C Format Specifiers Tutorial

The C programming language has some nice options when it comes to formatting output on the terminal and I’ve noticed that most tutorials covering format specifiers don’t show some of the nicer things you can do with them. This tutorial will show a list of format specifiers and will show examples for each one with what it outputs to the terminal.

List of format specifiers

From here I’ll show some example code and the output from that code at just underneath it. We’ll start with character and signed integer:

Character and signed integer

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char c = 'c';
int  i = 3;

printf("%c\n", c);
printf("%d\n", i);

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c
3

Float

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float f = 123.4567890f;

printf("%e\n", f);
printf("%E\n", f);
printf("%f\n", f);
printf("%g\n", f);
printf("%G\n", f);

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1.234568e+02
1.234568E+02
123.456787
123.457
123.457

Signed and unsigned short

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short s = -6;
unsigned short us = 6;

printf("%hi\n", s);
printf("%hu\n", us);

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-6
6

Unsigned integer

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unsigned int i = 3;

printf("%i\n", i);

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3

Long

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long l = 1234567;

printf("%l\n",  l);
printf("%ld\n", l);
printf("%li\n", l);

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%
1234567
1234567

Double

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double d = 987.654321;

printf("%lf\n", d);

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987.654321

Long double

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long double ld = 987.654321;

printf("%Lf\n", ld);

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987.654321

Long long and unsigned long long

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long long ll = -123456789;
unsigned long long ull= 123456789;

printf("%lli\n", ll);
printf("%lld\n", ll);

printf("%llu\n", ull);

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-123456789
-123456789
123456789

Octal representation

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printf("%o\n", 69);

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105

Pointer

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int i = 3;
int *ptr = &i;


printf("%p\n", ptr);

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0x7ffd24a1cdcc

String

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char *s = "Hello, world!";

printf("%s\n", s);

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Hello, world!

Unsigned integer

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unsigned int ui = 3;

printf("%u\n", ui);

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3

Hexadecimal representation

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int i = -3;

printf("%x\n", i);
printf("%X\n", i);

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fffffffd
FFFFFFFD

Other

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int i = 3;
int *ptr = &i;

printf("%n\n", ptr);
printf("%%\n");

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%

Formatting tricks

There are a few other flags you can add to a format specifier to format output to specific needs. These are ‘*’, ‘.’, and ‘-‘. You can also supply a number just after the % character to add spacing to the formatted output.

Left alignment

By supplying a numerical value after the % character you will align values from the left. All this does is add spacing in front of the formatted values so the line up. This only works if the supplied values are equal to or less than the supplied numerical value. Below is an example with three different strings all set to be left aligned.

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char *str_one       = "Hello, World!";
char *str_two       = "Hello there, World!";
char *str_three     = "Why hello again, World!";

printf("%20s ==\n", str_one);
printf("%20s ==\n", str_two);
printf("%20s ==\n", str_three);

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       Hello, World! ==
 Hello there, World! ==
Why hello again, World! ==

As you can see, the first two strings are aligned properly. The two equals signs have been added to the output to show how the alignment. The third string however is not correctly aligned because the length of the string is greater than 20 characters.

Right alignment

When you supply the minus (-) sign just after the % character this will align values to the right. It works exactly like the left alignment where it will put spaces after the formatted values.

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char *str_one       = "Hello, World!";
char *str_two       = "Hello there, World!";
char *str_three     = "Why hello again, World!";


printf("%-20s ==\n", str_one);
printf("%-20s ==\n", str_two);
printf("%-20s ==\n", str_three);

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Hello, World!        ==
Hello there, World!  ==
Why hello again, World! ==

Similarly to the left alignment, the first and second strings have been aligned correctly but the third string has not. Again this is due to the length of the third string being greater than the supplied length for the format specifier.

Variable length formatting

Instead of supplying a number after the % character, you can actually use values of variables. You can do this by supplying an asterisk (*) after the % character.

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char *str = "Hello, World!";

int len_one = 20;
int len_two = 15;

printf("%*s ==\n", len_one, str);
printf("%*s ==\n", len_two, str);

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     Hello, World! ==
Hello, World! ==

When using the asterisk, the first argument you’ll have to pass after the format string will have to be the variable length. As you can see from the output the left alignment for each printf call as been aligned differently. You can also combine the asterisk with the right alignment minus as well. It would look more like this:

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char *str = "Hello, World!";

printf("%-*s ==\n", 20, str);
printf("%-*s ==\n", 15, str);

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Hello, World!        ==
Hello, World!   ==

Truncate length

You also have the option to truncate or shorten the formatted value. By using the period (.) after the % character and then supplying a number the formatting will remove any trailing characters. This also works with the asterisk variable formatting.

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char *str_one       = "Hello, World!";
char *str_two       = "Hello there, World!";
char *str_three     = "Why hello again, World!";

printf("%.10s ==\n", str_one);
printf("%.10s ==\n", str_two);
printf("%.10s ==\n", str_three);

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Hello, Wor ==
Hello ther ==
Why hello  ==

In this example output as all of the supplied strings are longer than the length of the truncation length so none of them are printed in full. Here’s an example of the variable length with the truncation:

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char *str_one       = "Hello, World!";
char *str_two       = "Hello there, World!";
char *str_three     = "Why hello again, World!";

int len = 12;

printf("%.*s ==\n", len, str_one);
printf("%.*s ==\n", len, str_two);
printf("%.*s ==\n", len, str_three);

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Hello, World ==
Hello there, ==
Why hello ag ==