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I'm currently doing a nm like program, to print the symbols of an ELF file with their addresses and their type.

The typical output is something like :

$ nm ./my_exec      
0000000000003d28 d _DYNAMIC
0000000000003f28 d _GLOBAL_OFFSET_TABLE_
0000000000002000 R _IO_stdin_used
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable

The first column is the symbol address, the second is the symbol type and the third, the symbol name.

The man indicates the meaning for each character, upper-case is a global symbol, lower-case is local symbol etc...

But I don't understand how it choose which character to use.

I'm using the Gelf library and I didn't see a field from where I could determine the character.

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nm chooses the character mostly based on the section in which the symbol appears. ELF is specified in the System V ABI; pages 4-22 and following describe the symbol table, which include two fields, st_info and st_shndx, and these determine the character shown by nm.

The symbol binding (ELF32_ST_BIND(st_info)) determines whether the symbol is local (lowercase in nm), global (uppercase), or weak (v, w). sh_shndx points to the section containing the symbol, and many of nm’s characters reflect this, either as an explicit section (b, d...), or as a value with special meaning (e.g. u for SHN_UNDEF, undefined symbols).

So you won’t find a single value which directly corresponds to the character shown by nm, but the above should give you enough information to interpret the values described in the ABI specification.

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