Platform independent size_t Format specifiers in c?

94
votes

I want to print out a variable of type size_t in C but it appears that size_t is aliased to different variable types on different architectures. For example, on one machine (64-bit) the following code does not throw any warnings:

size_t size = 1;
printf("the size is %ld", size);

but on my other machine (32-bit) the above code produces the following warning message:

warning: format '%ld' expects type 'long int *', but argument 3 has type 'size_t *'

I suspect this is due to the difference in pointer size, so that on my 64-bit machine size_t is aliased to a long int ("%ld"), whereas on my 32-bit machine size_t is aliased to another type.

Is there a format specifier specifically for size_t?

3
cplatform-independentsize-tformat-stringformat-specifiers
Your warning message does not match the code. The warning mentions pointers, your code doesn't have any. Did you remove some & somewhere? – Jens
Pointers? No I don't get any warnings about pointers, in fact depending on what machine I run that code on sometimes I get no warnings at all. Try the following test code: #include <stdio.h> int main(){ size_t size = 1; printf("the size is %ld", size); return 0; } – Ethan Heilman
See also Cross platform format string for variables of type size_t?. – legoscia
@EthanHeilman He's referring to the fact that your warnings say warning: format '%ld' expects type 'long int *', but argument 3 has type 'size_t *' when it probably should be saying warning: format '%ld' expects type 'long int', but argument 3 has type 'size_t'. Were you maybe using scanf() instead when you got these warnings? – RastaJedi
Possible duplicate of What's the correct way to use printf to print a size_t? – phuclv

3 Answers

131
votes

Yes: use the z length modifier:

size_t size = sizeof(char);
printf("the size is %zu\n", size);  // decimal size_t ("u" for unsigned)
printf("the size is %zx\n", size);  // hex size_t

The other length modifiers that are available are hh (for char), h (for short), l (for long), ll (for long long), j (for intmax_t), t (for ptrdiff_t), and L (for long double). See §7.19.6.1 (7) of the C99 standard.

46
votes

Yes, there is. It is %zu (as specified in ANSI C99).

size_t size = 1;
printf("the size is %zu", size);

Note that size_t is unsigned, thus %ld is double wrong: wrong length modifier and wrong format conversion specifier. In case you wonder, %zd is for ssize_t (which is signed).

1
votes

MSDN, says that Visual Studio supports the "I" prefix for code portable on 32 and 64 bit platforms. 

size_t size = 10;
printf("size is %Iu", size);