Fix perfmap output

[tommcdon]

Fix a recent regression in perfmaps caused by #116203
The fix changes fprintf usage in PerfMap::WriteLine to using

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[Copilot]

Pull request overview

This PR fixes a regression in perfmap output caused by incorrect error checking in fprintf usage. The previous code treated successful writes as errors due to checking if the return value was not equal to zero, when fprintf actually returns the number of characters written (> 0) on success.

Key Changes

• Replaced fprintf(m_fp, "%s", line.GetUTF8()) with a direct write() system call using the file descriptor
• Fixed error checking to properly detect write failures and partial writes

[jkotas]

What is the problem with fprintf?

[tommcdon]

The bug was introduced by comparing the output of fprintf to 0. fprintf returns a value < 0 on failure, otherwise the number of bytes written to the stream.
Since we always use a formatting character of "%s", we can use write to accomplish the same task.

[huoyaoyuan]

It may be valuable to double check the return value handling of each stdio function. Some returns error code and some returns meaningful length.

Since we always use a formatting character of "%s", we can use write to accomplish the same task.

Or maybe fwrite to be consistent at stdio level?

[jkotas]

cc @huoyaoyuan

[jkotas]

Fix a recent regression in perfmaps caused by #116203
tommcdon added this to the 10.0.x milestone

#116203 was merge 3 weeks ago. It is not in .NET 10.

[tommcdon]

Fix a recent regression in perfmaps caused by #116203
tommcdon added this to the 10.0.x milestone

#116203 was merge 3 weeks ago. It is not in .NET 10.

Setting the milestone to 10.0 was an error on my part. The intent was to fix this in main only.

[jkotas]

• FILE does buffering for better perf. Using write sys-call directly is going to be slower. Would it be better to use fwrite to benefit from the buffering?
• If we want to use direct sycallls for some reason, the syscalls should be wrapped in EINTR dance like

  runtime/src/native/libs/System.Native/pal_io.c

  Line 1262 in 74cf618

  while ((bytesWritten = write(outFd, buffer + offset, (size_t)bytesRead)) < 0 && errno == EINTR);

  . Also, we can open/close the flle using open/close syscalls and avoid using FILE.

[jkotas]

SString::GetCount is going to convert the string to Unicode and return the number of unicode characters:

runtime/src/coreclr/inc/sstring.inl

Lines 1172 to 1174 in 74cf618

	ConvertToFixed();
	SS_RETURN SizeToCount(GetSize());

. SString is not very intuitive...