如果附加到 strace，挂起的进程将恢复

Question

我有一个使用 TCP 套接字用 C 语言编写的网络程序。有时客户端程序永远挂起等待来自服务器的输入。具体来说，客户端挂起在 fd 上设置的 select() 调用，该 fd 旨在读取服务器发送的字符。

我正在使用 strace 来了解进程卡在哪里。但是，有时当我将挂起的客户端进程附加到 strace 时，它​​会立即恢复执行并正确退出。并非所有挂起的进程都表现出这种行为，即使我将它们附加到 strace，某些进程也会卡在 select() 中。但是大多数进程在附加到 strace 时会恢复执行。

我很好奇是什么导致进程在附加到 strace 时恢复。它可能会给我一些线索，让我知道为什么客户端进程会挂起。

有什么想法吗？是什么导致挂起的进程在附加到 strace 时恢复执行？

更新：

这是 strace 在挂起进程上的输出。

> sudo strace -p 25645
Process 25645 attached - interrupt to quit
--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
[ Process PID=25645 runs in 32 bit mode. ]
select(6, [3 5], NULL, NULL, NULL)      = 2 (in [3 5])
read(5, "\0", 8192)                     = 1
write(2, "", 0)                         = 0
read(3, "====Setup set_oldtempbehaio"..., 8192) = 555
write(1, "====Setup set_oldtempbehaio"..., 555) = 555
select(6, [3 5], NULL, NULL, NULL)      = 2 (in [3 5])
read(5, "", 8192)                       = 0
read(3, "", 8192)                       = 0
close(5)                                = 0
kill(25652, SIGKILL)                    = 0
exit_group(0)                           = ?
Process 25645 detached

_

> sudo strace -p 14462
Process 14462 attached - interrupt to quit
[ Process PID=14462 runs in 32 bit mode. ]
read(0, 0xff85fdbc, 8192)               = -1 EIO (Input/output error)
shutdown(3, 1 /* send */)               = 0
exit_group(0)                           = ?

_

> sudo strace -p 7517
Process 7517 attached - interrupt to quit
--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
[ Process PID=7517 runs in 32 bit mode. ]
connect(3, {sa_family=AF_INET, sin_port=htons(300), sin_addr=inet_addr("100.64.220.98")}, 16) = -1 ETIMEDOUT (Connection timed out)
close(3)                                = 0
dup(2)                                  = 3
fcntl64(3, F_GETFL)                     = 0x1 (flags O_WRONLY)
close(3)                                = 0
write(2, "dsd13: Connection timed out\n", 30) = 30
write(2, "Error code : 110\n", 17)      = 17
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
exit_group(1)                           = ?
Process 7517 detached

不仅是 select()，而且（同一程序的）进程在我将它们附加到 strace 之前会卡在各种系统调用中。他们在附加到 strace 后突然恢复。如果我不将它们附加到 strace，它们就会永远挂在那里。

更新 2：

我了解到 strace 可以启动以前停止的进程（T 状态的进程）。现在我试图了解为什么这些过程会进入“T”状态，原因是什么。这是/proc//状态信息：

> cat /proc/12554/status
Name:   someone
State:  T (stopped)
SleepAVG:       88%
Tgid:   12554
Pid:    12554
PPid:   9754
TracerPid:      0
Uid:    5000    5000    5000    5000
Gid:    48986   48986   48986   48986
FDSize: 256
Groups: 9149 48986
VmPeak:     1992 kB
VmSize:     1964 kB
VmLck:         0 kB
VmHWM:       608 kB
VmRSS:       608 kB
VmData:      156 kB
VmStk:        20 kB
VmExe:        16 kB
VmLib:      1744 kB
VmPTE:        20 kB
Threads:        1
SigQ:   54/73728
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000006
SigCgt: 0000000000004000
CapInh: 0000000000000000
CapPrm: 0000000000000000
CapEff: 0000000000000000
Cpus_allowed:   00000000,00000000,00000000,0000000f
Mems_allowed:   00000000,00000001

Answer 1

strace 使用ptrace。 ptrace man page 有这个：

Since attaching sends SIGSTOP and the tracer usually suppresses it,
this may cause a stray EINTR return from the currently executing system
call in the tracee, as described in the "Signal injection and
suppression" section.

你看到select返回EINTR了吗？