Ok, omitting the last input appears to be acceptable.

$ perl -wE'say length pack "sslli", 0, 0, 0, 0;'
16

$ perl -wE'say length pack "sslli", 0, 0, 0, 0, 0;'
16
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However, that's far shorter than expected, at least on the 64-bit system I used.

$ cat a.c
#include <fcntl.h>
#include <stddef.h>
#include <stdio.h>

int main() {
   struct flock lock;
   printf("flock:          %lu\n",        sizeof(lock));
   printf("flock.l_type:   %lu @ %2lu\n", sizeof(lock.l_type  ), offse
+tof(struct flock, l_type  ));
   printf("flock.l_whence: %lu @ %2lu\n", sizeof(lock.l_whence), offse
+tof(struct flock, l_whence));
   printf("flock.l_start:  %lu @ %2lu\n", sizeof(lock.l_start ), offse
+tof(struct flock, l_start ));
   printf("flock.l_len:    %lu @ %2lu\n", sizeof(lock.l_len   ), offse
+tof(struct flock, l_len   ));
   printf("flock.l_pid:    %lu @ %2lu\n", sizeof(lock.l_pid   ), offse
+tof(struct flock, l_pid   ));
   return 0;
}

$ 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
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On that system, one would need

$ perl -wE'say length pack "s s x4 q q L x4", 0, 0, 0, 0, 0;'
32
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