What are Format Specifiers in C?

In C programming, format specifiers (also called conversion specifiers) are placeholder tokens used within formatted I/O functions like printf(), scanf(), sprintf(), and sscanf().

A format specifier begins with a % character followed by characters that define how data should be formatted or parsed.

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1. Common Format Specifiers Cheat Sheet

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2. Width, Precision, and Flag Modifiers

You can control field width, alignment, and decimal precision by placing modifiers between % and the conversion character:

• %05d: Pad integer with leading zeros to minimum 5 width (e.g. 00042).
• %-10s: Left-align string within a 10-character column.
• %.2f: Round floating-point value to exactly 2 decimal places.
• %+d: Force display of sign (+ or -).

c (ISO Standard)
Run in Compiler.cCopy
#include <stdio.h>

int main() {
    double price = 19.9542;
    int invoiceNum = 73;

    printf("Formatted Invoice: INV-%05d\n", invoiceNum);
    printf("Rounded Price: $%.2f\n", price);
    printf("Aligned Table: |%-10s|%8.2f|\n", "Apples", 4.5);

    return 0;
}

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3. Critical Scanf Traps & Best Practices

1. Always pass address (&) for primitive variables in scanf:

   c (ISO Standard)
   Run in Compiler.cCopy
   int age;
   scanf("%d", &age); /* Correct */

2. Prevent Buffer Overflows with %s: Specify field limits when reading strings to avoid stack smashing:

   c (ISO Standard)
   Run in Compiler.cCopy
   char username[32];
   scanf("%31s", username); /* Reads max 31 chars + null terminator */

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Related References & Guides

• Review fundamental types in the C Data Types Reference.
• Explore standard I/O library prototypes in C Standard Library Functions.
• Test formatting and parsing in the Online C Compiler.