vasprintf

Description

The printf family of functions produces output according to a format as described below. The printf and vprintf functions write output to stdout, the standard output stream; fprintf and vfprintf write output to the given output stream; sprintf, snprintf, vsprintf and vsnprintf write to the character string str; asprintf and vasprintf dynamically allocate a new string with reference:malloc.

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) are converted for output.

Also, they 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 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.

• An optional separator character( , | ; | : | _) used for separating multiple values when printing an AltiVec vector, or other multi-value unit.
  

NOTE: This is an AltiVec only extension onto the printf specification. Behaviour of these values for printf is only defined for operating systems conforming to the AltiVec Technology Programming Interface Manual. (At time of writing this includes only Mac OS X 10.2 and later.)

• 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:

Modifier	d,i	o,u,x,X	n
hh	signed char	unsigned char	signed char *
h	short	unsigned short	short *
l (ell)	long	unsigned long	long *
ll (ell ell)	long long	unsigned long long	long long *
j	intmax_t	uintmax_t	intmax_t *
t	ptrdiff_t	(see note)	ptrdiff_t *
z	(see note)	size_t	(see note)
q(deprecated)	quad_t	u_quad_t	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 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 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 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)	double (ignored, same behavior as without it)
L	long double

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

Modifier	c	s
l (ell)	wint_t	wchar_t *

The AltiVec Technology Programming Interface Manual also defines five additional length modifiers which can be used (in place of the conventional length modifiers) for the printing of AltiVec vectors:

v	Treat the argument as a vector value, unit length will be determined by the conversion specifier (default = 16 8-bit units for all integer conversions, 4 32-bit units for floating point conversions).
vh,hv	Treat the argument as a vector of 8 16-bit units.
vl,lv	Treat the argument as a vector of 4 32-bit units.

NOTE: The vector length specifiers are AltiVec only extensions onto the printf specification. Behaviour of these values for printf is only defined for operating systems conforming to the AltiVec Technology Programming Interface Manual. (At time of writing this includes only Mac OS X 10.2 and later.)

• 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 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:

