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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

#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, ...);

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

#include <stdarg.h>
#include <stdio.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);

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

The () 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 () and () functions write output to the standard output stream, ; () and () write output to the supplied stream pointer stream; () and () write output to the given file descriptor; sprintf(), snprintf(), vsprintf(), and vsnprintf() write to the character string str; () and () 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 va_start(3)) are converted for output.

() and () write at most size-1 characters to str, followed by a terminating ‘\0’. If size is zero, no characters are written and str may be a NULL pointer.

() and () effectively assume an infinite size; their use is not recommended.

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.

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.

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.

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.

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.

The int argument is converted to an unsigned char, and the resulting character is written.
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.
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.
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.

For all these functions if an output or encoding error occurs, a value less than 0 is returned.

The printf(), dprintf(), fprintf(), sprintf(), vprintf(), vdprintf(), 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’). A return value greater than or equal to the size argument indicates that the string was too small and some characters were discarded.

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 or some other error occurs, these functions return -1. The value of ret in this situation is implementation-dependent. On OpenBSD, ret is set to the NULL pointer, but other implementations may leave ret unchanged.

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 <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);
}

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.
[]
The return value would be too large to be represented by an int.

printf(1), scanf(3), wprintf(3)

The fprintf(), printf(), snprintf(), sprintf(), vfprintf(), vprintf(), vsnprintf(), and vsprintf() functions conform to ISO/IEC 9899:1999 (“ISO C99”). The dprintf() and vdprintf() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

The predecessors ftoa() and itoa() first appeared in Version 1 AT&T UNIX. The function printf() first appeared in Version 2 AT&T UNIX, and fprintf() and sprintf() in Version 7 AT&T UNIX.

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.

The functions dprintf() and vdprintf() first appeared in OpenBSD 5.3.

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 asprintf() interface is not available on all systems as it is not part of ISO/IEC 9899:1999 (“ISO C99”).

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:

int ret = snprintf(buffer, sizeof(buffer), "%s", string);
if (ret < 0 || ret >= sizeof(buffer))
	goto toolong;

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.

On systems other than OpenBSD, the LC_NUMERIC locale(1) category can cause erratic output; see CAVEATS in setlocale(3) for details.

August 30, 2019 OpenBSD-6.6