fix(stdio): correct position tracking on mode switch and free owned buffers on close

include/stdio.h

@@ -206,7 +206,13 @@ fseeko(FILE *stream, off_t offset, int whence);
 long
 ftell(FILE *stream);
 
-__attribute__((pure)) off_t
+/*
+ * Not marked pure: the implementation can allocate the stream buffer on
+ * first use (an observable side effect), so the compiler must not elide
+ * or reorder the call. (feof/ferror/fileno below are genuinely read-only
+ * and keep their pure attribute.)
+ */
+off_t
 ftello(FILE *stream);
 
 /* Rewind to the start and clear feof/ferror (equivalent to

src/stdio/stdio.c

@@ -278,6 +278,10 @@ stdio_alloc_file(int fd, int m)
     f->wstop = f->buf + BUFSIZ;
     f->fd = fd;
     f->flags = m | F_OWNBUF | F_HEAP;
+    if ((m & F_READ) && (m & F_WRITE))
+    {
+        f->flags |= F_RDWR;
+    }
     f->pos = 0;
     f->ungot = 0;
     f->next = open_list;
@@ -320,6 +324,10 @@ stdio_discard_read(FILE *f)
             f->flags |= F_ERR;
             return -1;
         }
+        /* The kernel now sits at the logical position; re-anchor `pos`
+         * there so the invariant holds after the buffer is emptied
+         * below (empty buffer => kernel == pos). */
+        f->pos += (off_t)(f->rpos - f->buf);
     }
     f->rpos = f->rstop = f->buf;
     return 0;
@@ -462,7 +470,11 @@ freopen(const char *restrict path, const char *restrict mode, FILE *restrict str
     if (path == NULL)
     {
         /* Change the mode only; keep the descriptor. */
-        stream->flags = (stream->flags & ~(F_READ | F_WRITE | F_APPEND)) | m;
+        stream->flags = (stream->flags & ~(F_READ | F_WRITE | F_APPEND | F_RDWR)) | m;
+        if ((m & F_READ) && (m & F_WRITE))
+        {
+            stream->flags |= F_RDWR;
+        }
         stream->flags &= ~(F_EOF | F_ERR | F_PUSHED);
         return stream;
     }
@@ -483,7 +495,12 @@ freopen(const char *restrict path, const char *restrict mode, FILE *restrict str
         return NULL;
     }
     stream->fd = newfd;
-    stream->flags = (stream->flags & ~(F_READ | F_WRITE | F_APPEND | F_EOF | F_ERR | F_PUSHED)) | m;
+    stream->flags =
+        (stream->flags & ~(F_READ | F_WRITE | F_APPEND | F_EOF | F_ERR | F_PUSHED | F_RDWR)) | m;
+    if ((m & F_READ) && (m & F_WRITE))
+    {
+        stream->flags |= F_RDWR;
+    }
     stream->rpos = stream->rstop = stream->buf;
     stream->wpos = stream->buf;
     stream->pos = 0;
@@ -522,6 +539,16 @@ fclose(FILE *stream)
         {
             *link = stream->next;
         }
+        if (stream->flags & F_OWNBUF)
+        {
+            /* The FILE struct is heap-owned, so its library-allocated
+             * buffer is too (a user buffer via setvbuf clears F_OWNBUF
+             * and must not be freed). The static std streams are not
+             * F_HEAP, so their lazily allocated buffers are
+             * intentionally never freed: process-lifetime storage,
+             * matching their registry/global nature. */
+            __libc_free(stream->buf);
+        }
         __libc_free(stream);
     }
     return rc;
@@ -675,14 +702,16 @@ int
 fputc(int c, FILE *stream)
 {
     stdio_init_if_needed(stream);
-    if (!(stream->flags & F_WRITE))
+    if (!(stream->flags & F_WRITE) && !(stream->flags & F_RDWR))
     {
+        /* The open mode does not allow writes. */
         stream->flags |= F_ERR;
         return EOF;
     }
     if (stream->rpos < stream->rstop)
     {
-        /* Pending read data: discard it before writing. */
+        /* Pending read data: discard it before writing (the read -> write
+         * mode switch on an update stream). */
         if (stdio_discard_read(stream) < 0)
         {
             return EOF;
@@ -808,6 +837,13 @@ fgets(char *restrict s, int n, FILE *restrict stream)
             {
                 return NULL;
             }
+            /* Mid-line end of input: a genuine EOF keeps the partial
+             * line, but a read error makes the array contents
+             * indeterminate and fgets must return NULL (C23 7.23.7.2). */
+            if (stream->flags & F_ERR)
+            {
+                return NULL;
+            }
             break;
         }
         s[i] = (char)c;

src/stdio/stdio_impl.h

@@ -17,9 +17,11 @@
  *
  * Mode switching: reading while write data is pending flushes it first;
  * writing while unread buffered data is pending seeks the kernel back to
- * the logical position (discarding the unread data). F_PUSHED marks the
- * single guaranteed ungetc pushback byte in `ungot`; a successful seek
- * discards it.
+ * the logical position (discarding the unread data). F_WRITE doubles as
+ * the current-mode marker and is cleared by the read switch; F_RDWR
+ * records the write capability of update streams so the write switch can
+ * re-set F_WRITE. F_PUSHED marks the single guaranteed ungetc pushback
+ * byte in `ungot`; a successful seek discards it.
  *
  * The helper prototypes below are shared with the formatted-I/O todos
  * (16: vfprintf, 17: vfscanf) so they operate on the same buffer state.
@@ -55,6 +57,7 @@ struct vlibc_FILE
 #define F_OWNBUF 0x080  /* buf was allocated by the library */
 #define F_HEAP 0x100    /* the FILE struct itself is heap-allocated */
 #define F_PUSHED 0x200  /* ungot holds a pushed-back byte */
+#define F_RDWR 0x400    /* update stream: reads and writes both enabled */
 
