CONTENTS

LIBRARY

SYNOPSIS

DESCRIPTION

These functions write the output under the control of a format string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for output.

These functions return the number of characters printed (not including the trailing ‘\0’ used to end output to strings) or a negative value if an output error occurs, except for snprintf and vsnprintf, which return the number of characters that would have been printed if the size were unlimited (again, not including the final ‘\0’).

The asprintf and vasprintf functions set *ret to be a pointer to a buffer sufficiently large to hold the formatted string. This pointer should be passed to free(3) to release the allocated storage when it is no longer needed. If sufficient space cannot be allocated, asprintf and vasprintf will return -1 and set ret to be a NULL pointer.

The snprintf and vsnprintf functions will write at most size -1 of the characters printed into the output string (the size ’th character then gets the terminating ‘\0’); if the return value is greater than or equal to the size argument, the string was too short and some of the printed characters were discarded. The output is always null-terminated.

The sprintf and vsprintf functions effectively assume an infinite size.

The format string is composed of zero or more directives: ordinary characters (not %), which are copied unchanged to the output stream; and conversion specifications, each of which results in fetching zero or more subsequent arguments. Each conversion specification is introduced by the % character. The arguments must correspond properly (after type promotion) with the conversion specifier. After the %, the following appear in sequence:

• An optional field, consisting of a decimal digit string followed by a $, specifying the next argument to access. If this field is not provided, the argument following the last argument accessed will be used. Arguments are numbered starting at 1. If unaccessed arguments in the format string are interspersed with ones that are accessed the results will be indeterminate.
• Zero or more of the following flags:

  '#'	The value should be converted to an "alternate form". For c, d, i, n, p, s, and u conversions, this option has no effect. For o conversions, the precision of the number is increased to force the first character of the output string to a zero (except if a zero value is printed with an explicit precision of zero). For x and X conversions, a non-zero result has the string ‘0x’ (or ‘0X’ for X conversions) prepended to it. For a, A, e, E, f, F, g, and G conversions, the result will always contain a decimal point, even if no digits follow it (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conversions, trailing zeros are not removed from the result as they would otherwise be.
  '0(zero)'	Zero padding. For all conversions except n, the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion ( d, i, o, u, i, x, and X), the 0 flag is ignored.
  '-'	A negative field width flag; the converted value is to be left adjusted on the field boundary. Except for n conversions, the converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A - overrides a 0 if both are given.
  '" "(space)'	A blank should be left before a positive number produced by a signed conversion ( a, A, d, e, E, f, F, g, G, or i).
  '+'	A sign must always be placed before a number produced by a signed conversion. A + overrides a space if both are used.
  '’'	Decimal conversions ( d, u, or i) or the integral portion of a floating point conversion ( f or F) should be grouped and separated by thousands using the non-monetary separator returned by localeconv(3).

• An optional decimal digit string specifying a minimum field width. If the converted value has fewer characters than the field width, it will be padded with spaces on the left (or right, if the left-adjustment flag has been given) to fill out the field width.
• An optional precision, in the form of a period . followed by an optional digit string. If the digit string is omitted, the precision is taken as zero. This gives the minimum number of digits to appear for d, i, o, u, x, and X conversions, the number of digits to appear after the decimal-point for a, A, e, E, f, and F conversions, the maximum number of significant digits for g and G conversions, or the maximum number of characters to be printed from a string for s conversions.
• An optional length modifier, that specifies the size of the argument. The following length modifiers are valid for the d, i, n, o, u, x, or X conversion:

  Modifier d, i o, u, x, X n hh Vt "signed char" Vt "unsigned char" Vt "signed char *" h Vt short Vt "unsigned short" Vt "short *" l (ell) Vt long Vt "unsigned long" Vt "long *" ll (ell ell) Vt "long long" Vt "unsigned long long" Vt "long long *" j Vt intmax_t Vt uintmax_t Vt "intmax_t *" t Vt ptrdiff_t (see note) Vt "ptrdiff_t *" z (see note) Vt size_t (see note) q (deprecated) Vt quad_t Vt u_quad_t Vt "quad_t *"

  Note: the t modifier, when applied to a o, u, x, or X conversion, indicates that the argument is of an unsigned type equivalent in size to a
  .Vt ptrdiff_t . The z modifier, when applied to a d or i conversion, indicates that the argument is of a signed type equivalent in size to a
  .Vt size_t . Similarly, when applied to an n conversion, it indicates that the argument is a pointer to a signed type equivalent in size to a
  .Vt size_t .

  The following length modifier is valid for the a, A, e, E, f, F, g, or G conversion:

  Modifier a, A, e, E, f, F, g, G l (ell) Vt double
  	(ignored, same behavior as without it)
  L Vt "long double"

  The following length modifier is valid for the c or s conversion:

  Modifier c s l (ell) Vt wint_t Vt "wchar_t *"

• A character that specifies the type of conversion to be applied.

A field width or precision, or both, may be indicated by an asterisk ‘*’ or an asterisk followed by one or more decimal digits and a ‘$’ instead of a digit string. In this case, an
.Vt int argument supplies the field width or precision. A negative field width is treated as a left adjustment flag followed by a positive field width; a negative precision is treated as though it were missing. If a single format directive mixes positional (nn$) and non-positional arguments, the results are undefined.

The conversion specifiers and their meanings are:

The decimal point character is defined in the program’s locale (category LC_NUMERIC).

To print pi to five decimal places:
#include <math.h>
#include <stdio.h>
fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

#ifdef BAD
/*
* This first sprintf is bad behavior. Do not use sprintf!
*/
sprintf(onstack, "%s, %s", arbitrary_string, and_another);
#else
/*
* The following two lines demonstrate better use of
* snprintf().
*/
snprintf(onstack, sizeof(onstack), "%s, %s", arbitrary_string,
and_another);
#endif
}

The printf and sprintf family of functions are also easily misused in a manner allowing malicious users to arbitrarily change a running program’s functionality by either causing the program to print potentially sensitive data ""left on the stack"", or causing it to generate a memory fault or bus error by dereferencing an invalid pointer.

%n can be used to write arbitrary data to potentially carefully-selected addresses. Programmers are therefore strongly advised to never pass untrusted strings as the format argument, as an attacker can put format specifiers in the string to mangle your stack, leading to a possible security hole. This holds true even if the string was built using a function like snprintf, as the resulting string may still contain user-supplied conversion specifiers for later interpolation by printf.

Always use the proper secure idiom:

"snprintf(buffer, sizeof(buffer), "%s", string);"

SEE ALSO

STANDARDS

HISTORY

BUGS