after dump, stderr output disappear

[ghost]

What is the issue you have?

json version: 3.7.3
source:
static int HanldeFailed(json& reply, json& result, std::string& err, const std::string& err_key, int has_err_key) {
fprintf(stderr, "xxxxx handler failed\n");
fprintf(stderr, "xxxxx call 44 %d field:%s,err:%d x\n", has_err_key, err_key.c_str(), err.size());
if (has_err_key) {
result.push_back(json {{err_key, err}});
reply["reply"] = result;
fprintf(stderr, "xxxxx handler failed 2\n");
std::string ss = result.dump();
fprintf(stderr, "xxxxx call 41 %s x\n", "soineweij");
return 0;
}
fprintf(stderr, "xxxxx call 42 %s x\n", "jiojonosnfie");
return -1;
}
output:
xxxxx handler failed
xxxxx call 44 1 field:ErrMsg,err:33 x
xxxxx handler failed 2
other:

1. remove line of "std::string ss = result.dump();" , it work ok, output:
   xxxxx handler failed
   xxxxx call 44 1 field:ErrMsg,err:33 x
   xxxxx handler failed 2
   xxxxx call 41 soineweij x
2. simple json demo cannot reproduce the issue
   i'm confused why it appears, please give some suggest to get where is wrong, thanks

Please describe the steps to reproduce the issue.

may not reproduce, but in my project is indeed exist every time

1. nginx fprintf(stderr, ...) output to file

Can you provide a small but working code example?

What is the expected behavior?

And what is the actual behavior instead?

Which compiler and operating system are you using?

• Compiler: ___
• Operating system: ___

Which version of the library did you use?

• latest release version 3.9.1
• other release - please state the version: ___
• the develop branch

If you experience a compilation error: can you compile and run the unit tests?

• yes
• no - please copy/paste the error message below