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Common mistakes in creating format strings include

• using invalid conversion specifiersspecifications
• using a length modifier on an incorrect specifierspecification
• mismatching the argument and conversion specifier typespecification type
• using an argument of type other than int for width, precision, or the n conversion specification
• providing insufficient arguments for all the conversion specifications in the format string
• using invalid character classes

The following are C99-compliant conversion specifiersspecifications \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. Using any other specifierspecification may result in [undefined behavior|BB. Definitions#undefined behavior].

d, i, o, u, x, X, f, F, e, E, g, G, a, A, c, s, p, n, %

In addition, ISO/IEC 9945-2003 permits the following conversion specifications as synonyms for lc and ls:

C, S

Only some of the conversion specifiers specifications can correctly take a length modifier. Using a length modifier on any specifier specification other than the following may result in undefined behavior.

d, i, o, u, x, X, a, A, e, E, f, F, g, G, c, s

Character class ranges must also be properly specified with a hyphen in between two printable characters. The two following lines are both properly specified. The first accepts any character from a to z, inclusive, while the second accepts anything that is not a to z, inclusive.

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Mismatches between arguments and conversion specifiers specifications may result in undefined behavior. Many compilers can diagnose type mismatches in formatted output function invocations.

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This compliant solution ensures that the format arguments match their respective format specifiersspecifications.

const char *error_msg = "Resource not available to user.";
int error_type = 3;
/* ... */
printf("Error (type %d): %s\n", error_type, error_msg);

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