comment on

This does look like the right answer.

There are 3 different cases to consider:

a 32 bit system with large file support (-D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64)
a 32 bit system without large file support
a 64 bit system

ikegami's c program for each of the three:

32 bit system with LFS:

$ gcc -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -Wall a.c -o a && a
flock:          24
flock.l_type:   2 @  0
flock.l_whence: 2 @  2
flock.l_start:  8 @  4
flock.l_len:    8 @ 12
flock.l_pid:    4 @ 20

32 bit system without LFS:

$ gcc  -Wall a.c -o a && a
flock:          16
flock.l_type:   2 @  0
flock.l_whence: 2 @  2
flock.l_start:  4 @  4
flock.l_len:    4 @  8
flock.l_pid:    4 @ 12

64 bit system:

$ gcc  -Wall a.c -o a && a
flock:          32
flock.l_type:   2 @  0
flock.l_whence: 2 @  2
flock.l_start:  8 @  8
flock.l_len:    8 @ 16
flock.l_pid:    4 @ 24

The test perls:

stock perl-5.10.0, running on a 32 bit system with large file support
debian perl-5.10.0, running on a 32 bit system with large file support
debian perl-5.10.1, running on a 64 bit system

Changing the op's code to:

my $flags = pack('sslli', F_WRLCK, SEEK_SET, 5, 0);

perl 1: length of flags: 16 bits, expected length: 24 bits

fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=0}, 0x9d827d0) = 0
=> this probably works by accident

perl 2: length of flags: 16 bits, expected length: 24 bits (running as a normal user)

fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=0}, 0x819b4a8) = 0
=> this probably works by accident

perl 2: length of flags: 16 bits, expected length: 24 bits (running as root)

fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2314885392840523776}, 0x81ed670) = 0
=> the length parameter does not contain the expected value

perl 3: length of flags: 16 bits, expected length: 32 bits

fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=0, len=0}) = 0
=> not correct, start is 0 but 'expected' value is 5

my $flags = pack('sslllli', F_WRLCK, SEEK_SET, 5, 0, 2, 0, 0);
- perl 1: length of flags: 24 bits, expected length: 24 bits
```
fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}, 0x970c7d0) = 0
```
- perl 2: length of flags: 24 bits, expected length: 24 bits (running as a normal user and as root)
```
fcntl64(3, F_SETLK64, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}, 0x819b4a8) = 0
```
- perl 3: length of flags: 24 bits, expected length: 32 bits
```
fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=8589934592, len=0}) = 0
```
my $flags = pack("s s x4 q q L x4", F_WRLCK, SEEK_SET, 5, 2, 0);
- perl 3: length of flags: 32 bits, expected length: 32 bits
```
fcntl(3, F_SETLK, {type=F_WRLCK, whence=SEEK_SET, start=5, len=2}) = 0
```

What this all means is that the value you need for $flags depends highly on the system it is running.. Which makes me wonder wheter or not there is a better way to accomplish what the op wants..

In reply to Re^3: fcntl failure after eval by Animator
in thread fcntl failure after eval by flipper

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