#0  0x00007f7ff789df92 in ?? () from /usr/lib/libc.so.12
#1  0x00007f7ff789e3d0 in ?? () from /usr/lib/libc.so.12
#2  0x00007f7ff789e689 in malloc () from /usr/lib/libc.so.12
#3  0x00007f7ff78ed28c in __smakebuf () from /usr/lib/libc.so.12
#4  0x00007f7ff78e24f8 in __srefill () from /usr/lib/libc.so.12
#5  0x00007f7ff78e2384 in __srget () from /usr/lib/libc.so.12
#6  0x00007f7ff7843997 in fgetc () from /usr/lib/libc.so.12

これはどういう時かな。確かに読んでいるファイルは大きいのだけれど。

% wc hoge.5itmp 
   43213  152201 1897759 hoge.5itmp

1.9MByte かな。でもそんなにやたらに大きということでもないと思う
次のような文字を教えても google は見向きもしない(ようだ)

in malloc ()
in __smakebuf ()
in __srefill ()
in __srget ()
in fgetc () from

ちなみに、次のところで説明されている問題は、まだ理解出来ていない。
fgetc が malloc を(勝手に)呼んでいるとして、そのメモリは、いつ free されるのだろうか ?

上記の答えは、「file descriptor を close すれば、開放される」
include/stdio.h

   112  typedef struct __sFILE {
   113          unsigned char *_p;      /* current position in (some) buffer */
   114          int     _r;             /* read space left for getc() */
   115          int     _w;             /* write space left for putc() */
   116          unsigned short _flags;  /* flags, below; this FILE is free if 0 */
   117          short   _file;          /* fileno, if Unix descriptor, else -1 */
   118          struct  __sbuf _bf;     /* the buffer (at least 1 byte, if !NULL) */
   119          int     _lbfsize;       /* 0 or -_bf._size, for inline putc */
   120  
   121          /* operations */
   122          void    *_cookie;       /* cookie passed to io functions */
   123          int     (*_close)(void *);
   124          int     (*_read) (void *, char *, int);
   125          __off_t (*_seek) (void *, __off_t, int);
   126          int     (*_write)(void *, const char *, int);
   127  
   128          /* file extension */
   129          struct  __sbuf _ext;
   130  
   131          /* separate buffer for long sequences of ungetc() */
   132          unsigned char *_up;     /* saved _p when _p is doing ungetc data */
   133          int     _ur;            /* saved _r when _r is counting ungetc data */
   134  
   135          /* tricks to meet minimum requirements even when malloc() fails */
   136          unsigned char _ubuf[3]; /* guarantee an ungetc() buffer */
   137          unsigned char _nbuf[1]; /* guarantee a getc() buffer */
   138  
   139          /* Formerly used by fgetln/fgetwln; kept for binary compatibility */
   140          struct  __sbuf _lb__unused;
   141  
   142          /* Unix stdio files get aligned to block boundaries on fseek() */
   143          int     _blksize;       /* stat.st_blksize (may be != _bf._size) */
   144          __off_t _offset;        /* current lseek offset */
   145  } FILE;
   146  

lib/libc/stdio/makebuf.c

    55  /*
    56   * Allocate a file buffer, or switch to unbuffered I/O.
    57   * Per the ANSI C standard, ALL tty devices default to line buffered.
    58   *
    59   * As a side effect, we set __SOPT or __SNPT (en/dis-able fseek
    60   * optimisation) right after the fstat() that finds the buffer size.
    61   */
    62  void
    63  __smakebuf(fp)
    64          FILE *fp;
    65  {
    66          void *p;
    67          int flags;
    68          size_t size;
    69          int couldbetty;
    70
    71          _DIAGASSERT(fp != NULL);
    72
    73          if (fp->_flags & __SNBF) {
    74                  fp->_bf._base = fp->_p = fp->_nbuf;
    75                  fp->_bf._size = 1;
    76                  return;
    77          }
    78          flags = __swhatbuf(fp, &size, &couldbetty);
    79          if ((p = malloc(size)) == NULL) {
    80                  fp->_flags |= __SNBF;
    81                  fp->_bf._base = fp->_p = fp->_nbuf;
    82                  fp->_bf._size = 1;
    83                  return;
    84          }
    85          __cleanup = _cleanup;
    86          flags |= __SMBF;
    87          fp->_bf._base = fp->_p = p;
    88          fp->_bf._size = size;
    89          if (couldbetty && isatty(__sfileno(fp)))
    90                  flags |= __SLBF;
    91          fp->_flags |= flags;
    92  }

上の中で size が気になる時には __swhatbuf も見る必要がある

    94  /*
    95   * Internal routine to determine `proper' buffering for a file.
    96   */
    97  int
    98  __swhatbuf(fp, bufsize, couldbetty)
    99          FILE *fp;
   100          size_t *bufsize;
   101          int *couldbetty;
   102  {
   103          struct stat st;
   104
   105          _DIAGASSERT(fp != NULL);
   106          _DIAGASSERT(bufsize != NULL);
   107          _DIAGASSERT(couldbetty != NULL);
   108
   109          if (__sfileno(fp) == -1 || fstat(__sfileno(fp), &st) < 0) {
   110                  *couldbetty = 0;
   111                  *bufsize = BUFSIZ;
   112                  return (__SNPT);
   113          }
   114
   115          /* could be a tty iff it is a character device */
   116          *couldbetty = S_ISCHR(st.st_mode);
   117          if (st.st_blksize == 0) {
   118                  *bufsize = BUFSIZ;
   119                  return (__SNPT);
   120          }
   121
   122          /*
   123           * Optimise fseek() only if it is a regular file.  (The test for
   124           * __sseek is mainly paranoia.)  It is safe to set _blksize
   125           * unconditionally; it will only be used if __SOPT is also set.
   126           */
   127          *bufsize = st.st_blksize;
   128          fp->_blksize = st.st_blksize;
   129          return ((st.st_mode & S_IFMT) == S_IFREG && fp->_seek == __sseek ?
   130              __SOPT : __SNPT);
   131  }

st というのは 103 行目 にあるが struct stat の構成要素(member)だ

modena@makoto 23:14:17/140627(/export/src)% (cd include;mkid; gid BUFSIZ)
stdio.h:181:#define     BUFSIZ  1024            /* size of buffer used by setbuf */

というのもある