This is my first post in 2026.
What exactly are ELF(Executable and Linkable Format) files?
ELF files are the default executable file format in Linux.
If you’re not familiar with Linux, consider it to be the equivalent of an exe file in Windows.
Back when I first learned Linux, I thought clicking on the little green triangle in Microsoft Visual Studio would create an ELF or a PE file.
It wasn’t technically incorrect, but there’s much more happening under the hood.
Computers can’t understand C code.
It can only understand binaries which consists of 0s and 1s.
But since we only know languages like C we need a program to translate the C code to a binary.
The program below prints my name.
// hello.c
#include <stdio.h>
int main(){
printf("hello hwkim301\n");
}
You can compile the C code with gcc without specifying the name of -o (output file) you’ll get an a.out file.
gcc hello.c
./a.out
hello hwkim301
Below are some explanations on why not passing the output file’s name would create an a.out.
So how does gcc take a .c or .cc file and produce an ELF file?
gcc is a compiler driver, when executing will invoke multiple programs from the GNU toolchain.
It invokes the cpp(C-preprocessor), cc1(C compiler phase 1), as(Assembler), and ld (Linker).
Let’s first run cpp on hello.c
cpp hello.c -o hello.i
# 0 "hello.c"
# 0 "<built-in>"
# 0 "<command-line>"
# 1 "/usr/include/stdc-predef.h" 1 3 4
# 0 "<command-line>" 2
# 1 "hello.c"
# 1 "/usr/include/stdio.h" 1 3 4
# 28 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 1 3 4
# 33 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 3 4
# 1 "/usr/include/features.h" 1 3 4
# 394 "/usr/include/features.h" 3 4
# 1 "/usr/include/features-time64.h" 1 3 4
# 20 "/usr/include/features-time64.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 21 "/usr/include/features-time64.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 1 3 4
# 19 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 20 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 2 3 4
# 22 "/usr/include/features-time64.h" 2 3 4
# 395 "/usr/include/features.h" 2 3 4
# 502 "/usr/include/features.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 1 3 4
# 576 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 577 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/long-double.h" 1 3 4
# 578 "/usr/include/x86_64-linux-gnu/sys/cdefs.h" 2 3 4
# 503 "/usr/include/features.h" 2 3 4
# 526 "/usr/include/features.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 1 3 4
# 10 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/gnu/stubs-64.h" 1 3 4
# 11 "/usr/include/x86_64-linux-gnu/gnu/stubs.h" 2 3 4
# 527 "/usr/include/features.h" 2 3 4
# 34 "/usr/include/x86_64-linux-gnu/bits/libc-header-start.h" 2 3 4
# 29 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/lib/gcc/x86_64-linux-gnu/13/include/stddef.h" 1 3 4
# 214 "/usr/lib/gcc/x86_64-linux-gnu/13/include/stddef.h" 3 4
# 214 "/usr/lib/gcc/x86_64-linux-gnu/13/include/stddef.h" 3 4
typedef long unsigned int size_t;
# 35 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/lib/gcc/x86_64-linux-gnu/13/include/stdarg.h" 1 3 4
# 40 "/usr/lib/gcc/x86_64-linux-gnu/13/include/stdarg.h" 3 4
typedef __builtin_va_list __gnuc_va_list;
# 38 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types.h" 1 3 4
# 27 "/usr/include/x86_64-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 28 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 1 3 4
# 19 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/wordsize.h" 1 3 4
# 20 "/usr/include/x86_64-linux-gnu/bits/timesize.h" 2 3 4
# 29 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
typedef unsigned char __u_char;
typedef unsigned short int __u_short;
typedef unsigned int __u_int;
typedef unsigned long int __u_long;
typedef signed char __int8_t;
typedef unsigned char __uint8_t;
typedef signed short int __int16_t;
typedef unsigned short int __uint16_t;
typedef signed int __int32_t;
