I am working on x86_64 machine. My linux kernel is also 64 bit kernel. As there are different ways to implement a system call (int 80, syscall, sysenter), i wanted to know what type of system call my machine is using. I am newbie to linux. I have written a demo program.
#include <stdio.h>
int main()
{
getpid();
return 0;
}
getpid() does one system call. Can anybody give me a method to find which type of system call will be used by my machine for this program.. Thank you….
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Answer
victory:~ # gcc getpid.c -o getpid -g victory:~ # gdb getpid <snip> (gdb) break main Breakpoint 1 at 0x400540: file getpid.c, line 4. (gdb) run Starting program: /root/getpid Breakpoint 1, main () at getpid.c:4 4 getpid(); (gdb) disassemble Dump of assembler code for function main: 0x000000000040053c <main+0>: push %rbp 0x000000000040053d <main+1>: mov %rsp,%rbp 0x0000000000400540 <main+4>: mov $0x0,%eax 0x0000000000400545 <main+9>: callq 0x400440 <getpid@plt> 0x000000000040054a <main+14>: mov $0x0,%eax 0x000000000040054f <main+19>: leaveq 0x0000000000400550 <main+20>: retq End of assembler dump.
Looks like our call to getpid() is actually a library call. Let’s set a breakpoint there and continue.
(gdb) break getpid Breakpoint 2 at 0x7ffff7b29c00 (gdb) cont Continuing. Breakpoint 2, 0x00007ffff7b29c00 in getpid () from /lib64/libc.so.6 (gdb) disassemble Dump of assembler code for function getpid: 0x00007ffff7b29c00 <getpid+0>: mov %fs:0x94,%edx 0x00007ffff7b29c08 <getpid+8>: cmp $0x0,%edx 0x00007ffff7b29c0b <getpid+11>: mov %edx,%eax 0x00007ffff7b29c0d <getpid+13>: jle 0x7ffff7b29c11 <getpid+17> 0x00007ffff7b29c0f <getpid+15>: repz retq 0x00007ffff7b29c11 <getpid+17>: jne 0x7ffff7b29c1f <getpid+31> 0x00007ffff7b29c13 <getpid+19>: mov %fs:0x90,%eax 0x00007ffff7b29c1b <getpid+27>: test %eax,%eax 0x00007ffff7b29c1d <getpid+29>: jne 0x7ffff7b29c0f <getpid+15> 0x00007ffff7b29c1f <getpid+31>: mov $0x27,%eax 0x00007ffff7b29c24 <getpid+36>: syscall 0x00007ffff7b29c26 <getpid+38>: test %edx,%edx 0x00007ffff7b29c28 <getpid+40>: mov %rax,%rsi 0x00007ffff7b29c2b <getpid+43>: jne 0x7ffff7b29c0f <getpid+15> 0x00007ffff7b29c2d <getpid+45>: mov %esi,%fs:0x90 0x00007ffff7b29c35 <getpid+53>: mov %esi,%eax 0x00007ffff7b29c37 <getpid+55>: retq End of assembler dump.
Buried in the getpid() library is the syscall assembler instruction. This is an AMD64 instruction that supports a fast context switch to ring0 for the purpose of system calls.