Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>This proposed structure:</p> <pre><code>typedef struct { char[14] command; float *pressure; char comma; float *temperature; char comma; uint32_t *time_stamp; char CR; }comStr; </code></pre> <p>Is not going to help you with your requirement:</p> <blockquote> <p>The only important thing here is just whether the memory is allocated continuously so that a hardware fetch lasting for 29 bytes starting from the memory address (<code>comStr-&gt;command</code>) gives me exactly the string I want.</p> </blockquote> <p>Note you can't have two members with the same name; you'd need to use <code>comma1</code> and <code>comma2</code> for example. Also, the array dimension is in the wrong place.</p> <p>One problem is that there will be padding bytes within the structure.</p> <p>Another problem is that the pointers will be holding addresses of something outside the structure (since there is nothing valid inside the structure for them to point at).</p> <p>It is not clear what you're after. Only a very limited range of floating point values can be represented by 4 bytes in a string. If you're after binary data I/O, then you can drop the pointers and the commas:</p> <pre><code>typedef struct { char command[14]; float pressure; float temperature; uint32_t time_stamp; }comStr; </code></pre> <p>If you want the commas present, then you're going to have to work harder:</p> <pre><code>typedef struct { char command[14]; char pressure[4]; char comma1; char temperature[4]; char comma2; char time_stamp[4]; char CR; } comStr; </code></pre> <p>You will have to load the data carefully:</p> <pre><code>struct comStr com; float pressure = ...; float temperature = ...; uint32_t time_stamp = ...; assert(sizeof(float) == 4); ... memmove(&amp;com.pressure, &amp;pressure, sizeof(pressure)); memmove(&amp;com.temperature, &amp;temperature, sizeof(temperature)); memmove(&amp;com.time_stamp, &amp;time_stamp, sizeof(time_stamp)); </code></pre> <p>You have to unpack with a similar set of memory copies. Note that you won't be able to use simple string manipulation on the structure; there could be zero bytes in any or all of the <code>pressure</code>, <code>temperature</code> and <code>time_stamp</code> sections of the structure.</p> <hr> <h3>Structure padding</h3> <pre><code>#include &lt;stddef.h&gt; #include &lt;stdio.h&gt; #include &lt;stdint.h&gt; typedef struct { char command[14]; float *pressure; char comma1; float *temperature; char comma2; uint32_t *time_stamp; char CR; } comStr; int main(void) { static const struct { char *name; size_t offset; } offsets[] = { { "command", offsetof(comStr, command) }, { "pressure", offsetof(comStr, pressure) }, { "comma1", offsetof(comStr, comma1) }, { "temperature", offsetof(comStr, temperature) }, { "comma2", offsetof(comStr, comma2) }, { "time_stamp", offsetof(comStr, time_stamp) }, { "CR", offsetof(comStr, CR) }, }; enum { NUM_OFFSETS = sizeof(offsets)/sizeof(offsets[0]) }; printf("Size of comStr = %zu\n", sizeof(comStr)); for (int i = 0; i &lt; NUM_OFFSETS; i++) printf("%-12s %2zu\n", offsets[i].name, offsets[i].offset); return 0; } </code></pre> <p>Output on Mac OS X:</p> <pre><code>Size of comStr = 64 command 0 pressure 16 comma1 24 temperature 32 comma2 40 time_stamp 48 CR 56 </code></pre> <p>Note how large the structure is on a 64-bit machine. Pointers are 8-bytes each and are 8-byte aligned.</p>
    singulars
    1. This table or related slice is empty.
    1. POContinuous memory allocation with different data type in C?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USJonathan Leffler
    1. USJonathan Leffler
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POContinuous memory allocation with different data type in C?
      singulars
      1. PTQuestion
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTAcceptedByOriginator
    1. CO4-byte string offers me room for a 32-bit floating point number, which has 23(+1) bits' significand. The float represents a quotient resulted from dividing one 16-bit integer by another, which two have close range. Thus it has sufficient precision.
      singulars
      1. PO
      1. USspringRoll
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload