CODEWITHSUNDEEP
c++variables
jg c
jg c

Reputation: 27

Where are variable names stored in a C++ program？

For example,I define a variable

int a=5;

I know the 5 is stored in the stack, but where is a stored? Please help

Upvotes: 2

Views: 838

Answers (3)

OrenIshShalom
OrenIshShalom

Reputation: 7132

Here is a complete minimal example that demonstrates @eerorika's answer about the debug information (here, in ELF files):

$ cat main.cpp
int main(int argc, char **argv)
{
    int whereIsMyVar = 5;
    return whereIsMyVar;
}

And compiling it without optimizations (-O0), and with debug info (-g):

$ g++ -g -O0 main.cpp -o main

Now let's see what went on with objdump:

$ objdump -D -S main > main.asm
$ sed -n "378,389p" main.asm
00000000000005fa <main>:
int main(int argc, char **argv)
{
 5fa:   55                      push   %rbp
 5fb:   48 89 e5                mov    %rsp,%rbp
 5fe:   89 7d ec                mov    %edi,-0x14(%rbp)
 601:   48 89 75 e0             mov    %rsi,-0x20(%rbp)
    int whereIsMyVar = 5;
 605:   c7 45 fc 05 00 00 00    movl   $0x5,-0x4(%rbp) <--- there is your assignment !
    return whereIsMyVar;
 60c:   8b 45 fc                mov    -0x4(%rbp),%eax
}

Now let's use the addr2line utility to easily extract the source code location corresponding to this address:

$ addr2line -e main 0x605
/home/oren/main.cpp:3

And finally, let's see what's on line 3 of our source file [[ drumroll ]]:

$ sed -n "3,3p" /home/oren/main.cpp
    int whereIsMyVar = 5;

So as you can see, the name whereIsMyVar is connected via file name + line to its original source file location.

Upvotes: 3

Ajay Rajan
Ajay Rajan

Reputation: 31

The variables names are non-existent after the compilation process(Exceptions: global variables in shared libs). The whole idea is to convert your source code to machine instructions.

int a = 5;

Assuming this is your global variable, it will be assigned a address in memory let's say 0xFFFFFFFF, by the compiler and linker & henceforth, all actions done pertaining to this variable (read/write/modify), will be done via accessing that address.

Upvotes: 0

eerorika
eerorika

Reputation: 238361

Where are variable names stored in a C++ program？

The variable names are stored in the source code.

The details of the produced program are outside the scope of the language, but typically, the variable names are stored - if they are stored at all - in "debug information".

Upvotes: 3

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