PRINTF(3) Linux Programmer's Manual PRINTF(3)
NAME
printf, fprintf, dprintf, sprintf, snprintf, vprintf, vfprintf, vdprintf, vsprintf, vsnprintf - formatted output conversion
SYNOPSIS
#include <stdio.h>
int printf(const char *format, ...);
int fprintf(FILE *stream, const char *format, ...);
int dprintf(int fd, const char *format, ...);
int sprintf(char *str, const char *format, ...);
int snprintf(char *str, size_t size, 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 vdprintf(int fd, 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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
snprintf(), vsnprintf():
_XOPEN_SOURCE >= 500 || _ISOC99_SOURCE ||
|| /* Glibc versions <= 2.19: */ _BSD_SOURCE
dprintf(), vdprintf():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_GNU_SOURCE
DESCRIPTION
The functions in the printf() family produce output according to a format as described below. The functions printf() and vprintf() 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.
The function dprintf() is the same as fprintf() except that it outputs to a file descriptor, fd, instead of to a stdio stream.
The functions snprintf() and vsnprintf() write at most size bytes (including the terminating null byte (' ')) to str.
The functions vprintf(), vfprintf(), vdprintf(), vsprintf(), vsnprintf() are equivalent to the functions printf(), fprintf(), dprintf(),
sprintf(), snprintf(), respectively, except that they are called with a va_list instead of a variable number of arguments. These functions
do not call the va_end macro. Because they invoke the va_arg macro, the value of ap is undefined after the call. See stdarg(3).
All of 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.
C99 and POSIX.1-2001 specify that the results are undefined if a call to sprintf(), snprintf(), vsprintf(), or vsnprintf() would cause
copying to take place between objects that overlap (e.g., if the target string array and one of the supplied input arguments refer to the
same buffer). See NOTES.
Format of the format string
The format string is a character string, beginning and ending in its initial shift state, if any. 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 %, and ends with a
conversion specifier. In between there may be (in this order) zero or more flags, an optional minimum field width, an optional precision
and an optional length modifier.
The arguments must correspond properly (after type promotion) with the conversion specifier. By default, the arguments are used in the
order given, where each '*' (see Field width and Precision below) and each conversion specifier asks for the next argument (and it is an
error if insufficiently many arguments are given). One can also specify explicitly which argument is taken, at each place where an argu-
ment is required, by writing "%m$" instead of '%' and "*m$" instead of '*', where the decimal integer m denotes the position in the argu-
ment list of the desired argument, indexed starting from 1. Thus,
printf("%*d", width, num);
and
printf("%2$*1$d", width, num);
are equivalent. The second style allows repeated references to the same argument. The C99 standard does not include the style using '$',
which comes from the Single UNIX Specification. If the style using '$' is used, it must be used throughout for all conversions taking an
argument and all width and precision arguments, but it may be mixed with "%%" formats, which do not consume an argument. There may be no
gaps in the numbers of arguments specified using '$'; for example, if arguments 1 and 3 are specified, argument 2 must also be specified
somewhere in the format string.
For some numeric conversions a radix character ("decimal point") or thousands' grouping character is used. The actual character used
depends on the LC_NUMERIC part of the locale. (See setlocale(3).) The POSIX locale uses '.' as radix character, and does not have a
grouping character. Thus,
printf("%'.2f", 1234567.89);
results in "1234567.89" in the POSIX locale, in "1234567,89" in the nl_NL locale, and in "1.234.567,89" in the da_DK locale.
Flag characters
The character % is followed by zero or more of the following flags:
# The value should be converted to an "alternate form". For o conversions, the first character of the output string is made zero (by
prefixing a 0 if it was not zero already). For x and X conversions, a nonzero result has the string "0x" (or "0X" for X conver-
sions) 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 dig-
its follow it (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conver-
sions, trailing zeros are not removed from the result as they would otherwise be. For other conversions, the result is undefined.
