Initializing entire array with memset

c++ arrays memset
ishan bansal picture ishan bansal · Sep 21, 2015 · Viewed 7.9k times · Source

I have initialised the entire array with value 1 but the output is showing some garbage value. But this program works correctly if i use 0 or -1 in place of 1. So are there some restrictions on what type of values can be initialised using memset. 

 int main(){
    int a[100];
    memset(a,1,sizeof(a));
    cout<<a[5]<<endl;
    return 0;
    }

Answer

Snaipe picture Snaipe · Sep 21, 2015

memset, as the other say, sets every byte of the array at the specified value.

The reason this works with 0 and -1 is because both use the same repeating pattern on arbitrary sizes:

(int) -1 is 0xffffffff
(char) -1 is 0xff

so filling a memory region with 0xff will effectively fill the array with -1.

However, if you're filling it with 1, you are setting every byte to 0x01; hence, it would be the same as setting every integer to the value 0x01010101, which is very unlikely what you want.

Related questions

Reset C int array to zero : the fastest way?

Assuming that we have a T myarray[100] with T = int, unsigned int, long long int or unsigned long long int, what is the fastest way to reset all its content to zero (not only for initialization but to reset the …

Read More
How to memset char array with null terminating character?

What is the correct and safest way to memset the whole character array with the null terminating character? I can list a few usages: ... char* buffer = new char [ARRAY_LENGTH]; //Option 1: memset( buffer, '\0', sizeof(buffer) ); //Option 2 before edit: …

Read More
How memset initializes an array of integers by -1?

The manpage says about memset: #include <string.h> void *memset(void *s, int c, size_t n) The memset() function fills the first n bytes of the memory area pointed to by s with the constant byte c. …

Read More