[clang][analyzer] Improve documentation of StreamChecker (NFC). (#83858)

Author: balazske

clang/docs/analyzer/checkers.rst

@@ -3120,44 +3120,82 @@ 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 (the single exception is
+  ``fflush`` where 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 (like ``ferror``, ``clearerr``, ``fseek``) are
+  allowed in this state.
+* Invalid 3rd ("``whence``") argument to ``fseek``.
+
+The checker does not track the correspondence 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)