Load Warning: Cannot Find Entry Symbol _Start

load warning: cannot find entry symbol _start

Use the label _start instead of main for the ELF entry point. main implies it's like the C main function, but this isn't even a function (e.g. you can't ret).

You don't say, but from the warning messages and code I assume you're building your 32-bit code with nasm -felf32 hello32.asm && ld -melf_i386 -o hello32 hello32.o

(If you're actually building 64-bit code, you're lucky that it happens to work, but it'll break as soon as you do anything with esp instead of rsp.)

The warning message is from ld, not from nasm. It says so right in the message. Tim's comment is correct: ld looks for a _start symbol in the files it links, but sets the entry point to the beginning of the text segment if it doesn't find one. (That's why this is a warning, not an error. If you had put some other code earlier in the file, execution would start there without global _start / _start:)

It doesn't matter what other global/external symbols you define. main has no relevance at all here, and could point anywhere you want. It's only useful for a disassembly output and stuff like that. Your code would work exactly the same if you took out the global main / main: lines, or changed them to any other name.

Labelling that as main is unwise because the ELF entry point is not a function. It's not main(), and doesn't receive argc and argv arguments in the standard way, and can't ret because ESP is pointing at argc instead of a return address.

Only use main if you link with gcc / glibc's CRT startup code that looks for a main symbol and calls it after initializing libc. (So functions like printf work. Technically dynamic linker hooks let libc initialize itself before your _start if you linked it, but generally don't do that unless you understand exactly what you're doing). Related: Assembling 32-bit binaries on a 64-bit system (GNU toolchain)

e.g. gcc -m32 -no-pie -o hello main.o if you do define a main:

instead of gcc -m32 -static -nostdlib -o hello start.o

(which is equivalent to your bare ld).

(For the past few years, Linux distros have configured GCC with -pie as the default, which wants position-independent code. But that's really inconvenient in 32-bit mode where you don't have x86-64 RIP-relative addressing (look at GCC asm output for example), and means ld won't convert call printf into call printf@plt for you. So for most hand-written asm following most tutorials, you want traditional non-PIE executables so no text relocations are needed.)

ld: warning: cannot find entry symbol _start; defaulting to 0000000000401000

ld by itself links no libraries or startup code. It's suitable for a program where you use _start as an entry point and do I/O via direct calls to the kernel instead of standard C library functions. But your program uses main as its entry point, so it expects to be called by C startup code, and it calls library functions like printf. Hence you should link it like a C program:

gcc -no-pie -o quadratic quadratic.o

The -no-pie option is needed because your code makes absolute references to static data, e.g. fld qword[b]. gcc by default assumes you want to build a position-independent executable, which can't do that; you'd need to write fld qword[rel b] to produce an rip-relative effective address. So -no-pie asks gcc to link a non-position-independent executable. See Why use RIP-relative addressing in NASM? for more on this.

warning: cannot find entry symbol _start; defaulting to 0000000008048060

It's really only in C (and C++) that the starting point is named main.

The default linker script usually uses start or _start for the actual entry point. If you rename or add the correct symbol then the linker should not complain about that.

warning: cannot find entry symbol _start - while compiling to .so

Removing -fPIE and -pie flags solved my problem.
-pie shouldn't be used when you are trying to create shared library,
and also -fPIE shouldn't be used in most of the cases.

ld: warning: cannot find entry symbol main; not setting start address

You are trashing your os.o (input same as output)

Change:

ld -m elf_i386 -nmagic -T os.ld os.o -o os.o

Into:

ld -m elf_i386 -nmagic -T os.ld os.o -o osx

Thanks, that fixed it. I was trying to overwrite it on purpose. Can you explain why it caused that error? – Felix B.

ld [and most such build commands] do not allow an input file to be specified as an output file.

ld may have to read os.o several times while writing the -o osx output file.

The simple construction is that we open a file for read and an output file for writing when the program starts.

For what you wanted, the program would have to detect that infile and outfile are the same and create a temp copy of infile.

Too difficult to guarantee in the general case. It's rarely used and would slow things down.

Also, if you put the commands in a makefile:

all: osx

os.o: os.c
    gcc -ffreestanding -nostdlib -gdwarf-4 -m32 -ggdb3 -c os.c -o os.o

osx: os.o os.ld
    ld -m elf_i386 -nmagic -T os.ld os.o -o osx

If ld produced an error, make would remove the osx file.

Consider that if os.o was the the output file, this would mess up the dependencies that make relies on.

Load Warning: Cannot Find Entry Symbol _Start