Do Identical String Literals Always Have the Same Address Across Translation Units in C?

String Literal Address Consistency Across Translation Units

In C programming, string literals are constant arrays of characters stored in the program's read-only memory. It is often assumed that identical string literals referencing the same text will have the same memory address. However, relying on this assumption across translation units (separate source files compiled into object files) is not guaranteed.

Standard Specifications

The C99 and C draft standards explicitly state that the distinctness of string literals is implementation-defined. This means that compilers can choose to store identical string literals in separate objects or pool them to save memory.

Compiler Behavior

Some compilers, such as GCC and Visual Studio, support string literal pooling across compilation units. However, this behavior may vary depending on optimization settings and compiler options. For example, GCC's -fmerge-constants flag enables literal merging, while Visual Studio's /GF option allows string pooling.

Reliability Within Translation Units

Within a single translation unit (a single source file compiled together), it is more reliable to assume that identical string literals will have the same address. However, even this can vary depending on the compiler and optimization level.

Reasons for Non-Portability

The decision not to mandate string literal uniqueness across translation units arose from the diverse implementation practices of the time. Some implementations stored string literals in ROM or constant data sections, making it impractical to guarantee uniqueness.

Practical Considerations

While relying on same-address assumptions can be convenient in some cases, it is safest to treat string literals as distinct objects. This ensures portability and avoids potential runtime issues.

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