typedef unsigned int __uint32_t;
typedef signed long int __int64_t;
typedef unsigned long int __uint64_t;
typedef __int8_t __int_least8_t;
typedef __uint8_t __uint_least8_t;
typedef __int16_t __int_least16_t;
typedef __uint16_t __uint_least16_t;
typedef __int32_t __int_least32_t;
typedef __uint32_t __uint_least32_t;
typedef __int64_t __int_least64_t;
typedef __uint64_t __uint_least64_t;
typedef long int __quad_t;
typedef unsigned long int __u_quad_t;
typedef long int __intmax_t;
typedef unsigned long int __uintmax_t;
# 141 "/usr/include/x86_64-linux-gnu/bits/types.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/typesizes.h" 1 3 4
# 142 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/time64.h" 1 3 4
# 143 "/usr/include/x86_64-linux-gnu/bits/types.h" 2 3 4
typedef unsigned long int __dev_t;
typedef unsigned int __uid_t;
typedef unsigned int __gid_t;
typedef unsigned long int __ino_t;
typedef unsigned long int __ino64_t;
typedef unsigned int __mode_t;
typedef unsigned long int __nlink_t;
typedef long int __off_t;
typedef long int __off64_t;
typedef int __pid_t;
typedef struct { int __val[2]; } __fsid_t;
typedef long int __clock_t;
typedef unsigned long int __rlim_t;
typedef unsigned long int __rlim64_t;
typedef unsigned int __id_t;
typedef long int __time_t;
typedef unsigned int __useconds_t;
typedef long int __suseconds_t;
typedef long int __suseconds64_t;
typedef int __daddr_t;
typedef int __key_t;
typedef int __clockid_t;
typedef void * __timer_t;
typedef long int __blksize_t;
typedef long int __blkcnt_t;
typedef long int __blkcnt64_t;
typedef unsigned long int __fsblkcnt_t;
typedef unsigned long int __fsblkcnt64_t;
typedef unsigned long int __fsfilcnt_t;
typedef unsigned long int __fsfilcnt64_t;
typedef long int __fsword_t;
typedef long int __ssize_t;
typedef long int __syscall_slong_t;
typedef unsigned long int __syscall_ulong_t;
typedef __off64_t __loff_t;
typedef char *__caddr_t;
typedef long int __intptr_t;
typedef unsigned int __socklen_t;
typedef int __sig_atomic_t;
# 40 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__fpos_t.h" 1 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__mbstate_t.h" 1 3 4
# 13 "/usr/include/x86_64-linux-gnu/bits/types/__mbstate_t.h" 3 4
typedef struct
{
int __count;
union
{
unsigned int __wch;
char __wchb[4];
} __value;
} __mbstate_t;
# 6 "/usr/include/x86_64-linux-gnu/bits/types/__fpos_t.h" 2 3 4
typedef struct _G_fpos_t
{
__off_t __pos;
__mbstate_t __state;
} __fpos_t;
# 41 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__fpos64_t.h" 1 3 4
# 10 "/usr/include/x86_64-linux-gnu/bits/types/__fpos64_t.h" 3 4
typedef struct _G_fpos64_t
{
__off64_t __pos;
__mbstate_t __state;
} __fpos64_t;
# 42 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/__FILE.h" 1 3 4
struct _IO_FILE;
typedef struct _IO_FILE __FILE;
# 43 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/FILE.h" 1 3 4
struct _IO_FILE;
typedef struct _IO_FILE FILE;
# 44 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/struct_FILE.h" 1 3 4
# 35 "/usr/include/x86_64-linux-gnu/bits/types/struct_FILE.h" 3 4
struct _IO_FILE;
struct _IO_marker;
struct _IO_codecvt;
struct _IO_wide_data;
typedef void _IO_lock_t;
struct _IO_FILE
{
int _flags;
char *_IO_read_ptr;
char *_IO_read_end;
char *_IO_read_base;
char *_IO_write_base;
char *_IO_write_ptr;
char *_IO_write_end;
char *_IO_buf_base;
char *_IO_buf_end;
char *_IO_save_base;
char *_IO_backup_base;
char *_IO_save_end;
struct _IO_marker *_markers;
struct _IO_FILE *_chain;
int _fileno;
int _flags2;
__off_t _old_offset;
unsigned short _cur_column;
signed char _vtable_offset;
char _shortbuf[1];
_IO_lock_t *_lock;
__off64_t _offset;
struct _IO_codecvt *_codecvt;
struct _IO_wide_data *_wide_data;
struct _IO_FILE *_freeres_list;
void *_freeres_buf;
size_t __pad5;
int _mode;
char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)];
};
