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PRINTF(3) Library Functions Manual PRINTF(3)

printf, fprintf, sprintf, snprintf, asprintf, dprintf, vprintf, vfprintf, vsprintf, vsnprintf, vasprintf, vdprintf — formatted output conversion

Standard C Library (libc, -lc)

#include <stdio.h>

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

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

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

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

int
asprintf(char **ret, const char *format, ...);

int
dprintf(int fd, const char * restrict format, ...);

#include <stdarg.h>

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

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

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

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

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

int
vdprintf(int fd, const char * restrict format, va_list ap);

The () family of functions produces output according to a format as described below. The printf() and () functions write output to stdout, the standard output stream; () and () write output to the given output stream; () and () write output to the given file descriptor; sprintf(), snprintf(), vsprintf(), and vsnprintf() write to the character string str; and asprintf() and vasprintf() dynamically allocate a new string with malloc(3).

Extended locale versions of these functions are documented in printf_l(3). See xlocale(3) for more information.

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.

The () and () 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 () and () 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, unless size is 0.

The () and () functions effectively assume a size of INT_MAX + 1.

For those routines that write to a user-provided character string, that string and the format strings should not overlap, as the behavior is undefined.

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:

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.
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.
The double argument is rounded and converted in the style [-]d.ddde±dd 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.

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.
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.
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, 0x1.92p+1, 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.

Treated as c with the l (ell) modifier.
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.

Treated as s with the l (ell) modifier.
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.

The void * pointer argument is printed in hexadecimal (as if by ‘%#x’ or ‘%#lx’).
The number of characters written so far is stored into the integer indicated by the int * (or variant) pointer argument. No argument is converted. The format argument must be in write-protected memory if this specifier is used; see SECURITY CONSIDERATIONS below.
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.

These functions return the number of characters printed (not including the trailing ‘\0’ used to end output to strings), 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’). These functions return a negative value if an error occurs.

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 pi 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 <stdio.h>
#include <stdlib.h>
#include <stdarg.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);
}

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.

In addition to the errors documented for the write(2) system call, the printf() family of functions may fail if:

[]
An invalid wide character code was encountered.
[]
Insufficient storage space is available.

printf(1), printf_l(3), fmtcheck(3), scanf(3), setlocale(3), stdarg(3), wprintf(3)

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”), while dprintf() and vdprintf() conform to IEEE Std 1003.1-2008 (“POSIX.1”).

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 OpenBSD 2.3 by Todd C. Miller <Todd.Miller@courtesan.com>. The dprintf() and vdprintf() functions were added in FreeBSD 8.0.

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

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. For example:

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 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(). For this reason, a format argument containing %n is assumed to be untrustworthy if located in writable memory (i.e. memory with protection PROT_WRITE; see mprotect(2)) and any attempt to use such an argument is fatal. Practically, this means that %n is permitted in literal format strings but disallowed in format strings located in normal stack- or heap-allocated memory.

Always use the proper secure idiom:

snprintf(buffer, sizeof(buffer), "%s", string);
December 2, 2009 macOS