0 The value should be zero padded. For d, i, o, u, x, X, a, A, e, E, f, F, g, and G conversions, the converted value is padded on the
left with zeros rather than blanks. If the 0 and - flags both appear, the 0 flag is ignored. If a precision is given with a
numeric conversion (d, i, o, u, x, and X), the 0 flag is ignored. For other conversions, the behavior is undefined.
- The converted value is to be left adjusted on the field boundary. (The default is right justification.) 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) A blank should be left before a positive number (or empty string) produced by a signed conversion.
+ A sign (+ or -) should always be placed before a number produced by a signed conversion. By default, a sign is used only for nega-
tive numbers. A + overrides a space if both are used.
The five flag characters above are defined in the C99 standard. The Single UNIX Specification specifies one further flag character.
' For decimal conversion (i, d, u, f, F, g, G) the output is to be grouped with thousands' grouping characters if the locale informa-
tion indicates any. (See setlocale(3).) Note that many versions of gcc(1) cannot parse this option and will issue a warning.
(SUSv2 did not include %'F, but SUSv3 added it.)
glibc 2.2 adds one further flag character.
I For decimal integer conversion (i, d, u) the output uses the locale's alternative output digits, if any. For example, since glibc
2.2.3 this will give Arabic-Indic digits in the Persian ("fa_IR") locale.
Field width
An optional decimal digit string (with nonzero first digit) 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). Instead of a deci-
mal digit string one may write "*" or "*m$" (for some decimal integer m) to specify that the field width is given in the next argument, or
in the m-th argument, respectively, which must be of type int. A negative field width is taken as a '-' flag followed by a positive field
width. In no case does a nonexistent 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.
Precision
An optional precision, in the form of a period ('.') followed by an optional decimal digit string. Instead of a decimal digit string one
may write "*" or "*m$" (for some decimal integer m) to specify that the precision is given in the next argument, or in the m-th argument,
respectively, which must be of type int. If the precision is given as just '.', the precision is taken to be zero. A negative precision
is taken as if the precision were omitted. 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 radix character 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 and S conversions.
Length modifier
Here, "integer conversion" stands for d, i, o, u, x, or X conversion.
hh A following integer conversion corresponds to a signed char or unsigned char argument, or a following n conversion corresponds to a
pointer to a signed char argument.
h A following integer conversion corresponds to a short int or unsigned short int argument, or a following n conversion corresponds to
a pointer to a short int argument.
l (ell) A following integer conversion corresponds to a long int or unsigned long int argument, or a following n conversion corre-
sponds to a pointer to a long int argument, or a following c conversion corresponds to a wint_t argument, or a following s conver-
sion corresponds to a pointer to wchar_t argument.
ll (ell-ell). A following integer conversion corresponds to a long long int or unsigned long long int argument, or a following n con-
version corresponds to a pointer to a long long int argument.
q A synonym for ll. This is a nonstandard extension, derived from BSD; avoid its use in new code.
L A following a, A, e, E, f, F, g, or G conversion corresponds to a long double argument. (C99 allows %LF, but SUSv2 does not.)
j A following integer conversion corresponds to an intmax_t or uintmax_t argument, or a following n conversion corresponds to a
pointer to an intmax_t argument.
z A following integer conversion corresponds to a size_t or ssize_t argument, or a following n conversion corresponds to a pointer to
a size_t argument.
Z A nonstandard synonym for z that predates the appearance of z. Do not use in new code.
t A following integer conversion corresponds to a ptrdiff_t argument, or a following n conversion corresponds to a pointer to a
ptrdiff_t argument.
SUSv3 specifies all of the above, except for those modifiers explicitly noted as being nonstandard extensions. SUSv2 specified only the
length modifiers h (in hd, hi, ho, hx, hX, hn) and l (in ld, li, lo, lx, lX, ln, lc, ls) and L (in Le, LE, Lf, Lg, LG).
