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adrianlizarraga
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Jan 12, 2024
mszhanyi
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Jan 15, 2024
### Description
In some environments the test code has undefined behavior. To prove it, save the following code as
test.cpp
```c++
#include <iostream>
#include <stdio.h>
int main(){
char buf[1024];
int ret = snprintf(buf, sizeof(buf), "%ls","abc");
if(ret <0){
std::cout<< ret<< std::endl;
} else{
std::cout<< "OK: ret="<<ret<< std::endl;
}
return 0;
}
```
Then compile it as
```
g++ -DNDEBUG -std=gnu++17 test.cpp -o /tmp/t
```
Or
```
g++ -O2 -DNDEBUG -std=gnu++17 test.cpp -o /tmp/t
```
The first command is without optimization. The second one turns on
optimization. Then the outputs are different.
When optimization is enabled, the output might be:
```
OK: ret=-1
```
You cannot explain why it would go to this branch when ret is "-1". It
might be a bug of a specific version of GCC. However, at this moment we
cannot change the version. It was found in GCC version 8.5.0 20210514
(Red Hat 8.5.0-18) (GCC) that is provided by UBI8. RHEL9 doesn't have
the problem. snprintf is a builtin function of GCC. So the problem was
not related to glibc.
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
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Description
The code has undefined behavior. To prove it, save the following code as test.cpp
Then compile it as
Or
The first command is without optimization. The second one turns on optimization. Then the outputs are different.
When optimization is enabled, the output might be:
You cannot explain why it would go to this branch when ret is "-1". It might be a bug of a specific version of GCC. However, at this moment we cannot change the version. It was found in GCC version 8.5.0 20210514 (Red Hat 8.5.0-18) (GCC) that is provided by UBI8. RHEL9 doesn't have the problem. snprintf is a builtin function of GCC. So the problem was not related to glibc.
Motivation and Context