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sizeof array....

Posted on 2000-03-21
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Last Modified: 2011-09-20
can someone pls help me how to figure out the size of the _environ array?

printf("%d", sizeof(*_environ)/sizeof(_environ[0]));

...this obviously doesn't work.... why?
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Question by:sa9813
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3 Comments
 
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Expert Comment

by:wylliker
ID: 2643431
The _environ[] array is not a fixed length array. It can change if your program adds an environment string, for example.  In that case it's address may even change in memory.  It is also an array of pointers to character strings.


So if you do something like ...


unsigned short EnvCnt = 0;

while(_environ[EnvCnt])
    ++EnvCnt;

You can know how many environment variables there are.


If you want to know the size of all of the strings then you would do something like ...


unsigned short EnvCnt = 0;
// Not including NULL Terminators
unsigned short EnvTotalStringSize = 0;
// including NULL terminators
unsigned short EnvTotalSize = 0;

while(_environ[EnvCnt])
{
    EnvTotalString += strlen(_environ[EnvCnt]);
    EnvTotalSize += strlen(_environ[EnvCnt])+1;
    ++EnvCnt;
}

// Environment ends with TWO (2) NULL terminators
EnvTotalSize += 2;






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Accepted Solution

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rbr earned 120 total points
ID: 2643461
It depends on how this array was created.

int array[1000];

printf ("%d",sizeof(array)/sizeof(array[0]));
will print 1000

int *array;

array = calloc (1000,sizeof(int));
printf ("%d",sizeof(array)/sizeof(array[0]));
will print the wrong result.
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Expert Comment

by:mohitdas
ID: 2643479
The sizeof keyword gives the amount of storage, in bytes, associated with a variable or a type (including aggregate types). This keyword returns a value of type size_t.

When applied to a statically dimensioned array, sizeof returns the size of the entire array. The sizeof operator cannot return the size of dynamically allocated arrays or external arrays.

int  array[] = { 1, 2, 3, 4, 5 };
size_t  sizearr = sizeof( array ) / sizeof( array[0] );

above will give the number of elements in the array.

sizeof(*_environ)/sizeof(_environ[0])
as this refers to some external array, it'll definitely not work.  :)
Mohit.
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