To achieve high-precision floating-point output in C, especially beyond standard double precision, it's essential to understand format specifiers, data types, and compiler-specific behaviors. The following examples demonstrate how to correctly print extended-precision values using long double and appropriate format strings.

Environment Considerations

Certain environments like Cygwin64 with GCC support higher precision for long double (typically 80-bit extended precision), whereas MinGW64 may not provide the same level of accuracy due to differences in runtime library implementations.

Basic Example: Printing High-Precision Values

#include <stdio.h>
#include <math.h>

int main(void) {
    // Print cosine with 19 decimal places
    printf("%.19f\n", cos(3.1415926 / 3));

    // Literal interpreted as double (limited precision)
    printf("%.49f\n", 1.3456789123456789123456789123456789123456789123456789);

    // Explicit long double literal
    long double val = 1.3456789123456789123456789123456789123456789123456789L;
    printf("%.49Lf\n", val);

    return 0;
}

Sample Output:

0.5000000154700405819
1.3456789123456789347699213976738974452018740000000
1.3456789123456789123269364272239556612476010000000

Note that regular double literals cannot retain more than ~17 significant decimal digits. Only long double literals (with L suffix) and proper %Lf formatting can expose extended precision.

Format Specifier Behavior

Mixing format specifiers and types leads to undefined behavior or incorrect output:

• %f, %lf: For double (they are equivalent in printf)
• %Lf: Required for long double
• %llf: Invalid — not a standard specifier and often treated as %lf or causes garbage output

Common Pitfalls and Corrections

The following corrected example shows proper usage:

#include <stdio.h>

int main(void) {
    long double ld = 1.23L;
    double d = 1.3456789123;

    printf("ld via %%Lf: %.15Lf\n", ld);
    printf("d via %%f: %.15f\n", d);

    // Correctly cast and print long double
    long double ld2 = 1.3456789123L;
    printf("ld2 via %%Lf: %.15Lf\n", ld2);

    // Size verification
    printf("Sizes: long double=%zu, double=%zu\n", 
           sizeof(long double), sizeof(double));

    return 0;
}

Key Observations:

• Always include <stdio.h> to avoid implicit function declaration warnings.
• Use L suffix for long double literals.
• Only %Lf reliably prints long double values with full precision.
• On Cygwin64/gcc, long double is typically 16 bytes (80-bit extended precision), offering ~19–21 decimal digits of precision.