Analysis and Solutions for 'assignment to expression with array type error' in C Struct Field Assignment

Problem Phenomenon and Error Analysis

In C programming practice, beginners often encounter the following error scenario: when attempting to directly assign values to character array fields within structures, the compiler reports <span style="font-family: monospace;">"error: assignment to expression with array type error"</span>. This error stems from misunderstandings about C language data types and assignment mechanisms.

C Language Standard Specification Analysis

According to section 6.5.16 of the C11 standard: <span style="font-family: monospace;">"assignment operator shall have a modifiable lvalue as its left operand"</span>. This means the left side of an assignment operator must be a modifiable lvalue.

Further referencing section 6.3.2.1 regarding the definition of modifiable lvalues: <span style="font-family: monospace;">"A modifiable lvalue is an lvalue that does not have array type"</span>. This specification clearly states that array types do not fall within the category of modifiable lvalues and therefore cannot appear directly on the left side of assignment operators.

Error Code Example and Correction

The following code demonstrates typical incorrect usage:

#include <stdio.h>
#include <string.h>

#define N 30

typedef struct {
    char name[N];
    char surname[N];
    int age;
} data;

int main() {
    data s1;
    // Error: array type cannot be directly assigned
    // s1.name = "Paolo";
    // s1.surname = "Rossi";
    
    // Correct: using strcpy function to copy strings
    strcpy(s1.name, "Paolo");
    strcpy(s1.surname, "Rossi");
    s1.age = 19;
    
    getchar();
    return 0;
}

Proper Use of Initialization Lists

Another correct approach is using initialization lists:

data s1 = {"Paolo", "Rossi", 19};

This approach works because it follows the initialization rules specified in section 6.7.9 of the C11 standard: <span style="font-family: monospace;">"Each brace-enclosed initializer list has an associated current object. When no designations are present, subobjects of the current object are initialized in order according to the type of the current object: array elements in increasing subscript order, structure members in declaration order"</span>. The initialization process does not involve assignment operators but directly assigns initial values to objects.

Memory Layout and Data Type Understanding

From a memory perspective, the structure <span style="font-family: monospace;">data</span> allocates contiguous memory space on the stack:

[name array(30 bytes), surname array(30 bytes), age(4 bytes)]

When executing <span style="font-family: monospace;">s1.name = "Paolo";</span>, it actually attempts to assign the address of a string literal to the array name, which is not permitted in C language. Array names degenerate to pointers to the first element of the array in most contexts but cannot serve as assignment targets.

Alternative Approach: Pointer Members

If direct assignment capability is needed, consider using pointer members:

typedef struct {
    char *name;
    char *surname;
    int age;
} data_ptr;

int main() {
    data_ptr s2;
    s2.name = "Paolo";    // Correct: assigning pointers
    s2.surname = "Rossi"; // Correct: assigning pointers
    s2.age = 19;
    return 0;
}

The cost of this design is the need for additional management of string memory lifecycle, as pointers may point to read-only string literals.

Best Practice Recommendations

In practical development, the following approaches are recommended:

1. For fixed-size string storage, use character arrays with <span style="font-family: monospace;">strcpy</span> or <span style="font-family: monospace;">strncpy</span>
2. Use initialization lists whenever possible during definition
3. For dynamic strings, use pointer members and properly manage memory
4. Always check string operation lengths to avoid buffer overflows

Conclusion

The special properties of array types in C language determine that they cannot serve as targets of assignment operations. Understanding this characteristic requires deep knowledge of C's value types, lvalue concepts, and memory management mechanisms. By correctly using string copy functions or initialization lists, developers can avoid such compilation errors and write more robust and standard-compliant C code.