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MySQL 5.7 Reference Manual  /  ...  /  mysql_fetch_lengths() mysql_fetch_lengths()

unsigned long *mysql_fetch_lengths(MYSQL_RES *result)


Returns the lengths of the columns of the current row within a result set. If you plan to copy field values, this length information is also useful for optimization, because you can avoid calling strlen(). In addition, if the result set contains binary data, you must use this function to determine the size of the data, because strlen() returns incorrect results for any field containing null characters.

The length for empty columns and for columns containing NULL values is zero. To see how to distinguish these two cases, see the description for mysql_fetch_row().

Return Values

An array of unsigned long integers representing the size of each column (not including any terminating null bytes). NULL if an error occurred.


mysql_fetch_lengths() is valid only for the current row of the result set. It returns NULL if you call it before calling mysql_fetch_row() or after retrieving all rows in the result.

unsigned long *lengths;
unsigned int num_fields;
unsigned int i;

row = mysql_fetch_row(result);
if (row)
    num_fields = mysql_num_fields(result);
    lengths = mysql_fetch_lengths(result);
    for(i = 0; i < num_fields; i++)
         printf("Column %u is %lu bytes in length.\n",
                i, lengths[i]);

User Comments
  Posted by Jennifer Fairchild on August 8, 2003
While the above example doesn't show it, it's probably a good idea to malloc() the lengths variable using lengths = malloc( sizeof( unsigned long ) * num_fields ) prior to packing your column lengths in, and free()'ing it after you're done with them.

I say 'probably' a good idea, because I'm honestly not sure. The MYSQL_RES type with mysql_store_result automatically allocated the memory.
  Posted by on September 23, 2003
No Jennifer, that's not necessary.

It appears that the mysql_fetch_lengths() returns a pointer to data inside 'result' - the memory is allocated and freed with 'result'.

My code is similar to the example and I haven't run into any problems.
  Posted by Chris Newey on February 6, 2005
malloc is definately not a good idea.

malloc acquires some memory and returns a pointer to that memory. The call to fetch_lengths is going to overwrite that pointer.

A subsequent call to free using the value now in the lengths stack variable is going to attempt to free some memory inside the result structure. Exactly what would happen then I can't be bothered to investigate. I think, stress think, that free will give up 'cos it won't recognize the address passed to it.

The bad news is the pointer to malloced memory has now gone walkies and a memory leak has been created - whoops <g>
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