[clang][analyzer] Improve documentation of StreamChecker (NFC).

[balazske]

No description provided.

[llvmbot]

@llvm/pr-subscribers-clang

@llvm/pr-subscribers-clang-static-analyzer-1

Author: Balázs Kéri (balazske)

Changes

---

Full diff: https://github.com/llvm/llvm-project/pull/83858.diff

1 Files Affected:

• (modified) clang/docs/analyzer/checkers.rst (+52-14)

diff --git a/clang/docs/analyzer/checkers.rst b/clang/docs/analyzer/checkers.rst
index fe211514914272..aa79792c64dc54 100644
--- a/clang/docs/analyzer/checkers.rst
+++ b/clang/docs/analyzer/checkers.rst
@@ -3020,44 +3020,82 @@ Check for misuses of stream APIs. Check for misuses of stream APIs: ``fopen, fcl
 
 alpha.unix.Stream (C)
 """""""""""""""""""""
-Check stream handling functions: ``fopen, tmpfile, fclose, fread, fwrite, fseek, ftell, rewind, fgetpos,``
-``fsetpos, clearerr, feof, ferror, fileno``.
+Check C stream handling functions:
+``fopen, fdopen, freopen, tmpfile, fclose, fread, fwrite, fgetc, fgets, fputc, fputs, fprintf, fscanf, ungetc, getdelim, getline, fseek, fseeko, ftell, ftello, fflush, rewind, fgetpos, fsetpos, clearerr, feof, ferror, fileno``.
+
+The checker maintains information about the C stream objects (``FILE *``) and
+can detect error conditions related to use of streams. The following conditions
+are detected:
+
+* The ``FILE *`` pointer passed to the function is NULL. (At ``fflush``
+  NULL is allowed.)
+* Use of stream after close.
+* Opened stream is not closed.
+* Read from a stream after end-of-file. (This is not a fatal error but reported
+  by the checker. Stream remains in EOF state and the read operation fails.)
+* Use of stream when the file position is indeterminate after a previous failed
+  operation. Some functions are allowed in this state.
+* Invalid 3rd ("``whence``") argument to ``fseek``.
+
+The checker is not capable of maintaining a relation between integer file
+descriptors and ``FILE *`` pointers. Operations on standard streams like
+``stdin`` are not treated specially and are therefore often not recognized
+(because these streams are usually not opened explicitly by the program, and
+are global variables).
 
 .. code-block:: c
 
- void test() {
+ void test1() {
    FILE *p = fopen("foo", "r");
  } // warn: opened file is never closed
 
- void test() {
+ void test2() {
    FILE *p = fopen("foo", "r");
    fseek(p, 1, SEEK_SET); // warn: stream pointer might be NULL
    fclose(p);
  }
 
- void test() {
+ void test3() {
    FILE *p = fopen("foo", "r");
+   if (p) {
+     fseek(p, 1, 3); // warn: third arg should be SEEK_SET, SEEK_END, or SEEK_CUR
+     fclose(p);
+   }
+ }
 
-   if (p)
-     fseek(p, 1, 3);
-      // warn: third arg should be SEEK_SET, SEEK_END, or SEEK_CUR
+ void test4() {
+   FILE *p = fopen("foo", "r");
+   if (!p)
+     return;
 
    fclose(p);
+   fclose(p); // warn: stream already closed
  }
 
- void test() {
+ void test5() {
    FILE *p = fopen("foo", "r");
+   if (!p)
+     return;
+
+   fgetc(p);
+   if (!ferror(p))
+     fgetc(p); // warn: possible read after end-of-file
+
    fclose(p);
-   fclose(p); // warn: already closed
  }
 
- void test() {
-   FILE *p = tmpfile();
-   ftell(p); // warn: stream pointer might be NULL
+ void test6() {
+   FILE *p = fopen("foo", "r");
+   if (!p)
+     return;
+
+   fgetc(p);
+   if (!feof(p))
+     fgetc(p); // warn: file position may be indeterminate after I/O error
+
    fclose(p);
  }
 
-
 .. _alpha-unix-cstring-BufferOverlap:
 
 alpha.unix.cstring.BufferOverlap (C)

[NagyDonat]

Looks reasonable.

[NagyDonat]

Suggested change

	The checker is not capable of maintaining a relation between integer file
	The checker does not track the correspondence between integer file

and re-apply line wrapping to the paragraph (if needed)

[benshi001]

after '' fclose '' ?

[balazske]

This is not meant to be a function name, just "close of a stream" (I do not know why it is displayed in blue color).

[benshi001]

... related to use of these streams.

English is not my first language, maybe my suggestion is not good.

[balazske]

I do not know but "can detect error conditions related to use of streams" is good in itself so I would leave it.

[benshi001]

The FILE * pointer passed to a function is NULL. (except ''fflush'' which allows that)

[benshi001]

Some functions (such as ...) are allowed in this state.

[steakhal]

I can see NULL and EOF (maybe others) spelled in the docs without double backticks.
Do you think we should escape those to have verbatim code highlighting?

[balazske]

I have the feeling that these words (NULL, EOF) are relatively commonly used (NULL is an abbrevation for null pointer, EOF = end-of-file). Probably NULL can be better as keyword (code format), but by EOF I did not mean the numeric constant, just "end of file".