printf deadlock - some printf messages ignored if printf used in signal handler

Question

Here is a slide from System Programming course I'm currently taking:

catch_child is SIGCHLD handler. How does the output correspond to the code? Why are some "Child #x started" messages not printed?

Presumably because of the deadlock the slide warns about. Does the rest of the slide deck not explain what they mean by that? — Oliver Charlesworth, Dec 11 '17 at 23:09
See also: https://stackoverflow.com/questions/3941271/why-are-malloc-and-printf-said-as-non-reentrant — Oliver Charlesworth, Dec 11 '17 at 23:13
@OliverCharlesworth No, the next slide discusses starvation. But the answers from the linked thread has answered all my questions. Thanks! — user1857492, Dec 14 '17 at 02:25

score 0 · Accepted Answer · answered Dec 14 '17 at 02:30

By definition, deadlock is a situation when no progress is made.

So it is not that some printf messages are not printed/ignored, but that after some particular printf call the program is waiting for something that will never happen and it keeps waiting there forever.

For more info about what causes such behavior see: Why are malloc() and printf() said as non-reentrant?

printf deadlock - some printf messages ignored if printf used in signal handler

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