# 45 "/usr/include/stdio.h" 2 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/types/cookie_io_functions_t.h" 1 3 4
# 27 "/usr/include/x86_64-linux-gnu/bits/types/cookie_io_functions_t.h" 3 4
typedef __ssize_t cookie_read_function_t (void *__cookie, char *__buf,
size_t __nbytes);
typedef __ssize_t cookie_write_function_t (void *__cookie, const char *__buf,
size_t __nbytes);
typedef int cookie_seek_function_t (void *__cookie, __off64_t *__pos, int __w);
typedef int cookie_close_function_t (void *__cookie);
typedef struct _IO_cookie_io_functions_t
{
cookie_read_function_t *read;
cookie_write_function_t *write;
cookie_seek_function_t *seek;
cookie_close_function_t *close;
} cookie_io_functions_t;
# 48 "/usr/include/stdio.h" 2 3 4
typedef __gnuc_va_list va_list;
# 64 "/usr/include/stdio.h" 3 4
typedef __off_t off_t;
# 78 "/usr/include/stdio.h" 3 4
typedef __ssize_t ssize_t;
typedef __fpos_t fpos_t;
# 129 "/usr/include/stdio.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/stdio_lim.h" 1 3 4
# 130 "/usr/include/stdio.h" 2 3 4
# 149 "/usr/include/stdio.h" 3 4
extern FILE *stdin;
extern FILE *stdout;
extern FILE *stderr;
extern int remove (const char *__filename) __attribute__ ((__nothrow__ , __leaf__));
extern int rename (const char *__old, const char *__new) __attribute__ ((__nothrow__ , __leaf__));
extern int renameat (int __oldfd, const char *__old, int __newfd,
const char *__new) __attribute__ ((__nothrow__ , __leaf__));
# 184 "/usr/include/stdio.h" 3 4
extern int fclose (FILE *__stream) __attribute__ ((__nonnull__ (1)));
# 194 "/usr/include/stdio.h" 3 4
extern FILE *tmpfile (void)
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
# 211 "/usr/include/stdio.h" 3 4
extern char *tmpnam (char[20]) __attribute__ ((__nothrow__ , __leaf__)) ;
extern char *tmpnam_r (char __s[20]) __attribute__ ((__nothrow__ , __leaf__)) ;
# 228 "/usr/include/stdio.h" 3 4
extern char *tempnam (const char *__dir, const char *__pfx)
__attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (__builtin_free, 1)));
extern int fflush (FILE *__stream);
# 245 "/usr/include/stdio.h" 3 4
extern int fflush_unlocked (FILE *__stream);
# 264 "/usr/include/stdio.h" 3 4
extern FILE *fopen (const char *__restrict __filename,
const char *__restrict __modes)
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
extern FILE *freopen (const char *__restrict __filename,
const char *__restrict __modes,
FILE *__restrict __stream) __attribute__ ((__nonnull__ (3)));
# 299 "/usr/include/stdio.h" 3 4
extern FILE *fdopen (int __fd, const char *__modes) __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
extern FILE *fopencookie (void *__restrict __magic_cookie,
const char *__restrict __modes,
cookie_io_functions_t __io_funcs) __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
extern FILE *fmemopen (void *__s, size_t __len, const char *__modes)
__attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (fclose, 1))) ;
# 334 "/usr/include/stdio.h" 3 4
extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__nonnull__ (1)));
extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
int __modes, size_t __n) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
size_t __size) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void setlinebuf (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int fprintf (FILE *__restrict __stream,
const char *__restrict __format, ...) __attribute__ ((__nonnull__ (1)));
extern int printf (const char *__restrict __format, ...);
extern int sprintf (char *__restrict __s,
const char *__restrict __format, ...) __attribute__ ((__nothrow__));
extern int vfprintf (FILE *__restrict __s, const char *__restrict __format,
__gnuc_va_list __arg) __attribute__ ((__nonnull__ (1)));
extern int vprintf (const char *__restrict __format, __gnuc_va_list __arg);
extern int vsprintf (char *__restrict __s, const char *__restrict __format,
__gnuc_va_list __arg) __attribute__ ((__nothrow__));
extern int snprintf (char *__restrict __s, size_t __maxlen,
const char *__restrict __format, ...)