As a nonstandard extension, the GNU implementations treats ll and L as synonyms, so that one can, for example, write llg (as a synonym for
the standards-compliant Lg) and Ld (as a synonym for the standards compliant lld). Such usage is nonportable.
Conversion specifiers
A character that specifies the type of conversion to be applied. The conversion specifiers and their meanings are:
d, i The int argument is converted to signed decimal notation. 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 default precision is 1. When 0 is
printed with an explicit precision 0, the output is empty.
o, u, x, X
The unsigned int argument is converted to 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 default
precision is 1. When 0 is printed with an explicit precision 0, the output is empty.
e, E 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 expo-
nent always contains at least two digits; if the value is zero, the exponent is 00.
f, F The double argument is rounded and converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the deci-
mal-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.
(SUSv2 does not know about F and says that character string representations for infinity and NaN may be made available. SUSv3 adds
a specification for F. The C99 standard specifies "[-]inf" or "[-]infinity" for infinity, and a string starting with "nan" for NaN,
in the case of f conversion, and "[-]INF" or "[-]INFINITY" or "NAN" in the case of F conversion.)
g, G 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 frac-
tional part of the result; a decimal point appears only if it is followed by at least one digit.
a, A (C99; not in SUSv2, but added in SUSv3) For a conversion, the double argument is converted to hexadecimal notation (using the let-
ters abcdef) in the style [-]0xh.hhhhp+-; for A conversion the prefix 0X, the letters ABCDEF, and the exponent separator P is used.
There is one hexadecimal digit before the decimal point, and the number of digits after it is equal to the precision. The default
precision suffices for an exact representation of the value if an exact representation in base 2 exists and otherwise is suffi-
ciently large to distinguish values of type double. The digit before the decimal point is unspecified for nonnormalized numbers,
and nonzero but otherwise unspecified for normalized numbers.
c If no l modifier is present, the int argument is converted to an unsigned char, and the resulting character is written. If an l
modifier is present, the wint_t (wide character) argument is converted to a multibyte sequence by a call to the wcrtomb(3) function,
with a conversion state starting in the initial state, and the resulting multibyte string is written.
s If no l modifier is present: the const 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 null byte (' '); if a precision is speci-
fied, no more than the number specified are written. If a precision is given, no null byte need be present; if the precision is not
specified, or is greater than the size of the array, the array must contain a terminating null byte.
If an l modifier is present: the const wchar_t * argument is expected to be a pointer to an array of wide characters. Wide charac-
ters from the array are converted to multibyte characters (each by a call to the wcrtomb(3) function, with a conversion state start-
ing in the initial state before the first wide character), up to and including a terminating null wide character. The resulting
multibyte characters are written up to (but not including) the terminating null byte. If a precision is specified, no more bytes
than the number specified are written, but no partial multibyte characters are written. Note that the precision determines the num-
ber of bytes written, not the number of wide characters or screen positions. The array must contain a terminating null wide charac-
ter, unless a precision is given and it is so small that the number of bytes written exceeds it before the end of the array is
reached.
C (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym for lc. Don't use.
S (Not in C99 or C11, but in SUSv2, SUSv3, and SUSv4.) Synonym for ls. Don't use.
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 pointed to by the corresponding argument. That argument shall be
an int *, or variant whose size matches the (optionally) supplied integer length modifier. No argument is converted. (This speci-
fier is not supported by the bionic C library.) The behavior is undefined if the conversion specification includes any flags, a
field width, or a precision.
m (Glibc extension; supported by uClibc and musl.) Print output of strerror(errno). No argument is required.
% A '%' is written. No argument is converted. The complete conversion specification is '%%'.
RETURN VALUE
Upon successful return, these functions return the number of characters printed (excluding the null byte used to end output to strings).
The functions snprintf() and vsnprintf() do not write more than size bytes (including the terminating null byte (' ')). If the output was
truncated due to this limit, then the return value is the number of characters (excluding the terminating null byte) which would have been
written to the final string if enough space had been available. Thus, a return value of size or more means that the output was truncated.
