ELF_FLAGELF(3)

💖 ELF_FLAGELF 💖 — from the Libelf Programmer's Manual

okay bestie, sit down, we're doing libelf flag manipulation and honestly? it's giving main character energy ✨

🌸 NAME 🌸

elf_flagelf, elf_flagdata, elf_flagehdr, elf_flagphdr, elf_flagscn, elf_flagshdr — set or clear flags on ELF descriptors and components. six little functions, one big vibe. 🫶

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✨ SYNOPSIS ✨

#include <libelf.h>

unsigned int elf_flagelf(Elf * elf, Elf_Cmd cmd, unsigned int flags);
unsigned int elf_flagehdr(Elf * elf, Elf_Cmd cmd, unsigned int flags);
unsigned int elf_flagphdr(Elf * elf, Elf_Cmd cmd, unsigned int flags);
unsigned int elf_flagscn(Elf_Scn * scn, Elf_Cmd cmd, unsigned int flags);
unsigned int elf_flagshdr(Elf_Scn * scn, Elf_Cmd cmd, unsigned int flags);
unsigned int elf_flagdata(Elf_Data * data, Elf_Cmd cmd, unsigned int flags);

like, look at them. all lined up. so tidy. 💅

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💜 DESCRIPTION 💜

So these functions let you, the caller, set or clear flags on the various ELF components that libelf is managing. They're the "hey bestie, I changed something" API. Typically you use them to mark a component as modified (aka dirty 🫧), or to control layout behavior when stuff gets written out.

• elf_flagelf() — modifies flags on the top-level Elf descriptor. the big boss. the CEO. 👑
• elf_flagdata() — modifies flags on an Elf_Data descriptor, usually one you got from elf_getdata(3) or made with elf_newdata(3).
• elf_flagehdr() — modifies flags on the ELF header for an ELF descriptor (like, the thing gelf_getehdr(3) hands you).
• elf_flagphdr() — modifies flags on the program header table of an ELF descriptor.
• elf_flagscn() — modifies flags on an Elf_Scn (section) descriptor.
• elf_flagshdr() — modifies flags on the section header of a given Elf_Scn.

The cmd argument is literally just deciding: are we setting these flags (ELF_C_SET) or clearing them (ELF_C_CLR)? that's it. that's the whole job. 🙈

The following flag values may be passed via the flags parameter:

• ELF_F_DIRTY — marks the object as modified. This makes sure changes to the component get written out during elf_update(3). if you changed it and you want it on disk, this is the one, no cap. ✍️
• ELF_F_LAYOUT — says that the layout of the ELF component is managed manually by you. Offsets and sizes will be preserved exactly as provided. This flag is only used with elf_flagelf() and is ignored when used with the other functions. If this flag is set, the user must set Ehdr e_phoff, e_shoff, as well as Elf_Data d_off and Shdr sh_offset. If it's not set, then those offsets will be recalculated when elf_update is called. And heads up: if ELF_F_LAYOUT is set, then changes to phdr offsets will prevent elf_update from automatically adjusting shdr offsets. 👀
• ELF_F_PERMISSIVE — enables relaxed validation of object structure. This flag permits nonstandard layouts without strict checks. Specifically, it removes a check in elf_update() that the section sh_size is a multiple of the section sh_entsize (if not zero). This flag is only used with elf_flagelf() and is ignored when used with the other functions. ELF_F_PERMISSIVE is an elfutils libelf extention and may not be available in other libelf implementations. (yes it says "extention" in the original, we don't fix, we report 🎀)

⚠️ real talk: ELF_F_LAYOUT and ELF_F_PERMISSIVE are only for elf_flagelf(). everywhere else they get ignored. don't be out here passing them to elf_flagshdr and expecting a moment. 💀

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🦋 PARAMETERS 🦋

• obj — the object to be modified. For elf_flagelf, elf_flagehdr, elf_flagphdr, this is an Elf * descriptor. For elf_flagscn and elf_flagshdr, this is an Elf_Scn * descriptor. For elf_flagdata, an Elf_Data * buffer.
• cmd — must be either: ELF_C_SET to set the given flags, or ELF_C_CLR to clear them. pick one, bestie, there's no third option. ⭐
• flags — bitmask of flags to modify. May include any combination of ELF_F_DIRTY, ELF_F_LAYOUT, and ELF_F_PERMISSIVE.

Note that ELF_F_LAYOUT and ELF_F_PERMISSIVE are only used with elf_flagelf() and are ignored for other functions. (say it with me one more time for the people in the back 📣)

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🌈 RETURN VALUE 🌈

Returns the new value of the flags after the set/clear operation. so you always know what you're working with. slay. 💖

Returns 0 on error, and sets an error code retrievable via elf_errmsg(3).

If the first argument is NULL then 0 is return and no error code is set.

If the last flag is cleared, 0 will be returned as the new flag value. In this case 0 is not an error indicator and no error code is set.

😭 this is the classic "is 0 bad or is 0 just empty" situation. read carefully. zero can mean "everything's cleared, we're chilling" OR "something went wrong". context is everything, iykyk.

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🧚 SEE ALSO 🧚

elf_errmsg(3), elf_update(3), libelf(3), elf(5)

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🎀 ATTRIBUTES 🎀

• Interface: elf_flagelf(), elf_flagehdr(), elf_flagphdr(), elf_flagscn(), elf_flagshdr(), elf_flagdata()
• Attribute: Thread safety
• Value: MT-Unsafe race

so like, all six of them, same energy: MT-Unsafe race. be careful out here in multithreaded land. 🫧

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🍓 REPORTING BUGS 🍓

Report bugs to <elfutils-devel@sourceware.org> or https://sourceware.org/bugzilla/.

found a bug? tell the girlies at sourceware. they'll handle it. 🫶

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and that's the whole page, bestie — brought to you by Libelf, dated 2025-06-23, living in section 3 of your friendly neighborhood manual. go forth and flag responsibly. periodt. 💅✨