sizeof(text) not equal to : fp_len + 1

Question

if

fp_len = ftell(fp);

prints 470

shouldn't this be printing "471" ? ( it prints 8. probably first line only )

char *text = malloc(sizeof(*text) * fp_len + 1);
int text_len;
text_len = sizeof(text);

printf("text-len: %d --- ",text_len);

full:

FILE *fp;
fp = fopen(path, "r");

fseek(fp, 0, SEEK_END);

int fp_len;
fp_len = ftell(fp); 
printf("%d---", fp_len);

fseek(fp, 0, SEEK_SET);

char *text = malloc(sizeof(*text) * fp_len + 1);

int text_len;
text_len = sizeof(text);
printf("text-len: %d --- ",text_len);

fread(text, fp_len, 1, fp);

printf("%s",text);

free(text);

Must be a duplicate, can't find one now. You take the size of a **pointer**. — , Jul 07 '17 at 09:31
`text_len = sizeof(text);` is the size of a `char` pointer, which on your system is 8, which is plausible. — Federico klez Culloca, Jul 07 '17 at 09:31
It shouldn't print 471. `text` is a pointer which uses 4 or 8 bytes on most machines. You cannot use `sizeof` to query the amount of allocated memory from `malloc` — Gerhardh, Jul 07 '17 at 09:31
@Vanada: seems that your previous question got solved. How about accepting an answer? — Jean-François Fabre, Jul 07 '17 at 09:33

score 1 · Answer 1 · answered Jul 07 '17 at 09:32

1

sizeof gives the size of the type, not a string length. In case of char *text, the type is a pointer value, and the size of a pointer value on a 64 bit machine is probably 8.

answered Jul 07 '17 at 09:32

Stephan Lechner

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score 0 · Accepted Answer · answered Jul 07 '17 at 09:37

The sizeof is an operator in C and not a function. It acts on the type of the expression inside the brackets.

It tells the size of the type. In general cases when you do sizeof of say a char array like,

char array[] = "Hello";
sizeof(array);

you get 6, because the type of array is char[6].

Now in this case you are doing sizeof on text.

text is defined as char* which is a pointer. Pointers on your system are 8 bytes. So it is giving out 8.

Trying to use sizeof which dynamically allocated buffers is not going to work. Instead you can use

sizeof(*text) * fp_len + 1

Because that is what you passed to malloc. That is ACTUALLY the size of the buffer pointed to by text.

sizeof(text) not equal to : fp_len + 1

2 Answers2