(See also below under NOTES.)
If an output error is encountered, a negative value is returned.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
+------------------------+---------------+----------------+
|Interface | Attribute | Value |
+------------------------+---------------+----------------+
|printf(), fprintf(), | Thread safety | MT-Safe locale |
|sprintf(), snprintf(), | | |
|vprintf(), vfprintf(), | | |
|vsprintf(), vsnprintf() | | |
+------------------------+---------------+----------------+
CONFORMING TO
fprintf(), printf(), sprintf(), vprintf(), vfprintf(), vsprintf(): POSIX.1-2001, POSIX.1-2008, C89, C99.
snprintf(), vsnprintf(): POSIX.1-2001, POSIX.1-2008, C99.
The dprintf() and vdprintf() functions were originally GNU extensions that were later standardized in POSIX.1-2008.
Concerning the return value of snprintf(), SUSv2 and C99 contradict each other: when snprintf() is called with size=0 then SUSv2 stipulates
an unspecified return value less than 1, while C99 allows str to be NULL in this case, and gives the return value (as always) as the number
of characters that would have been written in case the output string has been large enough. POSIX.1-2001 and later align their specifica-
tion of snprintf() with C99.
glibc 2.1 adds length modifiers hh, j, t, and z and conversion characters a and A.
glibc 2.2 adds the conversion character F with C99 semantics, and the flag character I.
NOTES
Some programs imprudently rely on code such as the following
sprintf(buf, "%s some further text", buf);
to append text to buf. However, the standards explicitly note that the results are undefined if source and destination buffers overlap
when calling sprintf(), snprintf(), vsprintf(), and vsnprintf(). Depending on the version of gcc(1) used, and the compiler options
employed, calls such as the above will not produce the expected results.
The glibc implementation of the functions snprintf() and vsnprintf() conforms to the C99 standard, that is, behaves as described above,
since glibc version 2.1. Until glibc 2.0.6, they would return -1 when the output was truncated.
BUGS
Because sprintf() and vsprintf() assume an arbitrarily long string, callers must be careful not to overflow the actual space; this is often
impossible to assure. Note that the length of the strings produced is locale-dependent and difficult to predict. Use snprintf() and
vsnprintf() instead (or asprintf(3) and vasprintf(3)).
Code such as printf(foo); often indicates a bug, since foo may contain a % character. If foo comes from untrusted user input, it may con-
tain %n, causing the printf() call to write to memory and creating a security hole.
EXAMPLE
To print Pi to five decimal places:
#include <math.h>
#include <stdio.h>
fprintf(stdout, "pi = %.5f
", 4 * atan(1.0));
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
",
weekday, month, day, hour, min);
Many countries use the day-month-year order. Hence, an internationalized version must be able to print the arguments in an order specified
by the format:
#include <stdio.h>
fprintf(stdout, format,
weekday, month, day, hour, min);
where format depends on locale, and may permute the arguments. With the value:
"%1$s, %3$d. %2$s, %4$d:%5$.2d
"
one might obtain "Sonntag, 3. Juli, 10:02".
To allocate a sufficiently large string and print into it (code correct for both glibc 2.0 and glibc 2.1):
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
char *
make_message(const char *fmt, ...)
{
int size = 0;
char *p = NULL;
va_list ap;
/* Determine required size */
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 0)
return NULL;
size++; /* For ' ' */
p = malloc(size);
if (p == NULL)
return NULL;
va_start(ap, fmt);
size = vsnprintf(p, size, fmt, ap);
va_end(ap);
if (size < 0) {
free(p);
return NULL;
}
return p;
}
If truncation occurs in glibc versions prior to 2.0.6, this is treated as an error instead of being handled gracefully.
SEE ALSO
printf(1), asprintf(3), puts(3), scanf(3), setlocale(3), strfromd(3), wcrtomb(3), wprintf(3), locale(5)
COLOPHON
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latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 PRINTF(3)