servers/procfs/buf.c

   1 /* ProcFS - buf.c - by Alen Stojanov and David van Moolenbroek */
   2
   3 #include "inc.h"
   4 #include <stdarg.h>
   5
   6 #define BUF_SIZE 4096
   7
   8 static char buf[BUF_SIZE + 1];
   9 static size_t off, left, used;
  10 static off_t skip;
  11
  12 /*===========================================================================*
  13  *                              buf_init                                     *
  14  *===========================================================================*/
  15 void buf_init(off_t start, size_t len)
  16 {
  17         /* Initialize the buffer for fresh use. The first 'start' bytes of the
  18          * produced output are to be skipped. After that, up to a total of
  19          * 'len' bytes are requested.
  20          */
  21
  22         skip = start;
  23         left = MIN(len, BUF_SIZE);
  24         off = 0;
  25         used = 0;
  26 }
  27
  28 /*===========================================================================*
  29  *                              buf_printf                                   *
  30  *===========================================================================*/
  31 void buf_printf(char *fmt, ...)
  32 {
  33         /* Add formatted text to the end of the buffer.
  34          */
  35         va_list args;
  36         ssize_t len, max;
  37
  38         if (left == 0)
  39                 return;
  40
  41         /* There is no way to estimate how much space the result will take, so
  42          * we need to produce the string even when skipping part of the start.
  43          * If part of the result is to be skipped, do not memcpy; instead, save
  44          * the offset of where the result starts within the buffer.
  45          *
  46          * The null terminating character is not part of the result, so room
  47          * must be given for it to be stored after completely filling up the
  48          * requested part of the buffer.
  49          */
  50         max = MIN(skip + left, BUF_SIZE);
  51
  52         va_start(args, fmt);
  53         len = vsnprintf(&buf[off + used], max + 1, fmt, args);
  54         va_end(args);
  55
  56         if (skip > 0) {
  57                 assert(off == 0);
  58                 assert(used == 0);
  59
  60                 if (skip >= len) {
  61                         skip -= len;
  62
  63                         return;
  64                 }
  65
  66                 off = skip;
  67                 if (left > BUF_SIZE - off)
  68                         left = BUF_SIZE - off;
  69                 len -= off;
  70                 skip = 0;
  71         }
  72
  73         assert(skip == 0);
  74         assert(len >= 0);
  75         assert((long) left >= 0);
  76
  77         if (len > (ssize_t) left)
  78                 len = left;
  79
  80         used += len;
  81         left -= len;
  82 }
  83
  84 /*===========================================================================*
  85  *                              buf_append                                   *
  86  *===========================================================================*/
  87 void buf_append(char *data, size_t len)
  88 {
  89         /* Add arbitrary data to the end of the buffer.
  90          */
  91
  92         if (left == 0)
  93                 return;
  94
  95         if (skip > 0) {
  96                 if (skip >= (ssize_t) len) {
  97                         skip -= len;
  98
  99                         return;
 100                 }
 101
 102                 data += skip;
 103                 len -= skip;
 104                 skip = 0;
 105         }
 106
 107         if (len > left)
 108                 len = left;
 109
 110         memcpy(&buf[off + used], data, len);
 111
 112         used += len;
 113         left -= len;
 114 }
 115
 116 /*===========================================================================*
 117  *                              buf_get                                      *
 118  *===========================================================================*/
 119 size_t buf_get(char **ptr)
 120 {
 121         /* Return the buffer's starting address and the length of the used
 122          * part, not counting the trailing null character for the latter.
 123          */
 124
 125         *ptr = &buf[off];
 126
 127         return used;
 128 }