I would expect strace run with the test code snippet to output the write call ahead of the open calls as suggested by the dissambly. Does puts perform the write asynchronously somehow or does strace reorder the output ?
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
int main() {
close(1);
printf("Interesting ordering\n");
int fd = open("temp.txt", O_RDONLY);
int fd1 = open("temp.txt", O_RDONLY);
}
Strace snippet:
munmap(0x7fe46ef45000, 115047) = 0
close(1) = 0
newfstatat(1, "", 0x7fffeea1b980, AT_EMPTY_PATH) = -1 EBADF (Bad file descriptor)
getrandom("\x96\xa6\x6b\xd0\x3c\xde\x09\x28", 8, GRND_NONBLOCK) = 8
brk(NULL) = 0xb18000
brk(0xb39000) = 0xb39000
openat(AT_FDCWD, "temp.txt", O_RDONLY) = 1
openat(AT_FDCWD, "temp.txt", O_RDONLY) = 3
write(1, "Interesting ordering\n", 21) = -1 EBADF (Bad file descriptor)
exit_group(0) = ?
Disassembly of main:
0000000000401146 <main>:
401146: 55 push %rbp
401147: 48 89 e5 mov %rsp,%rbp
40114a: 48 83 ec 10 sub $0x10,%rsp
40114e: bf 01 00 00 00 mov $0x1,%edi
401153: e8 e8 fe ff ff call 401040 <close@plt>
401158: bf 10 20 40 00 mov $0x402010,%edi
40115d: e8 ce fe ff ff call 401030 <puts@plt>
401162: be 00 00 00 00 mov $0x0,%esi
401167: bf 25 20 40 00 mov $0x402025,%edi
40116c: b8 00 00 00 00 mov $0x0,%eax
401171: e8 da fe ff ff call 401050 <open@plt>
401176: 89 45 fc mov %eax,-0x4(%rbp)
401179: be 00 00 00 00 mov $0x0,%esi
40117e: bf 25 20 40 00 mov $0x402025,%edi
401183: b8 00 00 00 00 mov $0x0,%eax
401188: e8 c3 fe ff ff call 401050 <open@plt>
40118d: 89 45 f8 mov %eax,-0x8(%rbp)
401190: b8 00 00 00 00 mov $0x0,%eax
401195: c9 leave
401196: c3 ret
The order shown by strace is correct: it is the order of the system calls, and not the library calls.
The call to printf is usually buffered, and is not really written until you issue a fflush.
The main problem with your code is that you are mixing calls from the stdio library with low-level calls such as write(); when you do that, the results are undefined and may surprise you.