Messages in this thread | | | Date | Sun, 3 Jun 2012 01:58:55 -0400 | Subject | futex system call in write | From | Xin Tong <> |
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I am a program with 2 threads, each of which calling printf in a loop. when i strace this programs. i see besides write() gets calls a good number of times. futex gets call many times as well. it seems to be related to the write system call ? how is the futex system call used here ? maybe write() is blocking and futex is used to wait the thread up when the write finishes ? if that is the case, what about other blocking system calls ?
[pid 9057] [ 377b4f542e] futex(0x377b78be10, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...> [pid 9058] [ 377b4f545f] futex(0x377b78be10, FUTEX_WAKE_PRIVATE, 1 <unfinished ...> [pid 9057] [ 377b4f542e] <... futex resumed> ) = -1 EAGAIN (Resource temporarily unavailable) [pid 9058] [ 377b4f545f] <... futex resumed> ) = 0 [pid 9057] [ 377b4d8a2d] write(1, "Hello World! It's me, thread #0!"..., 33Hello World! It's me, thread #0! <unfinished ...> [pid 9058] [ 377b4f542e] futex(0x377b78be10, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...> [pid 9057] [ 377b4d8a2d] <... write resumed> ) = 33 [pid 9057] [ 377b4f545f] futex(0x377b78be10, FUTEX_WAKE_PRIVATE, 1 <unfinished ...> [pid 9058] [ 377b4f542e] <... futex resumed> ) = 0 [pid 9057] [ 377b4f545f] <... futex resumed> ) = 1 [pid 9058] [ 377b4d8a2d] write(1, "Hello World! It's me, thread #1!"..., 33Hello World! It's me, thread #1! <unfinished ...> [pid 9057] [ 377b4f542e] futex(0x377b78be10, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...> [pid 9058] [ 377b4d8a2d] <... write resumed> ) = 33 [pid 9058] [ 377b4f545f] futex(0x377b78be10, FUTEX_WAKE_PRIVATE, 1) = 1 [pid 9057] [ 377b4f542e] <... futex resumed> ) = 0 [pid 9058] [ 377b4d8a2d] write(1, "Hello World! It's me, thread #1!"..., 33Hello World! It's me, thread #1! <unfinished ...>
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