CODEWITHSUNDEEP
clinuxsystem-callsstraceptrace
kidd0
kidd0

Reputation: 771

Using Ptrace to find out what exactly does the arguments signify for a system call

I already posted the following Question got a solution and moving forward
I am using ptrace to find what all are the arguments that are passed to the system call.
The program is fetching me values in ebx, ecx, edx. Now for a open system call I got the foll

SYSCALL 5: ebx :bf9748af ecx: 00008000 edx: 00000000 /open
SYSCALL 5: ebx :80485b3 ecx: 00000242 edx: 000001b6 /open

I used strace and it magically converts the above like this:

open("test.txt", O_RDONLY|O_LARGEFILE) = 3
open("test.txt", O_RDWR|O_CREAT|O_TRUNC, 0666) = 3

How can I do this manually? Is there any place I can find out the values for O_LARGEFILE?
I searched a lot and came across this But it doesnt have everything. Also tried reading strace code but did not come across the code for this conversion.
If someone can help me out it would be very helpful for me. Also If you know where this is written in strace I want to take a peek at it. Thanks in advance.

Upvotes: 1

Views: 543

Answers (2)

orlp
orlp

Reputation: 117681

You can read out those values from this header file:

#define O_ACCMODE      0003
#define O_RDONLY         00
#define O_WRONLY         01
#define O_RDWR           02
#define O_CREAT        0100 /* not fcntl */
#define O_EXCL         0200 /* not fcntl */
#define O_NOCTTY       0400 /* not fcntl */
#define O_TRUNC       01000 /* not fcntl */
#define O_APPEND      02000
#define O_NONBLOCK    04000
#define O_NDELAY    O_NONBLOCK
#define O_SYNC       010000
#define O_FSYNC      O_SYNC
#define O_ASYNC      020000

But the portable way of doing it is by using the macros for those values.

Upvotes: 1

ouah
ouah

Reputation: 145829

O_LARGEFILE is implementation specific and is defined in LSB (Linux Standard Base) as

0100000 (equal to 0x8000) for Linux x86 (in fcntl.h)

See LSB reference:

http://linuxbase.org/navigator/browse/constant.php?cmd=list-by-name&Cname=O_LARGEFILE

O_RDONLY value being 0, O_RDONLY | O_LARGEFILE is then equal to 0x8000.

Upvotes: 1

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