 /* Flush pending write data; returns 0, or -1 with F_ERR set. */
 hidden int

tests/test_stdio.c

@@ -25,6 +25,11 @@
  *      changes the mode only.
  *   9. remove/rename lifecycle; tmpfile write+rewind+read.
  *  10. Level 2: tmpnam/ctermid/setbuffer/setlinebuf/fopen64.
+ *  11. Read -> write mode switch: after a partial read the logical
+ *      position survives the buffered-data discard, so an overwrite
+ *      lands at the right offset.
+ *  12. fclose leak check: 200 open/write/close cycles leave no live
+ *      allocations (probed with the allocator's heap walk).
  *
  * Failure mode (-f): fseek past EOF then fread returns 0 with feof set
  * and no crash; fopen of a nonexistent path returns NULL; fdopen(-1)
@@ -50,6 +55,11 @@
 
 #include "../src/internal/syscall.h"
 
+/* The allocator's heap-walk consistency probe (hidden; linked in via
+ * malloc.c). Returns the live block count, or (size_t)-1 on disagreement. */
+extern size_t
+__vlibc_malloc_check(void); // NOLINT(bugprone-reserved-identifier)
+
 static int failures;
 
 /* Write a NUL-terminated string to fd via the raw syscall layer. */
@@ -586,6 +596,78 @@ name_scenario(void)
     }
 }
 
+/* 11. Read -> write mode switch keeps the logical position. */
+static void
+modeswitch_scenario(void)
+{
+    const char path[] = "/tmp/vlibc-test-stdio-modeswitch.bin";
+    char got[16];
+    FILE *f;
+    unsigned long r;
+
+    f = fopen(path, "w+b");
+    check(f != NULL, "modeswitch fopen w+b succeeds");
+    if (f == NULL)
+    {
+        return;
+    }
+    check(fwrite("abcdefgh", 1, 8, f) == 8, "modeswitch writes the 8-byte seed");
+    check(fflush(f) == 0, "modeswitch fflush returns 0");
+    check(fseek(f, 0, SEEK_SET) == 0, "modeswitch seeks back to the start");
+    check(fgetc(f) == 'a', "modeswitch reads 'a'");
+    check(fgetc(f) == 'b', "modeswitch reads 'b' (6 bytes left buffered)");
+    check(ftello(f) == 2, "modeswitch ftello == 2 after the two reads");
+    check(fputc('X', f) == 'X', "modeswitch fputc('X') switches back to writing");
+    check(ftello(f) == 3, "modeswitch ftello == 3 after the overwrite");
+    check(fseeko(f, 0, SEEK_CUR) == 0, "modeswitch fseeko(0, SEEK_CUR) returns 0");
+    check(ftello(f) == 3, "modeswitch ftello stays 3 after the no-op seek");
+    check(fclose(f) == 0, "modeswitch fclose returns 0");
+
+    f = fopen(path, "rb");
+    check(f != NULL, "modeswitch reopen rb succeeds");
+    if (f != NULL)
+    {
+        r = fread(got, 1, 8, f);
+        check(r == 8 && mem_eq((const unsigned char *)got, (const unsigned char *)"abXdefgh", 8),
+              "modeswitch: the overwrite landed at offset 2 (\"abXdefgh\")");
+        check(fclose(f) == 0, "modeswitch fclose after the verify returns 0");
+    }
+    check(remove(path) == 0, "remove deletes the modeswitch file");
+}
+
+/* 12. fclose releases the owned buffer (no leak across many cycles). */
+static void
+fclose_leak_scenario(void)
+{
+    const char path[] = "/tmp/vlibc-test-stdio-leak.txt";
+    size_t before;
+    size_t after;
+    int i;
+
+    before = __vlibc_malloc_check();
+    check(before != (size_t)-1, "allocator heap walk is consistent before the cycles");
+    if (before == (size_t)-1)
+    {
+        return;
+    }
+    for (i = 0; i < 200; i++)
+    {
+        FILE *f = fopen(path, "w");
+
+        if (f == NULL || fputc('x', f) == EOF || fclose(f) != 0)
+        {
+            (void)fclose(f); /* safe when f == NULL */
+            say(2, "FAIL: leak-cycle fopen/fputc/fclose\n");
+            failures++;
+            return;
+        }
+    }
+    after = __vlibc_malloc_check();
+    check(after == before,
+          "fclose frees the FILE struct and its owned buffer (no leak over 200 cycles)");
+    check(remove(path) == 0, "remove deletes the leak-cycle file");
+}
+
 #if VLIBC_LEVEL_GE(2)
 
 /* 10. Level 2 additions. */
@@ -702,6 +784,8 @@ main(int argc, char **argv)
     fdopen_scenario();
     freopen_scenario();
     name_scenario();
+    modeswitch_scenario();
+    fclose_leak_scenario();
 #if VLIBC_LEVEL_GE(2)
     level2_scenario();
 #endif