__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 4)));
extern int vsnprintf (char *__restrict __s, size_t __maxlen,
const char *__restrict __format, __gnuc_va_list __arg)
__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 3, 0)));
extern int vasprintf (char **__restrict __ptr, const char *__restrict __f,
__gnuc_va_list __arg)
__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 2, 0))) ;
extern int __asprintf (char **__restrict __ptr,
const char *__restrict __fmt, ...)
__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 2, 3))) ;
extern int asprintf (char **__restrict __ptr,
const char *__restrict __fmt, ...)
__attribute__ ((__nothrow__)) __attribute__ ((__format__ (__printf__, 2, 3))) ;
extern int vdprintf (int __fd, const char *__restrict __fmt,
__gnuc_va_list __arg)
__attribute__ ((__format__ (__printf__, 2, 0)));
extern int dprintf (int __fd, const char *__restrict __fmt, ...)
__attribute__ ((__format__ (__printf__, 2, 3)));
extern int fscanf (FILE *__restrict __stream,
const char *__restrict __format, ...) __attribute__ ((__nonnull__ (1)));
extern int scanf (const char *__restrict __format, ...) ;
extern int sscanf (const char *__restrict __s,
const char *__restrict __format, ...) __attribute__ ((__nothrow__ , __leaf__));
# 1 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 1 3 4
# 119 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 1 3 4
# 24 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 3 4
# 1 "/usr/include/x86_64-linux-gnu/bits/long-double.h" 1 3 4
# 25 "/usr/include/x86_64-linux-gnu/bits/floatn-common.h" 2 3 4
# 120 "/usr/include/x86_64-linux-gnu/bits/floatn.h" 2 3 4
# 438 "/usr/include/stdio.h" 2 3 4
# 463 "/usr/include/stdio.h" 3 4
extern int fscanf (FILE *__restrict __stream, const char *__restrict __format, ...) __asm__ ("" "__isoc99_fscanf")
__attribute__ ((__nonnull__ (1)));
extern int scanf (const char *__restrict __format, ...) __asm__ ("" "__isoc99_scanf")
;
extern int sscanf (const char *__restrict __s, const char *__restrict __format, ...) __asm__ ("" "__isoc99_sscanf") __attribute__ ((__nothrow__ , __leaf__))
;
# 490 "/usr/include/stdio.h" 3 4
extern int vfscanf (FILE *__restrict __s, const char *__restrict __format,
__gnuc_va_list __arg)
__attribute__ ((__format__ (__scanf__, 2, 0))) __attribute__ ((__nonnull__ (1)));
extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg)
__attribute__ ((__format__ (__scanf__, 1, 0))) ;
extern int vsscanf (const char *__restrict __s,
const char *__restrict __format, __gnuc_va_list __arg)
__attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__format__ (__scanf__, 2, 0)));
# 540 "/usr/include/stdio.h" 3 4
extern int vfscanf (FILE *__restrict __s, const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vfscanf")
__attribute__ ((__format__ (__scanf__, 2, 0))) __attribute__ ((__nonnull__ (1)));
extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vscanf")
__attribute__ ((__format__ (__scanf__, 1, 0))) ;
extern int vsscanf (const char *__restrict __s, const char *__restrict __format, __gnuc_va_list __arg) __asm__ ("" "__isoc99_vsscanf") __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__format__ (__scanf__, 2, 0)));
# 575 "/usr/include/stdio.h" 3 4
extern int fgetc (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern int getc (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern int getchar (void);
extern int getc_unlocked (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern int getchar_unlocked (void);
# 600 "/usr/include/stdio.h" 3 4
extern int fgetc_unlocked (FILE *__stream) __attribute__ ((__nonnull__ (1)));
# 611 "/usr/include/stdio.h" 3 4
extern int fputc (int __c, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern int putc (int __c, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern int putchar (int __c);
