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In this noncompliant code example, the value of length is read from a network connection and passed as an argument to a wrapper to malloc() to allocate the appropriate data block. Provided that the size of an unsigned long is equal to the size of an unsigned int, and both sizes are equal to or smaller than the size of size_t, this code runs as expected. However, if the size of an unsigned long is greater than the size of an unsigned int, the value stored in length may be truncated when passed as an argument to alloc().  Both read functions return zero on success and nonzero on failure.

void *alloc(unsigned int blocksize) {
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  unsigned long length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length) < 0) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1;  /* Indicate failure */
  }

  if (read_network_data(fd, data, length) < 0) {
    free(data);
    return -1;
  }
  data[length] = '\0';

  /* ... */
  free( data);
  return 0;
}

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void *alloc(rsize_t blocksize) {
  if (blocksize == 0 || blocksize > RSIZE_MAX) {
    return NULL;  /* Indicate failure */
  }
  return malloc(blocksize);
}

int read_counted_string(int fd) {
  rsize_t length;
  unsigned char *data;

  if (read_integer_from_network(fd, &length) < 0) {
    return -1;
  }

  data = (unsigned char*)alloc(length+1);
  if (data == NULL) {
    return -1; /* Indicate failure */
  }

  if (read_network_data(fd, data, length) < 0) {
    free(data);
    return -1;
  }
  data[length] = '\0';

  /* ... */
  free( data);
  return 0;
}

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