CODEWITHSUNDEEP
carrays
folibis
folibis

Reputation: 12864

project struct to a array of bytes in c

Suppose I have array of chars:

char buffer[1024];

The array in fact contains chain of structures in sequence. 

struct MyStruct {
    char name[4];
    int num1;
    int num2; 
}

I want to loop through the array:

MyStruct *p;
for(int i = 0;i < sizeof(buffer);i += sizeof(MyStruct))
{
    // how can I point p to some place in buffer here?    
}

I want to point p to start of buffer, the to buffer + 12 etc.

Upvotes: 1

Views: 59

Answers (3)

nmatt
nmatt

Reputation: 495

One issue to consider is that the char buffer might not be properly aligned for a struct (and, in this case, its int members num1 and num2). Depending on the platform and implementation, a 4-byte, 8-byte or 16-byte alignment might be required. For that reason, one alternative is to declare the buffer in terms of MyStruct initially and then access it via a char pointer:

MyStruct buffer[1024 / sizeof(MyStruct)];
char * cp = (char *) buffer;
// fill the buffer via cp
for (size_t i = 0; i < sizeof(buffer); ++i)
{
    // do stuff with buffer[i]
}

If that approach is not possible, the buffer needs to be copied to another buffer with safe alignment; For example:

size_t n = sizeof(buffer) / sizeof(MyStruct);
MyStruct * p = (MyStruct *) malloc(n * sizeof(MyStruct));
if (!p) { exit(EXIT_FAILURE); }
memcpy(p, buffer, n * sizeof(MyStruct));  // copy buffer to p
for (size_t i = 0; i < n; ++i)
{
    // do stuff with p[i]
}

Upvotes: 3

Paul Ogilvie
Paul Ogilvie

Reputation: 25286

You let p point to the first struct in the buffer, then increment it on each iteration:

MyStruct *p= (struct MyStruct *) buffer;
for(int i = 0; i < sizeof(buffer); i += sizeof(MyStruct), p++)
{
    // your code 
}

...and yes, this assumes the structs are contiguous in memory, with no padding in between.

Upvotes: 0

pm100
pm100

Reputation: 50180

first note that you are assuming that this will work. That there is no padding between the elements of the struct. Having said that do this:

MyStruct *s = (MyStruct*)(buffer + i)

Upvotes: 1

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