Thread: sizeof operator

Originally Posted by nzagajunior

hello!
Am a beginner on programming in c, i would like to know the uses of sizeof operator

Well, it tells you how many bytes an object contains (with no assumptions of the number of bit in a byte, mind you), so for instance making a "shallow" copy of an object (ie: bitwise; no accounting for the values referred to by pointers of a data structure), or generic bit-twiddling algorithms for integers of arbitrary size, etc.

The most common use is as an argument to malloc

Code:

int main(void)
{
    int *x = malloc(sizeof *x);
    *x = 1234;
    printf("x is %d\n", *x);
    return 0;
}

Originally Posted by c99tutorial

The most common use is as an argument to malloc

Code:

int main(void)
{
    int *x = malloc(sizeof *x);
    *x = 1234;
    printf("x is %d\n", *x);
    return 0;
}

I know that's just a trivial demonstrative snippet, but for the sake of good programming habits examples of malloc should always be followed up with an appropriate number of free calls.

I'll go ahead and amend that for the OP:

Code:

int main(void)

{

    int *x = malloc(sizeof *x);
    *x = 1234;
    printf("x is %d\n", *x);
    free(x);
    return 0;

}

True. Actually to be correct you must also handle failed memory allocations as well

Code:

int main(void)
{
    // allocate x
    int *x;
    if ((x = malloc(sizeof *x)) == NULL) {
        fprintf(stderr, "Failed to allocate memory");
        return 1;
    }

    // do something with x
    *x = 1234;
    printf("x is %d\n", *x);

    // clean up
    free(x);
    return 0;
}

Originally Posted by c99tutorial

True. Actually to be correct you must also handle failed memory allocations as well

Code:

int main(void)
{
    // allocate x
    int *x;
    if ((x = malloc(sizeof *x)) == NULL) {
        fprintf(stderr, "Failed to allocate memory");
        return 1;
    }

    // do something with x
    *x = 1234;
    printf("x is %d\n", *x);

    // clean up
    free(x);
    return 0;
}

Good point.

One other common use is to get the number of elements in an array. This can be used like so:

Code:

int x[10];
size_t i;

for (i = 0; i < sizeof x / sizeof x[0]; i++)
          x[i] = 1;