--mxv5cy4qt+RJ9ypb
Content-Type: text/plain; charset=us-ascii
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I have encountered what appears to be a bug when reading the output of
a command via a pipe from popen() into memory allocated by malloc()
and realloc().  Perhaps I am overlooking something, but the same piece
of code behaves correctly when I test it on FreeBSD.  I am wondering
if I should send a problem report.  I searched the NetBSD mailing
lists, google, and the NetBSD GNATS Database and found no reference to
this problem.

Here is my kernel information:

# uname -a
NetBSD frodo.hightek.org 2.0_RC4 NetBSD 2.0_RC4 (FRODO_GENERIC) #0:
Sat Oct 30 00:33:17 CDT 2004
vince@frodo.hightek.org:/home/netbsd/src/sys/arch/i386/compile/FRODO
i386

I also confirmed the problem exists on this kernel:

NetBSD frodo.hightek.org 2.0_BETA NetBSD 2.0_BETA (GENERIC_LAPTOP) #0:
Fri Jul 30 02:29:12 UTC 2004
autobuild@tgm.netbsd.org:/autobuild/netbsd-2-0/i386/OBJ/autobuild/netbsd-2-0/src/sys/arch/i386/compile/GENERIC_LAPTOP
i386


I am allocating memory as needed to store the output of a command.  If
I allocate and try to read any more than 1024 bytes at a time, it will
only read 1024 bytes on the first read, then when I call realloc() and
do a second read, read() returns a count that indicates it read the
full amount specified, but the pointer I get back from realloc() does
not point to the beginning of the data allocated by the first malloc()
as it should.  Instead, it appears to point to the beginning plus
1024.  All subsequent calls to realloc() appear to return a pointer to
the same place.

It works correctly, even with larger chunks, if I read directly from
the file rather than from the output of the cat command using popen().
It also works correctly using popen() as long as I only read 1024
bytes or less.

I have attached a small shell script, called mktestfile.sh, that
generates test data and the C source of a small routine called
test-malloc-pipe.c that demonstrates the problem.

Here is the result with block_size set to 1028 in test-malloc-pipe.c:

# ./mktestfile.sh > testfile
# ./test-malloc-pipe "cat testfile"

*** Read 1024 bytes ***        !!! Notice it only read 1024 bytes !!!
---------------------------------------------
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70 71 72
73 74 75 76 77 78 79 80 81 82 83 84
85 86 87 88 89 90 91 92 93 94 95 96
97 98 99 100 101 102 103 104 105 106 107 108
109 110 111 112 113 114 115 116 117 118 119 120
121 122 123 124 125 126 127 128 129 130 131 132
133 134 135 136 137 138 139 140 141 142 143 144
145 146 147 148 149 150 151 152 153 154 155 156
157 158 159 160 161 162 163 164 165 166 167 168
169 170 171 172 173 174 175 176 177 178 179 180
181 182 183 184 185 186 187 188 189 190 191 192
193 194 195 196 197 198 199 200 201 202 203 204
205 206 207 208 209 210 211 212 213 214 215 216
217 218 219 220 221 222 223 224 225 226 227 228
229 230 231 232 233 234 235 236 237 238 239 240
241 242 243 244 245 246 247 248 249 250 251 252
253 254 255 256 257 258 259 260 261 262 263 264
265 266 267 268 269 270 271 272 273 274 275 276
277 2

*** Read 1028 bytes ***
---------------------------------------------
78 279 280 281 282 283 284 285 286 287 288
289 290 291 292 293 294 295 296 297 298 299 300
301 302 303 304 305 306 307 308 309 310 311 312
313 314 315 316 317 318 319 320 321 322 323 324
325 326 327 328 329 330 331 332 333 334 335 336
337 338 339 340 341 342 343 344 345 346 347 348
349 350 351 352 353 354 355 356 357 358 359 360
361 362 363 364 365 366 367 368 369 370 371 372
373 374 375 376 377 378 379 380 381 382 383 384
385 386 387 388 389 390 391 392 393 394 395 396
397 398 399 400 401 402 403 404 405 406 407 408
409 410 411 412 413 414 415 416 417 418 419 420
421 422 423 424 425 426 427 428 429 430 431 432
433 434 435 436 437 438 439 440 441 442 443 444
445 446 447 448 449 450 451 452 453 454 455 456
457 458 459 460 461 462 463 464 465 466 467 468
469 470 471 472 473 474 475 476 477 478 479 480
481 482 483 484 485 486 487 488 489 490 491 492
493 494 495 496 497 498 499 500 501 502 503 504
505 506 507 508 509 510 511 512 513 514 515 516
517 518 519 520 521 522 523 524 525 526 527 528
529

...  [ part of output omitted to keep it short ]

===================
Total bytes read = 3976

//// End of first test ////


Here is the output I get on FreeBSD, which is what I expected:

# ./test-malloc-pipe "cat testfile"

*** Read 1028 bytes ***
---------------------------------------------
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70 71 72
73 74 75 76 77 78 79 80 81 82 83 84
85 86 87 88 89 90 91 92 93 94 95 96
97 98 99 100 101 102 103 104 105 106 107 108
109 110 111 112 113 114 115 116 117 118 119 120
121 122 123 124 125 126 127 128 129 130 131 132
133 134 135 136 137 138 139 140 141 142 143 144
145 146 147 148 149 150 151 152 153 154 155 156
157 158 159 160 161 162 163 164 165 166 167 168
169 170 171 172 173 174 175 176 177 178 179 180
181 182 183 184 185 186 187 188 189 190 191 192
193 194 195 196 197 198 199 200 201 202 203 204
205 206 207 208 209 210 211 212 213 214 215 216
217 218 219 220 221 222 223 224 225 226 227 228
229 230 231 232 233 234 235 236 237 238 239 240
241 242 243 244 245 246 247 248 249 250 251 252
253 254 255 256 257 258 259 260 261 262 263 264
265 266 267 268 269 270 271 272 273 274 275 276
277 278 2

