PRINTF(3) AerieBSD 1.0 Refernce Manual PRINTF(3)

NAME

printffprintf, sprintf, snprintf, asprintf, vprintf, vfprintf, vsprintf, vsnprintf, vasprintf formatted output conversion

SYNOPSIS

#include <stdio.h>

int printf(const char *format);...

int fprintf(FILE *stream, const char *format);...

int sprintf(char *str, const char *format);...

int snprintf(char *str, size_t size, const char *format);...

int asprintf(char **ret, const char *format);... #include <stdarg.h>

int vprintf(const char *format, va_list ap);

int vfprintf(FILE *stream, const char *format, va_list ap);

int vsprintf(char *str, const char *format, va_list ap);

int vsnprintf(char *str, size_t size, const char *format, va_list ap);

int vasprintf(char **ret, const char *format, va_list ap);

DESCRIPTION

The printf(); family of functions produce output according to the given format as described below. This format may contain “conversion specifiers”; the results of such conversions, if any, depend on the arguments following the format string.

The printf(); and vprintf(); functions write output to the standard output stream, stdout; fprintf(); and vfprintf(); write output to the supplied stream pointer stream; sprintf();, snprintf();, vsprintf();, and vsnprintf(); write to the character string str; asprintf(); and vasprintf(); write to a dynamically allocated string that is stored in ret.

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.

snprintf(); and vsnprintf(); will write at most size of the characters printed into the output string (the size character then gets the terminating "\e0")); 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. If size is zero, str may be a null pointer and no characters will be written; the number of bytes that would have been written excluding the terminating "\e0" byte, or \-1 on error, will be returned.

sprintf(); and vsprintf(); 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:

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 .Pf ( Cm d and -i)), unsigned octal (-o), unsigned decimal (-u), or unsigned hexadecimal .Pf ( Cm x and -X) notation. The letters -abcdef are used for -x conversions; 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 .Sm off .Pf [\-]d Cm \&. No ddd Cm e No \\*(Pmdd .Sm on 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.

If the argument is infinity, it will be converted to [-]inf (-e) or [-]INF (-E), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (-e) or [-]NAN (-E), respectively.
-fF
The double argument is rounded and converted to decimal notation in the style .Sm off .Pf [-]ddd Cm \&. No ddd , .Sm on 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.

If the argument is infinity, it will be converted to [-]inf (-f) or [-]INF (-F), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (-f) or [-]NAN (-F), respectively.
-gG
The double argument is converted in style -f or -e (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.

If the argument is infinity, it will be converted to [-]inf (-g) or [-]INF (-G), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (-g) or [-]NAN (-G), respectively.
-aA
The double argument is rounded and converted to hexadecimal notation in the style .Sm off .Pf [\-]0xh Cm \&. No hhh Cm p No [\\*(Pm]d .Sm on 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.

If the argument is infinity, it will be converted to [-]inf (-a) or [-]INF (-A), respectively. If the argument is not-a-number (NaN), it will be converted to [-]nan (-a) or [-]NAN (-A), respectively.
-c
The int argument is converted to an unsigned char, and the resulting character is written.
-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 NUL 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.
-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 "%%".

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

RETURN VALUES

For all these functions if an output or encoding error occurs, a value of \-1 is returned.

The printf();, fprintf();, sprintf();, vprintf();, vfprintf();, vsprintf();, asprintf();, and vasprintf(); functions return the number of characters printed (not including the trailing "\e0" used to end output to strings).

The snprintf(); and vsnprintf(); functions return the number of characters that would have been output if the size were unlimited ( again, not including the final "\e0". ).

The asprintf(); and vasprintf(); functions return the number of characters that were output to the newly allocated string (excluding the final "\e0")). A pointer to the newly allocated string is returned in ret; it should be passed to free(3) to release the allocated storage when it is no longer needed. If sufficient space cannot be allocated, these functions will return \-1. The value of ret in this situation is implementation-dependent (on OpenBSD, ret will be set to the null pointer, but this behavior should not be relied upon).

EXAMPLES

To print a date and time in the form `Sunday, July 3, 10:02', where weekday and month are pointers to strings:

#include 

fprintf(stdout, "%s, %s %d, %.2d:%.2d\en",
    weekday, month, day, hour, min);

To print \*(Pi to five decimal places:

#include 
#include 

fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0));

To allocate a 128-byte string and print into it:

#include 
#include 
#include 

char *
newfmt(const char *fmt, ...)
{
	char *p;
	va_list ap;

	if ((p = malloc(128)) == NULL)
		return (NULL);
	va_start(ap, fmt);
	(void) vsnprintf(p, 128, fmt, ap);
	va_end(ap);
	return (p);
}

SEE ALSO

printf(1), scanf(3)

STANDARDS

The fprintf();, printf();, sprintf();, vprintf();, vfprintf();, and vsprintf(); functions conform to

HISTORY

The functions snprintf(); and vsnprintf(); first appeared in 4.4BSD.

The functions asprintf(); and vasprintf(); first appeared in the GNU C library. This implementation first appeared in OpenBSD 2.3.

CAVEATS

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.

Because sprintf(); and vsprintf(); assume an infinitely long string, callers must be careful not to overflow the actual space; this is often impossible to assure. For safety, programmers should use the snprintf(); and asprintf(); family of interfaces instead. Unfortunately, the snprintf(); interface is not available on older systems and the asprintf(); interface is not portable.

It is important never to pass a string with user-supplied data as a format without using "%s". An attacker can put format specifiers in the string to mangle the stack, leading to a possible security hole. This holds true even if the string has been built “by hand” using a function like snprintf();, as the resulting string may still contain user-supplied conversion specifiers for later interpolation by printf();.

Be sure to use the proper secure idiom:

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

There is no way for printf(); to know the size of each argument passed. If positional arguments are used, care must be taken to ensure that all parameters, up to the last positionally specified parameter, are used in the format string. This allows for the format string to be parsed for this information. Failure to do this will mean the code is non-portable and liable to fail.


AerieBSD 1.0 Reference Manual May 26 2009 PRINTF(3)