pointers - C - Can return value of malloc be used array-like (alignment)? -

often malloc used allocate memory n elements of primitive datatype (int, float, char). this:

#define n 10  double *mem = malloc(n * sizeof(double));  (int = 0; < n; ++i) {   mem[i] = 10.0;       // write   double d = mem[i];   // read } 

this work if declared mem char * , used cast later on:

char *mem = malloc(n * sizeof(double));  (int = 0; < n; ++i) {   *(double*)(mem + * sizeof(double)) = 10.0;   double d = *(double *)(mem + * sizeof(double)); } 

currently i'm wondering if defined , working every datatype (even complex type struct). lets assume struct a anything. following defined?

char *mem = malloc(n * sizeof(struct a));  (int = 0; < 10; ++i)) {   *(struct *)(mem + * sizeof(struct a)) = /* valid */;   struct x = *(struct *)(mem + * sizeof(struct a)); } 

since return value of malloc suitably aligned kind of variable case i = 0 defined , working, how i = 1, 2, ...? alignment screw up? in example:

char char_arr[4] = /* value */; int *d = &char_arr[0]; 

the second line not guaranteed work, since char_arr may not aligned on 4-byte boundary (required int).

is following defined?

char *mem = malloc(n * sizeof(struct a));  (int = 0; < n /* 10 */; ++i)) {   *(struct *)(mem + * sizeof(struct a)) = /* valid */;   // struct = *(struct *)(mem + * sizeof(struct a));   struct x = *(struct *)(mem + * sizeof(struct a)); } 

almost always.

in terms of alignment, *alloc() returns pointer memory valid fundamental alignments op noted. (struct *)(mem + * sizeof(struct a)) provide aligned pointer 0 <= <= n.

not concern op, yet on rare machines i * sizeof(struct a) overflow size_t math (with large enough i , sizeof(struct a), whereas mem[i] not. not seen on common implementations use flat memory address.

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also robust code checks memory allocation failures.

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candidate simplified code. notice type ptr points not relevant, assuming not defined void *ptr. @usr.

ptr = malloc(sizeof *ptr * n); if (ptr == null) handle_oom();  (size_t = 0; < n; ++i)) {   ptr[i] = /* valid */; } 

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