fork后直接执行语句不能被ptrace跟踪，而调用execve后可以是什么原因。

请问这是什么情况

pid_t pid = fork();
if (pid == 0) {
    if(ptrace(PTRACE_TRACEME, 0, NULL, NULL) == -1) {
        printf("error in child: %s\n", strerror(errno));
        exit(2);
    }
    write(2, "hello\n", 6);  //情况A
    //execlp("/home/bman/HelloWolrd", ""); //情况B
} else {
    while(1) {
        int status = 0;
        wait(&status);
        if (WIFEXITED(status)) {
            break;
        }
        user_regs_struct reg;
        int ret = ptrace(PTRACE_GETREGS, pid, NULL, &reg );
        if (ret == -1) {
            printf("%s\n", strerror(errno));
            exit(2);
        }
        printf("ret %d\n", ret);
        printf("syscall %d\n", (int)reg.orig_eax);
        getchar();
        ptrace(PTRACE_SYSCALL, pid, NULL, NULL);
    }
}

情况A的子进程直接输出hello后退出

pid_t pid = fork();
if (pid == 0) {
    if(ptrace(PTRACE_TRACEME, 0, NULL, NULL) == -1) {
        printf("error in child: %s\n", strerror(errno));
        exit(2);
    }
    write(2, "hello\n", 6);  //情况A
    //execlp("/home/bman/HelloWolrd", ""); //情况B
} else {
    while(1) {
        int status = 0;
        wait(&status);
        if (WIFEXITED(status)) {
            break;
        }
        user_regs_struct reg;
        int ret = ptrace(PTRACE_GETREGS, pid, NULL, &reg );
        if (ret == -1) {
            printf("%s\n", strerror(errno));
            exit(2);
        }
        printf("ret %d\n", ret);
        printf("syscall %d\n", (int)reg.orig_eax);
        getchar();
        ptrace(PTRACE_SYSCALL, pid, NULL, NULL);
    }
}