CODEWITHSUNDEEP
cpointersindexing
Jpwang
Jpwang

Reputation: 143

Using [] on pointers in C?

Right now I'm looking over some C code and they have some pointer syntax that I'm confused about. So first they declared a pointer like so:

int32_t *p_tx_buf=NULL;

Then later on they wrote:

p_tx_buf = malloc(...math... );

The stuff in the middle is just math to calculate the size of a file, I'm assuming it's not important. After that, they wrote:

p_tx_buf[0] = 0

I'm rather confused about the []. How does that work with an integer pointer? I thought the [] was for indexing things, so I'm confused on how you can use this with a pointer.

Upvotes: 1

Views: 75

Answers (4)

Steve
Steve

Reputation: 1431

the malloc command I assume was some multiplication of the sizeof(int32_t)

say:

p_tx_buf = malloc(2*sizeof(int32_t));

you now have allocated enough memory space for 2 int32_t variables you can access them by:

*(p_tx_buf) and *(p_tx_buf+1)

or

p_tx_buf[0] and p_tx_buf[1]

Upvotes: 1

jamesblacklock
jamesblacklock

Reputation: 859

Just to expound a bit on the answers already provided, the "math" you are referring to is probably not calculating the size of a single file, rather it is multiplying the size of a file times some integer.

The result is that enough space is allocated for several file structures. The pointer winds up pointing to the first of several file structures laid out sequentially in memory. The index [0] means point to the first of those file structures (i.e., an offset of 0)

Upvotes: 1

Carey Gregory
Carey Gregory

Reputation: 6846

In C (and also C++), array[n] is equivalent to *(array+n). They mean exactly the same thing and produce exactly the same result.

Upvotes: 4

haccks
haccks

Reputation: 106102

p_tx_buf[0] = 0 means assign 0 to the location p_tx_buf + 0.

Upvotes: 1

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