GELF_UPDATE_SYMSHNDX(3)

💖 GELF_UPDATE_SYMSHNDX 💖

okay bestie, so this one lives in the Libelf Programmer's Manual, and it's GELF_UPDATE_SYMSHNDX(3) — say that three times fast, I dare you 🫶

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✨ NAME ✨

gelf_update_symshndx — update class-independent symbol information and extended section index in the symbol table 💅

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🌸 SYNOPSIS 🌸

#include <gelf.h>

int gelf_update_symshndx (Elf_Data *symdata, Elf_Data *shndxdata, int ndx, GElf_Sym *sym, Elf32_Word xshndx);

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🦋 DESCRIPTION 🦋

So what this cutie does: it updates symdata and shndxdata, at index ndx, with the class-independent symbol info from sym plus the section index xshndx. Two-for-one deal, we love to see it 💖

Real talk on the requirements though — symdata needs to be an Elf_Data pointer for an SHT_SYMTAB or SHT_DYNSYM section, and shndxdata needs to be an Elf_Data pointer for an SHT_SYMTAB_SHNDX section. No mixing and matching, okay? 👑

Wanna know how many symbols are in your symbol table? Divide the size of symdata by the size of a symbol table entry, easy peasy 🍓 And where do you get those numbers?

• The size of symdata comes from symdata->d_size, or from the corresponding section header's sh_size.
• The size of a symbol table entry comes from gelf_fsize() with ELF_T_SYM, or from the corresponding section header's sh_entsize.

Now here's the lore, iykyk: an SHT_SYMTAB_SHNDX section shows up in ELF binaries when there are too many sections to fit in an unsigned 16-bit value 😭 When that happens, the st_shndx field of a GElf_Sym gets set to SHN_XINDEX to flag that the symbol's section index is actually chillin' over in the SHT_SYMTAB_SHNDX section. You can grab shndxdata for that section using elf_scnshndx(3), elf_getscn(3), and elf_getdata(3) — the holy trinity ✨

Important, no cap: the number of entries in shndxdata must be equal to the number of entries in symdata. And entry ndx in shndxdata corresponds to entry ndx in symdata. They're matching, periodt 💅

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🎀 PARAMETERS 🎀

• symdata — Pointer to Elf_Data for a section with type SHT_SYMTAB or SHT_DYNSYM. 🌈
• shndxdata — Pointer to Elf_Data for a section with type SHT_SYMTAB_SHNDX. shndxdata may be NULL. And if shndxdata is NULL, then xshndx must be zero. Rules are rules, bestie 🙈
• ndx — Zero-based index of the symbol table entry and the extended section index entry that's getting updated. Zero-based, so the first one is 0, not 1 — don't come for me 💀
• sym — Pointer to a caller-provided symbol table entry. For ELFCLASS32 binaries, st_value and st_size should be able to fit within unsigned 32-bit values. Also, sym must not be NULL. Like, ever. 🚫
• xshndx — The updated extended section index entry to be associated with sym. Must be zero if shndxdata is NULL. And if sym->st_shndx is not SHN_XINDEX, then xshndx should be SHN_UNDEF. Keep that straight, k? 💜

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👑 RETURN VALUE 👑

On success? You get a non-zero value back, and the entries at index ndx in symdata and shndxdata (if it's not NULL) get updated with sym and xshndx, respectively 🎉 The sections associated with symdata and shndxdata also get flagged with ELF_F_DIRTY.

On failure? Zero comes back, and elf_errno is set. Sad girl hours 😭

Weird little plot twist: if sym is NULL, then NULL is returned without setting elf_errno. Not a typo, that's just how it is 💀

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🌟 SEE ALSO 🌟

elf_getdata(3), elf_getscn(3), elf_scnshndx(3), gelf_fsize(3), gelf_getsymshndx(3), gelf_update_sym(3), libelf(3), elf(5)

Go read those too, they're all part of the same friend group 🫶

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💅 ATTRIBUTES 💅

• Interface: gelf_update_symshndx()
• Attribute: Thread safety
• Value: MT-Safe

Yup, she's MT-Safe — thread safe and unbothered, main character energy fr fr ✨

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📢 REPORTING BUGS 📢

Found a bug? Report it to <elfutils-devel@sourceware.org> or hit up https://sourceware.org/bugzilla/ — they wanna hear about it, promise 🐛

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okay that's the whole page, you're literally a libelf girlie now, I'm so proud 😭💖

this was Libelf, dated 2025-12-30, and yes it's still GELF_UPDATE_SYMSHNDX(3) — slay, bye bestie! 👑🌸