Linker adds extra padding in section? - c

Question

I am using GCC and am trying to get the linker to create an array of data objects from different files in a custom section. I have added this section to the linker script:
.mydata :
{
__mydata_start = .;
KEEP (*(.mydata))
__mydata_end = .;
}
Here is my data structure:
struct my_type_t {
unsigned char a;
size_t b;
void *c;
const void *d;
const void *e;
const char *f;
};
I am on a 64-bit platform, and sizeof(struct my_type_t) gives 48. However, the linker seems to want to align these structures to 64-bytes.
My declarations:
struct my_type_t a1 __attribute__((section(".mydata"))) = {
1, 2, (void *)3, (void *)4, (void *)5, (void *)6,
};
struct my_type_t a2 __attribute__((section(".mydata"))) = {
7, 8, (void *)9, (void *)10, (void *)11, (void *)12,
};
Linker output:
$ gcc -o ldtest ldtest.c -Wl,-Tlink.ld
$ objcopy -Obinary -j .mydata ldtest mydata
$ hd mydata
00000000 01 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 |................|
00000010 03 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 |................|
00000020 05 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 |................|
00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
00000040 07 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 |................|
00000050 09 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 |................|
00000060 0b 00 00 00 00 00 00 00 0c 00 00 00 00 00 00 00 |................|
If I instead declare it as an array:
struct my_type_t a1[2] __attribute__((section(".mydata"))) = {
{
1, 2, (void *)3, (void *)4, (void *)5, (void *)6,
},
{
7, 8, (void *)9, (void *)10, (void *)11, (void *)12,
},
};
There is no padding, as expected:
00000000 01 00 00 00 00 00 00 00 02 00 00 00 00 00 00 00 |................|
00000010 03 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 |................|
00000020 05 00 00 00 00 00 00 00 06 00 00 00 00 00 00 00 |................|
00000030 07 00 00 00 00 00 00 00 08 00 00 00 00 00 00 00 |................|
00000040 09 00 00 00 00 00 00 00 0a 00 00 00 00 00 00 00 |................|
00000050 0b 00 00 00 00 00 00 00 0c 00 00 00 00 00 00 00 |................|
Why is the linker adding extra padding beyond what the compiler thinks is necessary, and how can I get rid of it?

Answer 1

I don't know why it wants it aligned on 64-byte, but you can do:
struct my_type_t {
unsigned char a;
size_t b;
void *c;
const void *d;
const void *e;
const char *f;
} __attribute__ ((aligned (64)));
To make gcc automatically pad each struct to the nearest higher 64-byte address and make sizeof(struct my_type_t) also include the padding. This makes operations like this work correctly:
struct my_type_t *s = &__mydata_start;
s++; // Jump to next entry
s[0]; // First entry
s[1]; // Second entry