.globl

    .globl 

For the sake of sdcc:
Declare the named Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel to be global.

The Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel is either defined in the current Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal scope after this declaration or it is referenced there.

Undefined global labels can be picked up later by the '#include: #include library
#include: #include library#include library' assembler directive.

Define a global label

Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#local
        .globl foo
        ...
foo:    nop
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal

The same effect can be achieved by putting two colons after the label definition:

Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#local
        ...
foo::   nop
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal

Refer to a global label

Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#local
        .globl foo
        ...
        jp  foo
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal

Unless you have a nested Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal context, this is identically to:

Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#local
        ...
        jp  foo
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal

In almost all cases the .globl declaration is superfluous. zasm automatically adds the Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel to the list of undefined Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabels and pushes it at each Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal to the outer context until it finds a context where it is defined – finally the global context where it can be picked up by the '#include: #include library
#include: #include library#include library' assembler directive Assembler directives: #if, #elif, #else, #endif
Pseudo instructions: if, endifif it is still undefined then.

Actually all references to symbols in system libraries as included by the c compiler are handled this way, without the need to define them all ahead.

.globl is useful in nested Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal contexts where the same name might be defined in an outer but not the global context.

.globl may also be useful Assembler directives: #if, #elif, #else, #endif
Pseudo instructions: if, endifif you want to redefine a redefinable Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel. Inside a Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal context it is impossible to redefine a Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel, because this would define a new Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocallocal Pseudo instructions: Label definition
Numeric expressions: Labels
8080 Assembler: Labelslabel instead, except Assembler directives: #if, #elif, #else, #endif
Pseudo instructions: if, endifif you define it .globl.

foo     defl    0
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#local
        ...
        .globl  foo
foo     defl    1
        ...
Assembler directives: #local, #endlocal, .local and .endlocal
Pseudo instructions: #local, #endlocal, .local and .endlocal
Including C Source Files: #local, #endlocal, .local and .endlocal#endlocal