Appendix C: Important System Calls

These are some of the more important system calls to use when dealing with Linux. For most cases, however, it is best to use library functions rather than direct system calls, because the system calls were designed to be minimalistic while the library functions were designed to be easy to program with. For information about the Linux C library, see the manual at http://www.gnu.org/software/libc/manual/

Remember that %eax holds the system call numbers, and that the return values and error codes are also stored in %eax.

Table C-1: Important Linux System Calls

%eax

Name

%ebx

%ecx

%edx

Notes

1

exit

return value (int)

Exits the program

3

read

file descriptor

buffer start

buffer size (int)

Reads into the given buffer

4

write

file descriptor

buffer start

buffer size (int)

Writes the buffer to the file descriptor

5

open

null-terminate file name

option list

permission mode

Opens the given file. Returns the file descriptor or an error number.

6

close

file descriptor

Closes the give file descriptor

12

chdir

null-terminated directory name

Changes the current directory of your program.

19

lseek

file descriptor

offset

mode

Repositions where you are in the given file. The mode (called the "whence") should be 0 for absolute positioning, and 1 for relative positioning.

20

getpid

Returns the process ID of the current process.

39

mkdir

null-terminated directory name

permission mode

Creates the given directory. Assumes all directories leading up to it already exist.

40

rmdir

null-terminated directory name

Removes the given directory.

41

dup

file descriptor

Returns a new file descriptor pat works just like the existing file descriptor.

42

pipe

pipe array

Creates two file descriptors, where writing on one produces data to read on the other and vice-versa. %ebx is a pointer to two words of storage to hold the file descriptors.

45

brk

new system break

Sets the system break (i.e. -the end of the data section). If the system break is 0, it simply returns the current system break.

54

ioctl

file descriptor

request

arguments

This is used to set parameters on device files. Its actual usage varies based on the type of file or device your descriptor references.

A more complete listing of system calls, along with additional information is available at http://www.lxhp.in-berlin.de/lhpsyscal.html You can also get more information about a system call by typing in man 2 SYSCALLNAME which will return you the information about the system call from section 2 of the UNIX manual. However, this refers to the usage of the system call from the C programming language, and may or may not be directly helpful.

For information on how system calls are implemented on Linux, see the Linux Kernel 2.4 Internals section on how system calls are implemented at http://www.faqs.org/docs/kernel_2_4/lki-2.html#ss2.11

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