*** Read 1028 bytes ***
---------------------------------------------
1 2 3 4 5 6 7 8 9 10 11 12
13 14 15 16 17 18 19 20 21 22 23 24
25 26 27 28 29 30 31 32 33 34 35 36
37 38 39 40 41 42 43 44 45 46 47 48
49 50 51 52 53 54 55 56 57 58 59 60
61 62 63 64 65 66 67 68 69 70 71 72
73 74 75 76 77 78 79 80 81 82 83 84
85 86 87 88 89 90 91 92 93 94 95 96
97 98 99 100 101 102 103 104 105 106 107 108
109 110 111 112 113 114 115 116 117 118 119 120
121 122 123 124 125 126 127 128 129 130 131 132
133 134 135 136 137 138 139 140 141 142 143 144
145 146 147 148 149 150 151 152 153 154 155 156
157 158 159 160 161 162 163 164 165 166 167 168
169 170 171 172 173 174 175 176 177 178 179 180
181 182 183 184 185 186 187 188 189 190 191 192
193 194 195 196 197 198 199 200 201 202 203 204
205 206 207 208 209 210 211 212 213 214 215 216
217 218 219 220 221 222 223 224 225 226 227 228
229 230 231 232 233 234 235 236 237 238 239 240
241 242 243 244 245 246 247 248 249 250 251 252
253 254 255 256 257 258 259 260 261 262 263 264
265 266 267 268 269 270 271 272 273 274 275 276
277 278 279 280 281 282 283 284 285 286 287 288
289 290 291 292 293 294 295 296 297 298 299 300
301 302 303 304 305 306 307 308 309 310 311 312
313 314 315 316 317 318 319 320 321 322 323 324
325 326 327 328 329 330 331 332 333 334 335 336
337 338 339 340 341 342 343 344 345 346 347 348
349 350 351 352 353 354 355 356 357 358 359 360
361 362 363 364 365 366 367 368 369 370 371 372
373 374 375 376 377 378 379 380 381 382 383 384
385 386 387 388 389 390 391 392 393 394 395 396
397 398 399 400 401 402 403 404 405 406 407 408
409 410 411 412 413 414 415 416 417 418 419 420
421 422 423 424 425 426 427 428 429 430 431 432
433 434 435 436 437 438 439 440 441 442 443 444
445 446 447 448 449 450 451 452 453 454 455 456
457 458 459 460 461 462 463 464 465 466 467 468
469 470 471 472 473 474 475 476 477 478 479 480
481 482 483 484 485 486 487 488 489 490 491 492
493 494 495 496 497 498 499 500 501 502 503 504
505 506 507 508 509 510 511 512 513 514 515 516
517 518 519 520 521 522 523 524 525 526 527 528
529 530

...  [ part of output Omitted ]

===================
Total bytes read = 3976

--mxv5cy4qt+RJ9ypb
Content-Type: application/x-sh
Content-Disposition: attachment; filename="mktestfile.sh"
Content-Transfer-Encoding: quoted-printable

#!/bin/sh=0A#=0A# Usage:     mktestfile.sh [count] > testfile=0A# Function:=
  Output numbers to generate a text file to use as test data=0A=0Acount=3D$=
1                        # Output numbers from 1 to $count=0Awidth=3D12    =
                    # How many numbers per line=0Ai=3D1=0Ax=3D0=0A=0A[ -z "=
$count" ] && count=3D1000=0A=0Awhile [ $i -le $count ]=0Ado=0A    echo -n "=
$i "=0A    i=3D$(($i + 1))=0A    x=3D$(($x + 1))=0A=0A    if [ $x -ge $widt=
h ]=0A    then=0A        echo=0A        x=3D0=0A    fi=0Adone=0A=0A
--mxv5cy4qt+RJ9ypb
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment; filename="test-malloc-pipe.c"

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <err.h>

int
main(int argc, char *argv[])
{
  FILE   *cmd;
  int     cmd_fd;
  char   *output;
  char   *bufp;
  ssize_t count;
  size_t  bytes_read = 0;
  size_t  block_size = 1028;

  output = NULL;
  if ((cmd = popen(argv[1], "r")) == NULL)  return(-1);
  cmd_fd = fileno(cmd);
  
  if ((output = malloc(block_size)) == NULL)
      {
      warn("Could not allocate memory");
      return(-1);
      }

  bufp = output;
  bufp[0] = 0;
  
  while ((count = read(cmd_fd, bufp, block_size)) > 0)
      {
      bytes_read += count; 
      printf("*** Read %i bytes ***\n", count); // debug xxxx

      if (count == block_size)
          {
          if ((bufp = realloc(output, bytes_read + block_size)) == NULL)
              {
              warn("Could not allocate memory");
              output[bytes_read] = 0;
              return(-1);
              }
          output = bufp;
          bufp += bytes_read;
          }          

      printf("---------------------------------------------\n"); // debug xxxx
      printf("%s\n\n", output); // debug xxxx
      }

  if (count == -1)
      {
      warn("Error reading file");
      output[bytes_read] = 0;
      return( -1);
      }

  printf("===================\n"); // debug xxxx
  printf("Total bytes read = %d\n", bytes_read); // debug xxxx
  
  output[bytes_read] = 0;
  pclose(cmd);
  return(0);
}

--mxv5cy4qt+RJ9ypb--