COutput with printf()

Output with printf()

printf(), declared in <stdio.h>, is the workhorse function for producing formatted output in C. It takes a format string describing what to print, followed by zero or more additional arguments whose values get substituted into placeholders (format specifiers) within that string.

C
#include <stdio.h>

int main(void) {
    int age = 25;
    double price = 19.99;
    char grade = 'A';

    printf("Age: %d, Price: %.2f, Grade: %c\n", age, price, grade);
    return 0;
}
Format Specifiers
Each %-prefixed placeholder in the format string (like %d, %f, %s, %c) must match the type of the corresponding argument. A full reference of the available specifiers lives on the Format Specifiers page — this page focuses on how printf itself works.
printf Returns a Value
The return value is rarely used, but it exists
printf() returns an int: the number of characters successfully printed, or a negative value if an output error occurred. Most beginner code ignores this return value, but it is occasionally useful for error checking or for computing how much text was written.

C
int charsPrinted = printf("Hello, World!\n");
printf("That printed %d characters\n", charsPrinted); // 14 (includes the newline)
Format-String Mismatches Are Undefined Behavior
The types must match — the compiler usually cannot fully check this
printf is a variadic function: it has no way of knowing at runtime how many arguments were actually passed or what their real types are. It trusts the format string completely. If you write %d but pass a double, or you provide fewer arguments than the format string expects, the behavior is undefined — it may print garbage, crash, or appear to work by coincidence. Modern GCC/Clang can catch many of these mistakes at compile time with -Wall -Wformat, but the language itself does not enforce it.

C
double price = 9.99;
printf("%d\n", price); // UNDEFINED BEHAVIOR: %d expects an int, not a double

/* Correct */
printf("%f\n", price);
Printing a Literal Percent Sign
Since % introduces a format specifier, printing a literal percent character requires doubling it: %%.

C
printf("Discount: 20%%\n"); // prints: Discount: 20%

  • printf(format, ...) substitutes arguments into %-prefixed placeholders in the format string

  • It returns the number of characters printed (or negative on error) — rarely checked, but available

  • Format specifiers must match argument types exactly; mismatches are undefined behavior

  • Compile with -Wall -Wformat so the compiler flags obvious specifier/argument mismatches

  • Use %% to print a literal % character