# 627 "/usr/include/stdio.h" 3 4
extern int fputc_unlocked (int __c, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern int putc_unlocked (int __c, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern int putchar_unlocked (int __c);
extern int getw (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern int putw (int __w, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
__attribute__ ((__access__ (__write_only__, 1, 2))) __attribute__ ((__nonnull__ (3)));
# 694 "/usr/include/stdio.h" 3 4
extern __ssize_t __getdelim (char **__restrict __lineptr,
size_t *__restrict __n, int __delimiter,
FILE *__restrict __stream) __attribute__ ((__nonnull__ (4)));
extern __ssize_t getdelim (char **__restrict __lineptr,
size_t *__restrict __n, int __delimiter,
FILE *__restrict __stream) __attribute__ ((__nonnull__ (4)));
extern __ssize_t getline (char **__restrict __lineptr,
size_t *__restrict __n,
FILE *__restrict __stream) __attribute__ ((__nonnull__ (3)));
extern int fputs (const char *__restrict __s, FILE *__restrict __stream)
__attribute__ ((__nonnull__ (2)));
extern int puts (const char *__s);
extern int ungetc (int __c, FILE *__stream) __attribute__ ((__nonnull__ (2)));
extern size_t fread (void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __stream)
__attribute__ ((__nonnull__ (4)));
extern size_t fwrite (const void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __s) __attribute__ ((__nonnull__ (4)));
# 766 "/usr/include/stdio.h" 3 4
extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __stream)
__attribute__ ((__nonnull__ (4)));
extern size_t fwrite_unlocked (const void *__restrict __ptr, size_t __size,
size_t __n, FILE *__restrict __stream)
__attribute__ ((__nonnull__ (4)));
extern int fseek (FILE *__stream, long int __off, int __whence)
__attribute__ ((__nonnull__ (1)));
extern long int ftell (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern void rewind (FILE *__stream) __attribute__ ((__nonnull__ (1)));
# 803 "/usr/include/stdio.h" 3 4
extern int fseeko (FILE *__stream, __off_t __off, int __whence)
__attribute__ ((__nonnull__ (1)));
extern __off_t ftello (FILE *__stream) __attribute__ ((__nonnull__ (1)));
# 829 "/usr/include/stdio.h" 3 4
extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos)
__attribute__ ((__nonnull__ (1)));
extern int fsetpos (FILE *__stream, const fpos_t *__pos) __attribute__ ((__nonnull__ (1)));
# 860 "/usr/include/stdio.h" 3 4
extern void clearerr (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int feof (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int ferror (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void clearerr_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int feof_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int ferror_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void perror (const char *__s) __attribute__ ((__cold__));
extern int fileno (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int fileno_unlocked (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
# 897 "/usr/include/stdio.h" 3 4
extern int pclose (FILE *__stream) __attribute__ ((__nonnull__ (1)));
extern FILE *popen (const char *__command, const char *__modes)
__attribute__ ((__malloc__)) __attribute__ ((__malloc__ (pclose, 1))) ;
extern char *ctermid (char *__s) __attribute__ ((__nothrow__ , __leaf__))
__attribute__ ((__access__ (__write_only__, 1)));
# 941 "/usr/include/stdio.h" 3 4
extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
# 959 "/usr/include/stdio.h" 3 4
extern int __uflow (FILE *);
extern int __overflow (FILE *, int);
# 983 "/usr/include/stdio.h" 3 4
# 2 "hello.c" 2
# 3 "hello.c"
int main(){
printf("hello hwkim301\n");
}
Although hello.c code was extremely short hello.i has a lot of code.
