purpose
start	Calls main() and ensures exit(0) is called if main() returns.
strcpy	Copies a string from source to destination.
strcmp	Compares two strings.
strlen	Returns the length of a string.
memset	Fills a memory area with a constant value.
strchr	Finds the first occurrence of a character in a string.
gets	Reads a line of input from standard input.
stat	Retrieves file metadata by opening and calling fstat.
atoi	Converts a string to an integer.
memmove	Copies memory, handling overlapping regions safely.
memcmp	Compares two memory areas.
memcpy	Copies memory from source to destination (calls memmove).

Initcode

There’s code that starts up a user process, by setting up init, and argv, in initcode.S.

#include "syscall.h"
 
# exec(init, argv)
.globl start
start:
        la a0, init
        la a1, argv
        li a7, SYS_exec
        ecall
 
# for(;;) exit();
exit:
        li a7, SYS_exit
        ecall
        jal exit
 
# char init[] = "/init\0";
init:
  .string "/init\0"
 
# char *argv[] = { init, 0 };
.p2align 2
argv:
  .quad init
  .quad 0