fread/fwrite/fflush bug

I’ve found what I consider to be a stream IO bug.  The code below can be compiled with g++ to easily illustrate the problem. Here’s what’s happening:

 

1. Open a file for writing (using fopen(…, “w”) ) and write some data to the file
2. Flush the writer using fflush()
3. Open a second stream to the file for reading only using fopen(…., “r”)
4. Read some bytes using fseek() & fread() to confirm the reader can see the data written in step 1
5. Have the writer update 1 byte using fwrite()
6. Flush the writer using fflush()
7. Using the read stream, read the byte written by the writer in step 5 using fseek() / fread()
8. You will get the wrong answer here.  The read should return the byte written in step 5 but instead returns the old value written in step 1.

 

Is this is a bug in Darwin?  Are there any work arounds? 

 

Here’s my test case code.  BTW, it passes on Solaris.

 

Norm Green

 

#include <stdlib.h>

#include <stdio.h>

#include <string.h>

 

int main(int argc, char *argv[]) {

 

  const char *fileName = "/tmp/testfile";

  FILE *writer = fopen(fileName, "w");

  if (writer == NULL) {

    perror("Failed opening the writer");

    exit(1);

  }

 

  const char *myString = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789\n";

  int length = strlen(myString);

 

  // write the string to the file 20 times

  for (int i = 0; i < 20; i++) {

    fwrite(myString, 1, length, writer);

    if (ferror(writer)) {

      perror("Failed when calling fwrite()");

      exit(1);

    }

  }

 

  // flush the writer

  if (fflush(writer) != 0) {

    perror("fflush() failed on the writer");

    exit(1);

  }

 

  // open a read stream on the same file

  FILE *reader = fopen(fileName, "r");

  if (reader == NULL) {

    perror("Failed opening the reader");

    exit(1);

  }

 

  // position the reader to 2nd char of 10th line

  if (fseek(reader, 630, SEEK_SET)) {

    perror("Failed seeking the reader");

    exit(1);

  }

 

  char dest[80];

  memset(dest, 0, sizeof(dest));

 

  // read the first 2 bytes of the line, which should be "ab"

  int res = fread(dest, 1, 2, reader);

  if (res != 2) {

    perror("Failed when calling fread() on the reader");

    exit(1);

  }

 

  if (dest[0] != 'a' || dest[1] != 'b') {

    printf("Unexpected results from read. Expected 'ab', found '%s'\n", dest);

    exit(1);

  }

 

  // Now have the writer update 1 byte at position 631

  if (fseek(writer, 631, SEEK_SET)) {

    perror("Failed seeking the writer");

    exit(1);

  }

 

  // change 'b' to 'B'

  if (fwrite("B", 1, 1, writer) != 1) {

    perror("Failed writing 1 byte");

    exit(1);

  }

 

  // flush the writer

  if (fflush(writer) != 0) {

    perror("fflush() failed on the writer");

    exit(1);

  }

 

  // Now read it back using the reader

  if (fseek(reader, 631, SEEK_SET)) {

    perror("Failed seeking the reader");

    exit(1);

  }

 

  memset(dest, 0, sizeof(dest));

  res = fread(dest, 1, 1, reader);

 

  if (res != 1) {

    printf("Failed when calling fread() on the reader.  Expected 1, found %d", res);

    exit(1);

  }

 

  if (dest[0] != 'B') {

    // where did the 'B' go that was written?

    printf("Wrong answer found by reader.  Expected 'B', found '%s'\n", dest);

    exit(1);

  }

 

  printf("Test passed\n");

  exit(0);