Display strace's debug information

-d option can be used to show strace's own debug information. Check following simple program:

# cat fork.c
#include <sys/types.h>
#include <unistd.h>

int main(void)
{
        int pid = fork();
        while (1)
        {
            sleep(1);
        }
        return 0;
}

Build and use strace's -d option to track it:

# gcc fork.c -o fork
# strace -d ./fork
strace: ptrace_setoptions = 0x51
strace: PTRACE_GET_SYSCALL_INFO: Input/output error
strace: PTRACE_GET_SYSCALL_INFO does not work
strace: new tcb for pid 583, active tcbs:1
strace: [wait(0x80137f) = 583] WIFSTOPPED,sig=SIGSTOP,EVENT_STOP (128)
strace: pid 583 has TCB_STARTUP, initializing it
strace: [wait(0x80057f) = 583] WIFSTOPPED,sig=SIGTRAP,EVENT_STOP (128)
strace: [wait(0x00127f) = 583] WIFSTOPPED,sig=SIGCONT
strace: [wait(0x00857f) = 583] WIFSTOPPED,sig=133
execve("./fork", ["./fork"], 0x7ffcc9f68b48 /* 21 vars */strace: [wait(0x04057f) = 583] WIFSTOPPED,sig=SIGTRAP,EVENT_EXEC (4)
......
strace: [wait(0x00857f) = 583] WIFSTOPPED,sig=133
clone(strace: [wait(0x00857f) = 583] WIFSTOPPED,sig=133
......

The debug information is interleaved with normal output. Open another terminal to check the process relationship, and this can help knowing debug information better:

# pstree -p `pidof strace`
strace(580)---fork(583)---fork(584)

Besides child processes' exit status, strace cares about events such as forking process, exit process, etc (please refer related code).