21.3: Appendix C - System Services

This appendix provides a listing and brief description of a subset of the system service calls. This list is for 64-bit Ubuntu systems. A more complete list can be obtained from multiple web sources.

23.1 Return Codes

The system call will return a code in the rax register. If the value returned is less than 0, that is an indication that an error has occurred. If the operation is successful, the value returned will depend on the specific system service. Refer to the Error Codes section for additional information regarding the values of the error codes.

23.2 Basic System Services

The following table summarizes the more common system services.

Call Code (rax)	System Service	Description
0	SYS_read	Read characters
		rdi = file descriptor (of where to read from)
		rsi = address of where to store characters
		rdx = count of characters to read
	If unsuccessful, returns negative value. If successful, returns count of characters actually read.
1	SYS_write	Write characters
		rdi = file descriptor (of where to write to)
		rsi = address of characters to write
		rdx = count of characters to write
	If unsuccessful, returns negative value. If successful, returns count of characters actually written.
2	SYS_open	Open a file
		rdi = address of NULL terminated file name
		rsi = file status flags (typically O_RDONLY)
	If unsuccessful, returns negative value. If successful, returns file descriptor.
3	SYS_close	Close an open file
		rdi = file descriptor of open file to close
	If unsuccessful, returns negative value.
8	SYS_lseek	Reposition the file read/write file offset.
		rdi = file descriptor (of where to write to)
		rsi = offset
		rdx = origin
	If unsuccessful, returns negative value.
57	SYS_fork	Fork current process.
59	SYS_execve	Execute a program
		rdi = Address of NULL terminated string for name of program to execute.
60	SYS_exit	Terminate executing process.
		rdi = exit status (typically 0)
85	SYS_creat	Open/Create a file.
		rdi = address of NULL terminated file name
		rsi = file mode flags
	If unsuccessful, returns negative value. If successful, returns file descriptor.
96	SYS_gettimeofday	Get date and time of day
		rdi = address of time value structure
		rsi = address of time zone structure
	If unsuccessful, returns negative value. If successful, returns information in the passed structures.

23.3 File Modes

When performing file operations, the file mode provides information to the operating system regarding the file access permissions that will be allowed.

When opening an existing file, one of the following file modes must be specified.

Mode	Value	Description
O_RDONLY	0	Read only. Allow reading from the file, but to not allow writing to the file. Most common operation.
O_WRONLY	1	Write only. Typically used if information is to be appended to a file.
O_RDWR	2	Allow simultaneous reading and writing.

When creating a new file, the file permissions must be specified. Below are the complete set of file permissions. As is standard for Linux file systems, the permission values are specified in Octal.

Mode	Value	Description
S_IRWXU	00700q	User (file owner) has read, write, and execute permission.
S_IRUSR	00400q	User (file owner) has read permission.
S_IWUSR	00200q	User (file owner) has write permission.
S_IXUSR	00100q	User (file owner) has execute permission.
S_IRWXG	00070q	Group has read, write, and execute permission.
S_IRGRP	00040q	Group has read permission.
S_IWGRP	00020q	Group has write permission.
S_IXGRP	00010q	Group has execute permission.
S_IRWXO	00007q	Others have read, write, and execute permission.
S_IROTH	00004q	Others have read permission.
S_IWOTH	00002q	Others have write permission.
S_IXOTH	00001q	Others have execute permission.

The text examples only address permissions for the user or owner of the file.

23.4 Error Codes

If a system service returns an error, the value of the return code will be negative. The following is a list of the error code. The code value is provided along with the Linux symbolic name. High-level languages typically use the name which is not used at the assembly level and is only provided for reference.

Error Code	Symbolic Name	Description
-1	EPERM	Operation not permitted.
-2	ENOENT	No such file or directory.
-3	ESRCH	No such process.
-4	EINTR	Interrupted system call.
-5	EIO	I/O Error.
-6	ENXIO	No such device or address.
-7	E2BIG	Argument list too long.
-8	ENOEXEC	Exec format error.
-9	EBADF	Bad file number.
-10	ECHILD	No child process.
-11	EAGAIN	Try again.
-12	ENOMEM	Out of memory.
-13	EACCES	Permission denied.
-14	EFAULT	Bad address.
-15	ENOTBLK	Block device required.
-16	EBUSY	Device or resource busy.
-17	EEXIST	File exists.
-18	EXDEV	Cross-device link.
-19	ENODEV	No such device.
-20	ENOTDIR	Not a directory.
-21	EISDIR	Is a directory.
-22	EINV AL	Invalid argument.
-23	ENFILE	File table overflow.
-24	EMFILE	Too many open files.
-25	ENOTTY	Not a typewriter.
-26	ETXTBSY	Text file busy.
-27	EFBIG	File too large.
-28	ENOSPC	No space left on device.
-29	ESPIPE	Illegal seek.
-30	EROFS	Read-only file system.
-31	EMLINK	Too many links.
-32	EPIPE	Broken pipe.
-33	EDOM	Math argument out of domain of function.
-34	ERANGE	Math result not representable.

Only the most common error codes are shown. A complete list can be found via the Internet or by looking on the current system includes files. For Ubuntu, this is typically located in /usr/include/asm-generic/errno-base.h.