PRINTF(3)

NAME
printf, fprintf, 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-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. 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 ‘\0’ 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:

•   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.

•   Zero or more of the following flags:

    -   A hash ‘#’ character specifying that 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.

    -   A zero ‘0’ character specifying 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, x, and X), the ‘0’ flag is ignored.

    -   A negative field width flag ‘-’ indicates 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.

    -   A space, specifying that a blank should be left before a positive number produced by a signed conversion (d, a, A, e, E, f, F, g, G, or i).

    -   A ‘+’ character specifying that a sign always be placed before a number produced by a signed conversion. A ‘+’ overrides a space if both are used.

•   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 | 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 an 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 | 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 *

•   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 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 [-]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.

    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 [-]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.

    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 [-]0xh.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.

    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 ‘\0’ 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 ‘\0’.).

The asprintf() and vasprintf() functions return the number of characters that were output to the newly allocated string (excluding the final ‘\0’). 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 <stdio.h>
    fprintf(stdout, "%s, %s %d, %.2d:%.2d\n",
        weekday, month, day, hour, min);

To print to five decimal places:

    #include <math.h>
    #include <stdio.h>
    fprintf(stdout, "pi = %.5f\n", 4 * atan(1.0));

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

    #include <stdarg.h>
    #include <stdio.h>
    #include <stdlib.h>
    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 ansiC.

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.