diouxX	The int (or appropriate variant) argument is converted to signed decimal ( d and i), unsigned octal (o,) unsigned decimal (u,) or unsigned hexadecimal ( x and X) notation. The letters abcdef are used for x; the letters ABCDEF are used for X conversions. The precision, if any, gives the minimum number of digits that must appear; if the converted value requires fewer digits, it is padded on the left with zeros.
DOU	The long int argument is converted to signed decimal, unsigned octal, or unsigned decimal, as if the format had been ld, lo, or lu respectively. These conversion characters are deprecated, and will eventually disappear.
eE	The double argument is rounded and converted in the style[-]d.dddedd where there is one digit before the decimal-point character and the number of digits after it is equal to the precision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. An E conversion uses the letter E (rather than e) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. For a, A, e, E, f, F, g, and G conversions, positive and negative infinity are represented as inf and -inf respectively when using the lowercase conversion character, and INF and -INF respectively when using the uppercase conversion character. Similarly, NaN is represented as nan when using the lowercase conversion, and NAN when using the uppercase conversion.
fF	The double argument is rounded and converted to decimal notation in the style[-]ddd. ddd where the number of digits after the decimal-point character is equal to the precision specification. If the precision is missing, it is taken as 6; if the precision is explicitly zero, no decimal-point character appears. If a decimal point appears, at least one digit appears before it.
gG	The double argument is converted in style f or e (or F or E for G conversions). The precision specifies the number of significant digits. If the precision is missing, 6 digits are given; if the precision is zero, it is treated as 1. Style e is used if the exponent from its conversion is less than -4 or greater than or equal to the precision. Trailing zeros are removed from the fractional part of the result; a decimal point appears only if it is followed by at least one digit.
aA	The double argument is rounded and converted to hexadecimal notation in the style[-]0x h. hhhp []d<, where the number of digits after the hexadecimal-point character is equal to the precision specification. If the precision is missing, it is taken as enough to represent the floating-point number exactly, and no rounding occurs. If the precision is zero, no hexadecimal-point character appears. The p is a literal character p, and the exponent consists of a positive or negative sign followed by a decimal number representing an exponent of 2. The A conversion uses the prefix 0X (rather than 0x), the letters ABCDEF (rather than abcdef) to represent the hex digits, and the letter P (rather than p) to separate the mantissa and exponent. Note that there may be multiple valid ways to represent floating-point numbers in this hexadecimal format. For example, 0x3.24p+0, 0x6.48p-1 and 0xc.9p-2 are all equivalent. The format chosen depends on the internal representation of the number, but the implementation guarantees that the length of the mantissa will be minimized. Zeroes are always represented with a mantissa of 0 (preceded by a '-' if appropriate) and an exponent of +0.
C	Treated as c with the l (ell) modifier.
c	The int argument is converted to an unsigned char, and the resulting character is written. If the l (ell) modifier is used, the wint_t argument shall be converted to a wchar_t, and the (potentially multi-byte) sequence representing the single wide character is written, including any shift sequences. If a shift sequence is used, the shift state is also restored to the original state after the character.
S	Treated as s with the <l (ell) modifier.
s	The char * argument is expected to be a pointer to an array of character type (pointer to a string). Characters from the array are written up to (but not including) a terminating NUL character; if a precision is specified, no more than the number specified are written. If a precision is given, no null character need be present; if the precision is not specified, or is greater than the size of the array, the array must contain a terminating NUL character. If the l (ell) modifier is used, the wchar_t * argument is expected to be a pointer to an array of wide characters (pointer to a wide string). For each wide character in the string, the (potentially multi-byte) sequence representing the wide character is written, including any shift sequences. If any shift sequence is used, the shift state is also restored to the original state after the string. Wide characters from the array are written up to (but not including) a terminating wide NUL character; if a precision is specified, no more than the number of bytes specified are written (including shift sequences). Partial characters are never written. If a precision is given, no null character need be present; if the precision is not specified, or is greater than the number of bytes required to render the multibyte representation of the string, the array must contain a terminating wide NUL character.
p	The void * pointer argument is printed in hexadecimal (as if by '%#x' or '%#lx').
n	The number of characters written so far is stored into the integer indicated by the int * (or variant) pointer argument. No argument is converted.
%	A '%' is written. No argument is converted. The complete conversion specification is '%%'.

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

In no case does a non-existent or small field width cause truncation of a numeric field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result.

#include <stdio.h>
fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
	weekday, month, day, hour, min);

Security Considerations

The sprintf and vsprintf functions are easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack. Because sprintf and vsprintf assume an infinitely long string, callers must be careful not to overflow the actual space; this is often hard to assure. For safety, programmers should use the snprintf interface instead.

void
foo(const char *arbitrary_string, const char *and_another)
{
	char onstack[8];
#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 \c 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);

Errors

The printf family of functions may fail if:

EILSEQ	An invalid wide character code was encountered.
ENOMEM	Insufficient storage space is available.

Standards

Subject to the caveats noted in the BUGS section below, the fprintf, printf, sprintf, vprintf, vfprintf and vsprintf functions conform to ANSI X3.159-1989 ("ANSI C89") and ISO/IEC 9899:1999 ("ISO C99"). With the same reservation, the snprintf and vsnprintf functions conform to ISO/IEC 9899:1999 ("ISO C99").

History

The functions asprintf and vasprintf first appeared in the GNU C library. These were implemented by Peter Wemm peter@FreeBSD.org in FreeBSD 2.2 but were later replaced with a different implementation from Todd C. Miller Todd.Miller@courtesan.com for OpenBSD 2.3

Bugs

The conversion formats %D, %O, and %U are not standard and are provided only for backward compatibility. The effect of padding the %p format with zeros (either by the 0 flag or by specifying a precision), and the benign effect (i.e., none) of the # flag on %n and %p conversions, as well as other nonsensical combinations such as <%Ld, are not standard; such combinations should be avoided.

The printf family of functions do not correctly handle multibyte characters in the \c format argument.