Goto-cc Parsing Error With Attributes Before Contract Clauses
When working with formal verification tools like CBMC, developers often use contract clauses such as __CPROVER_requires to specify preconditions for functions. These clauses are crucial for ensuring the correctness of the code under verification. However, certain syntax patterns can lead to parsing errors, especially when combining GCC attributes with these contract clauses. This article delves into a specific issue where goto-cc, a compiler frontend used by CBMC, fails to parse function declarations that have GCC attributes placed before __CPROVER_* contract clauses. Understanding this issue and its workarounds is essential for developers aiming to write verifiable C code.
The Problem: GCC Attributes and Contract Clauses
The core problem arises when a function declaration includes GCC attributes (e.g., __attribute__((const))) before the __CPROVER_requires clause. The goto-cc parser encounters a syntax error in such cases, preventing the code from being correctly processed for verification. This issue is specific to GCC attributes; C23 attributes (e.g., [[gnu::const]]) are parsed without problems. This discrepancy highlights a parsing nuance within goto-cc that developers must be aware of. The following sections will elaborate on this issue, provide examples, and discuss potential solutions and workarounds.
Detailed Explanation of the Parsing Error
To fully grasp the issue, let's dissect the scenario that triggers the parsing error. Consider the following C code snippet:
int test_decl(int a) __attribute__((const)) __CPROVER_requires(a != 0);
In this declaration, the function test_decl is declared with an integer argument a. The __attribute__((const)) is a GCC attribute that signifies that the function is a pure function (i.e., its return value depends only on its input arguments and it has no side effects). The __CPROVER_requires(a != 0) is a contract clause that specifies a precondition: the function should only be called when a is not equal to 0.
When goto-cc attempts to parse this declaration, it encounters the __attribute__((const)) first, which it handles correctly. However, when it then encounters __CPROVER_requires(a != 0), it fails to recognize this construct in the context following the GCC attribute. This results in a syntax error, halting the compilation process. The error message typically indicates a syntax error before __CPROVER_requires, which can be perplexing if one is not aware of this specific parsing behavior. This issue is not a general problem with either GCC attributes or __CPROVER_requires clauses in isolation; rather, it is the specific ordering and combination that triggers the error.
Contrasting with C23 Attributes
A key observation is that C23 attributes do not exhibit the same problem. Consider the equivalent declaration using the C23 attribute syntax:
int test_decl(int a) [[gnu::const]] __CPROVER_requires(a != 0);
Here, [[gnu::const]] is the C23 equivalent of __attribute__((const)). When goto-cc parses this declaration, it correctly interprets both the attribute and the contract clause. This contrast underscores that the issue is specific to the GCC attribute syntax and how goto-cc handles it in relation to __CPROVER_* clauses. The successful parsing of C23 attributes suggests that the underlying parsing mechanism in goto-cc is capable of handling attributes in conjunction with contract clauses, but a specific rule or exception seems to be in play for GCC attributes.
Practical Implications of the Issue
This parsing issue has practical implications for developers using CBMC and related tools. If developers inadvertently place GCC attributes before __CPROVER_requires clauses, their code will fail to parse, preventing verification. This can lead to frustration and wasted time as developers debug what appears to be a syntax error. It is crucial for developers to be aware of this behavior and to adopt coding practices that avoid this problematic syntax. This might involve reordering attributes and clauses, or using C23 attributes instead of GCC attributes.
Example and Error Message
To illustrate the issue, let's revisit the initial example and the resulting error message. The following C code snippet demonstrates the problem:
int test_decl(int a) __attribute__((const)) __CPROVER_requires(a != 0);
When this code is processed with goto-cc, the following error message is produced:
$ goto-cc -o test.goto -c test.c
test.c:1:1: error: syntax error before '__CPROVER_requires'
int test_decl(int a) __attribute__((const)) __CPROVER_requires(a != 0);
PARSING ERROR
This error message clearly indicates that the parser stumbled upon an unexpected token (__CPROVER_requires) following the GCC attribute. The