CODEWITHSUNDEEP
cglobal-variables
Mohammad
Mohammad

Reputation: 63

Duplicate global variable

I need to use a global variable in multiple translation units, so I put

int g_exit_status;

in a header file and included that header in my source files. The default value for global variables is zero, but I need to use a specific value. I tried to specifying a value (e.g. 90), but then I received this error while compiling:

duplicate symbol '_g_exit_status' in:
    /var/folders/zz/zyxvpxvq6csfxvn_n0000kch0004l4/T/main-422e60.o
    /var/folders/zz/zyxvpxvq6csfxvn_n0000kch0004l4/T/parsing-328bc4.o
ld: 1 duplicate symbol for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [minishell] Error 1

Upvotes: 0

Views: 302

Answers (1)

Andreas Wenzel
Andreas Wenzel

Reputation: 24726

The line

int g_exit_status;

is a tentative definition, which will act as a definition unless that variable is defined somewhere else in the same translation unit (i.e. in the same .c file). In the latter case, it will instead act as a declaration.

The line

int g_exit_status = 90;

is a definition.

The line

extern int g_exit_status;

is a declaration.

A definition is also a declaration, but not vice-versa.

You must declare the variable g_exit_status in all translation units in which it is used. However, according to the one-definition rule, you must define it in exactly one translation unit. You appear to be violating this rule by putting a definition inside a header file that is included by multiple translation units.

For the reasons stated above, the declaration

extern int g_exit_status;

should be inside the header file, but the definition

int g_exit_status = 90;

should only be inside a single .c file, in order to not violate the one-definition rule.

Upvotes: 2

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