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In this noncompliant code example, the function {{clear()}} zeros the elements in an array. The function has one parameter declared as {{int array\[\]}} and is passed a static array consisting of 12 {{int}} as the argument. The function {{clear()}} uses the idiom {{sizeof(array) / sizeof(array\[0\])}} to determine the number of elements in the array. However, {{array}} has a pointer type because it is a parameter. As a result, {{sizeof(array)}} is equal to the {{sizeof(int \*)}}. For example, on an architecture (such as IA-32) where the {{sizeof(int) == 4}} and the {{sizeof(int *) == 4}}, the expression {{sizeof(array) / sizeof(array\[0\])}} evaluates to 1, regardless of the length of the array passed, leaving the rest of the array unaffected. |
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void clear(int array[]) {
for (size_t i = 0; i < sizeof(array) / sizeof(array[0]); ++i) {
array[i] = 0;
}
}
void dowork(void) {
int dis[12];
clear(dis);
/* ... */
}
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In this compliant solution, the size of the array is determined inside the block in which it is declared and passed as an argument to the function.
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void clear(int array[], size_t len) {
for (size_t i = 0; i < len; i++) {
array[i] = 0;
}
}
void dowork(void) {
int dis[12];
clear(dis, sizeof(dis) / sizeof(dis[0]));
/* ... */
}
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In this noncompliant code example, the sizeof a does not equal 100 * sizeof(int). This is because the sizeof operator, when applied to a parameter declared to have array or function type, yields the size of the adjusted (pointer) type, even if the parameter declaration specifies a length.
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enum {ARR_LEN = 100};
void clear(int a[ARR_LEN]) {
memset(a, 0, sizeof(a)); /* error */
}
int main(void) {
int b[ARR_LEN];
clear(b);
assert(b[ARR_LEN / 2]==0); /* may fail */
return 0;
}
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In this compliant solution, the size is specified using the expression len * sizeof(int).
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enum {ARR_LEN = 100};
void clear(int a[], size_t len) {
memset(a, 0, len * sizeof(int));
}
int main(void) {
int b[ARR_LEN];
clear(b, ARR_LEN);
assert(b[ARR_LEN / 2]==0); /* cannot fail */
return 0;
}
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