The ELF Auxiliary Table

Every process has a stack, but the system defines no fixed stack address. Furthermore, a program’s stack address can change from one system to another, and even from one process invocation to another. Thus the process initialization code must use the stack address in general purpose register r1. Data in the stack segment at addresses below the stack pointer contain undefined values.

Whereas the argument and environment vectors transmit information from one application program to another, the auxiliary vector conveys information from the operating system to the program. This vector is an array of structures, defined as follows:

typedef struct
{
int a_type;
union
{
long a_val;
void a_ptr;
void (a_fcn)();
} a_un;
} auxv_t; Name Value a_un field

AT_NULL 0 ignored
AT_IGNORE 1 ignored
AT_EXECFD 2 a_val
AT_PHDR 3 a_ptr
AT_PHENT 4 a_val
AT_PHNUM 5 a_val
AT_PAGESZ 6 a_val
AT_BASE 7 a_ptr
AT_FLAGS 8 a_val
AT_ENTRY 9 a_ptr
AT_HWCAP 16 a_val
AT_DCACHEBSIZE 19 a_val
AT_ICACHEBSIZE 20 a_val
AT_UCACHEBSIZE 21 a_val

AT_NULL: The auxiliary vector has no fixed length; instead an entry of this type denotes the end of the vector. The corresponding value of a_un is undefined.
AT_IGNORE: This type indicates the entry has no meaning. The corresponding value of a_un is undefined.
AT_EXECFD: As Chapter 5 in the System V ABI describes, exec may pass control to an interpreter program. When this happens, the system places either an entry of type AT_EXECFD or one of type AT_PHDR in the auxiliary vector. The entry for type AT_EXECFD uses the a_val member to contain a file descriptor open to read the application program’s object file.
AT_PHDR: Under some conditions, the system creates the memory image of the application program before passing control to an interpreter program. When this happens, the a_ptr member of the AT_PHDR entry tells the interpreter where to find the program header table in the memory image. If the AT_PHDR entry is present, entries of types AT_PHENT, AT_PHNUM, and AT_ENTRY must also be present. See the section Program Header in Chapter 5 of the System V ABI and the chapter called Program Loading and Dynamic Linking of this processor supplement for more information about the program header table.
AT_PHENT: The a_val member of this entry holds the size, in bytes, of one entry in the program header table to which the AT_PHDR entry points.
AT_PHNUM: The a_val member of this entry holds the number of entries in the program header table to which the AT_PHDR entry points.
AT_PAGESZ: If present, this entry’s a_val member gives the system page size in bytes. The same information is also available through the sysconf system call.
AT_BASE: The a_ptr member of this entry holds the base address at which the interpreter program was loaded into memory. See the section Program Header in Chapter 5 of the System V ABI for more information about the base address.
AT_FLAGS: If present, the a_val member of this entry holds 1-bit flags. Bits with undefined semantics are set to zero.
AT_ENTRY: The a_ptr member of this entry holds the entry point of the application program to which the interpreter program should transfer control.
AT_DCACHEBSIZE: The a_val member of this entry gives the data cache block size for processors on the system on which this program is running. If the processors have unified caches, AT_DCACHEBSIZE is the same as AT_UCACHEBSIZE.
AT_ICACHEBSIZE: The a_val member of this entry gives the instruction cache block size for processors on the system on which this program is running. If the processors have unified caches, AT_DCACHEBSIZE is the same as AT_UCACHEBSIZE.
AT_UCACHEBSIZE: The a_val member of this entry is zero if the processors on the system on which this program is running do not have a unified instruction and data cache. Otherwise, it gives the cache block size.
AT_HWCAP: The a_val member of this entry is bit map of hardware capabilities. Some bit mask values include:
PPC_FEATURE_32 0x80000000 / Always set for powerpc64 /
PPC_FEATURE_64 0x40000000 / Always set for powerpc64 /
PPC_FEATURE_HAS_ALTIVEC 0x10000000
PPC_FEATURE_HAS_FPU 0x08000000
PPC_FEATURE_HAS_MMU 0x04000000
PPC_FEATURE_UNIFIED_CACHE 0x01000000

Other auxiliary vector types are reserved. No flags are currently defined for AT_FLAGS on the 64-bit PowerPC Architecture.

When a process receives control, its stack holds the arguments, environment, and auxiliary vector from exec. Argument strings, environment strings, and the auxiliary information appear in no specific order within the information block; the system makes no guarantees about their relative arrangement. The system may also leave an unspecified amount of memory between the null auxiliary vector entry and the beginning of the information block. The back chain word of the first stack frame contains a null pointer (0).