Question

Possible Duplicate:
fork() and output

by running:

#include<stdio.h>
int main()
{
    fork();
    printf("b");
    if (fork() == 0) {
        write(1, "a", 1);
    }else{
        write(1, "c", 1);
    }
    return 0;
}

I got cbcabbab, could someone explain the output to me? And if possible, is there a tool to see the running procedure step by step?

Answer 1

Try running it again, you’ll probably get a different output.

As for a tool to see the procedure step by step, I think strace -f might help a bit:

$ strace -f ./weirdfork
execve("./weirdfork", ["./weirdfork"], [/* 35 vars */]) = 0
... uninteresting boiler plate removed ...
clone(Process 8581 attached
child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fe1c7d0b9d0) = 8581
[pid  8580] fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
[pid  8581] fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
[pid  8580] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
[pid  8581] mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0 <unfinished ...>
[pid  8580] <... mmap resumed> )        = 0x7fe1c7d22000
[pid  8581] <... mmap resumed> )        = 0x7fe1c7d22000
[pid  8581] clone( <unfinished ...>
[pid  8580] clone(Process 8582 attached
 <unfinished ...>
[pid  8581] <... clone resumed> child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fe1c7d0b9d0) = 8582
Process 8583 attached
[pid  8580] <... clone resumed> child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7fe1c7d0b9d0) = 8583
[pid  8580] write(1, "c", 1 <unfinished ...>
[pid  8581] write(1, "c", 1cc)            = 1
[pid  8580] <... write resumed> )       = 1
[pid  8581] write(1, "b", 1b <unfinished ...>
[pid  8580] write(1, "b", 1 <unfinished ...>
[pid  8581] <... write resumed> )       = 1
b[pid  8581] exit_group(0)               = ?
Process 8581 detached
[pid  8580] <... write resumed> )       = 1
[pid  8580] exit_group(0)               = ?
[pid  8583] write(1, "a", 1 <unfinished ...>
[pid  8582] write(1, "a", 1a)            = 1
a[pid  8582] write(1, "b", 1 <unfinished ...>
[pid  8583] <... write resumed> )       = 1
[pid  8583] write(1, "b", 1b)            = 1
[pid  8583] exit_group(0)               = ?
Process 8583 detached
b<... write resumed> )                   = 1
exit_group(0)                           = ?
Process 8582 detached