Improve ELF structure definitions and comments

[DrXiao]

Since the compiler now supports handling the 'short' data type, this commit adjusts the data types of the following data members from 'char [2]' to 'short':

• elf32_hdr_t: e_type, e_machine, e_ehsize, e_phentsize, e_phnum, e_shentsize, e_shnum, e_shstrndx
• elf32_sym_t: st_shndx

With these changes, the ELF structure definitions are closer to the ELF specification.

---

Summary by cubic

Aligns ELF 16-bit fields with the ELF32 spec by switching from char[2] to short and adds spec-aligned comments; simplifies header generation with no change to on-disk layout.

• Replaced char[2] with short for elf32_hdr_t: e_type, e_machine, e_ehsize, e_phentsize, e_phnum, e_shentsize, e_shnum, e_shstrndx, and for elf32_sym_t: st_shndx.
• Updated elf_generate_header to assign shorts directly.
• Added inline comments documenting ELF typedef sizes/alignments and noting temporary use of signed types until unsigned is supported.

^{Written for commit 56f60cd. Summary will update on new commits.}

[jserv]

Consider to add comments, addressing ELF spec.

[DrXiao]

Consider to add comments, addressing ELF spec.

/* In the ELF specification, the following data types are defined
 * for data representation:
 *
 * +-------------+------+-----------+--------------------------+
 * | Name        | Size | Alignment | Purpose                  |
 * +-------------+------+-----------+--------------------------+
 * | Elf32_Addr  | 4    | 4         | Unsigned program address |
 * +-------------+------+-----------+--------------------------+
 * | Elf32_Half  | 2    | 2         | Unsigned medium integer  |
 * +-------------+------+-----------+--------------------------+
 * | Elf32_Off   | 4    | 4         | Unsigned file offset     |
 * +-------------+------+-----------+--------------------------+
 * | Elf32_Sword | 4    | 4         | Signed large integer     |
 * +-------------+------+-----------+--------------------------+
 * | Elf32_Word  | 4    | 4         | Unsigned large integer   |
 * +-------------+------+-----------+--------------------------+
 * | unsigned    | 1    | 1         | Unsigned small integer   |
 * | char        |      |           |                          |
 * +-------------+------+-----------+--------------------------+
 *
 * However, since the current implementation doesn't support unsigned
 * data type definitions, such as 'unsigned int', 'unsigned short', and
 * so on, the ELF structures are now implemented primarily using the
 * 'signed' data type.
 * - Elf32_Addr    -> int
 * - Elf32_Half    -> short
 * - Elf32_Off     -> int
 * - Elf32_Word    -> int
 * - unsigned char -> char
 *
 * TODO: Use correct unsigned types for these ELF structures after
 * the 'unsigned' specifier is supported.
 */

/* ELF header
 *
 * #define EI_NIDENT 16
 * typedef struct {
 *     unsigned char e_ident[EI_NIDENT];
 *     Elf32_Half    e_type;
 *     Elf32_Half    e_machine;
 *     Elf32_Word    e_version;
 *     Elf32_Addr    e_entry;
 *     Elf32_Off     e_phoff;
 *     Elf32_Off     e_shoff;
 *     Elf32_Word    e_flags;
 *     Elf32_Half    e_ehsize;
 *     Elf32_Half    e_phentsize
 *     Elf32_Half    e_phnum;
 *     Elf32_Half    e_shentsize;
 *     Elf32_Half    e_shnum;
 *     Elf32_Half    e_shstrndx;
 * } Elf32_Ehdr;
 */
typedef struct {
    char e_ident[16];
    short e_type;
    short e_machine;
    int e_version;
    int e_entry;
    int e_phoff;
    int e_shoff;
    int e_flags;
    short e_ehsize;
    short e_phentsize;
    short e_phnum;
    short e_shentsize;
    short e_shnum;
    short e_shstrndx;
} elf32_hdr_t;

How about adding comments like the ones above?

ELF data types are summarized in a table, and each ELF structure (e.g.: elf32_hdr_t, elf32_phdr_t ...) includes a comment showing its original definition from the ELF specification.

[jserv]

Use inlined comments instead.

[jserv]

Thank @DrXiao for contributing!