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Subjectfutex system call in write
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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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