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UB15⚓︎

Author: PeiHong Dai

Definition⚓︎

  • A program requires the formation of a composite type from a variable length array type whose size is specified by an expression that is not evaluated (6.2.7).
  • 程序需要从变长数组类型形成复合类型，而该数组长度由未求值的表达式指定

Description⚓︎

  • A composite type can be constructed from two types that are compatible; it is a type that is compatible with both of the two types and satisfies conditions in 6.2.7. Specifically, the undefined situation is clarified below:
  • If both types are array types, the following rules are applied:
    • If one type is an array of known constant size, the composite type is an array of that size.
    • Otherwise, if one type is a variable length array whose size is specified by an expression that is not evaluated, the behavior is undefined.

Code⚓︎

#include  <stdio.h>
int m;
int main(void)
{
    char arr[m];
    m = 3;
    printf("%d\n", sizeof(arr));
    return 0;
}

Configurations⚓︎

OS: Microsoft Windows 11 22H2

gcc -v : gcc version 7.4.0, x86_64-w64-mingw32

compile and run commands: gcc -o UB15.exe UB15.c && ./UB15.exe

OS: Microsoft Windows 11 22H2

clang -v : clang version 6.0.0, x86_64-w64-mingw32

compile and run commands: clang -o UB15.exe UB15.c && ./UB15.exe

Behaviors⚓︎

compilation successful
output:
0

compilation successful
output:
0

Advice⚓︎

  • VLAs in C99 and forth are dependent on other variables, if user interactions are involved during the process, the compiler cannot determine the size during compilation, which could cause UBs.

  • c int n; scanf("%d",&n); int str[n];

  • For better stability and robustness, just try the old-school approach: malloc

  • c int n; scanf("%d", &n); int* str = malloc(n * sizeof(int)); ... free(str);

  • By the way, the sizeof evaluation of VLA is problematic since its implementation is during compilation, our teacher is working on this matter now.