What exactly is all this code in the hello.i file?
Notice that hello.i contains a lot of code, but it’s only the content of stdio.h and its dependencies. The preprocessor includes only what you explicitly asked for.
It also removes comments, and passes preprocessor directives wich start with a # like #if, #ifdef #endif.
Then passes only the code blocks necessary for compilation to the compiler.
The next step compilation transforms the preprocessed C code to assembly.
You can use either the gcc -S option or cc1 to generate the assembly.
Most people use gcc -S instead of directly invoking cc1 because using cc1 requires you to know a lot of information on where the header files are therefore making it’s use a bit cumbersome.
gcc -S hello.i -o hello.s
Here’s the assembly gcc -S produced.
It’s in AT&T syntax.
If you dislike the AT&T syntax pass the -masm=intel flag to produce intel flavor assembly.
.file "hello.c"
.text
.section .rodata
.LC0:
.string "hello hwkim301"
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
endbr64
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
leaq .LC0(%rip), %rax
movq %rax, %rdi
call puts@PLT
movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.ident "GCC: (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0"
.section .note.GNU-stack,"",@progbits
.section .note.gnu.property,"a"
.align 8
.long 1f - 0f
.long 4f - 1f
.long 5
0:
.string "GNU"
1:
.align 8
.long 0xc0000002
.long 3f - 2f
2:
.long 0x3
3:
.align 8
4:
After that the assembler gets run.
as hello.s -o hello.o
The assembler creates an object file, from now on the file is binary but you can’t execute it yet because it doesn’t have the final memory layout and resolved addresses for external symbols.
file hello.o
hello.o: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped
Finally the linker will link the programs and resolve symbolic references into fixed virtual memory addresses so that the computer can execute the binary.
How the linker sets the virtual memory addresss so that the actual functions and variables get executed is a massive topic by itself.
If you’re interested search for ELF symbols and Relocation.
Sadly, I’m no expert on GCC nor ELF files so this is as far as I can explain.
Anyways, running the linker as well as cpp, as directly is a bit diffcult.
The linker complains that it can’t find the entry symbol _start.
ld hello.o -o hello
ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000
ld: hello.o: in function `main':
hello.c:(.text+0x13): undefined reference to `puts'
In order to run ld manually you need to pass the location of the crt1.o, crti.o, the dynamic linker and the crtn.o to resolve the problem above.
/usr/bin/ld /usr/lib/x86_64-linux-gnu/crt1.o /usr/lib/x86_64-linux-gnu/crti.o hello.o -lc -dynamic-linker /lib64/ld-linux-x86-64.so.2 -o hello /usr/lib/x86_64-linux-gnu/crtn.o
Here are some explanations on why you need to pass crt1.o, crti.o and crtn.o.
Finally, an ELF(Executable and Linkable Format) is created.
file hello
hello: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /lib64/ld-linux-x86-64.so.2, for GNU/Linux 3.2.0, not stripped
Let’s run the program, it works nicely.
./hello
hello hwkim301
Since manually running cpp, gcc -S , as and ld is a lot of work gcc automatically executes these 4 programs in a row to create the ELF automatically.
You can also save each intermediate file as well using the -save-temps flag.
gcc -save-temps hello.c -o hello
Omitting -save-temps will only produce the final ELF executable.
gcc hello.c -o hello