C
Compiler support | ||||
Language | ||||
Headers | ||||
Type support | ||||
Program utilities | ||||
Variadic function support | ||||
Error handling | ||||
Dynamic memory management | ||||
Strings library | ||||
Algorithms | ||||
Numerics | ||||
Date and time utilities | ||||
Input/output support | ||||
Localization support | ||||
Concurrency support (C11) | ||||
Technical Specifications | ||||
Symbol index |
[edit]
File input/output
Types and objects | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Functions | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Defined in header | ||
(1) | ||
int printf( const char* format, ... ); | (until C99) | |
int printf( const char* restrict format, ... ); | (since C99) | |
(2) | ||
int fprintf( FILE* stream, const char* format, ... ); | (until C99) | |
int fprintf( FILE* restrict stream, const char* restrict format, ... ); | (since C99) | |
(3) | ||
int sprintf( char* buffer, const char* format, ... ); | (until C99) | |
int sprintf( char* restrict buffer, const char* restrict format, ... ); | (since C99) | |
int snprintf( char* restrict buffer, size_t bufsz, | (4) | (since C99) |
int printf_s( const char* restrict format, ... ); | (5) | (since C11) |
int fprintf_s( FILE* restrict stream, const char* restrict format, ... ); | (6) | (since C11) |
int sprintf_s( char* restrict buffer, rsize_t bufsz, | (7) | (since C11) |
int snprintf_s( char* restrict buffer, rsize_t bufsz, | (8) | (since C11) |
Loads the data from the given locations, converts them to character string equivalents and writes the results to a variety of sinks/streams:
1) Writes the results to the output stream stdout.
2) Writes the results to the output stream stream.
3) Writes the results to a character string buffer. The behavior is undefined if the string to be written (plus the terminating null character) exceeds the size of the array pointed to by buffer.
4) Writes the results to a character string buffer. At most bufsz - 1 characters are written. The resulting character string will be terminated with a null character, unless bufsz is zero. If bufsz is zero, nothing is written and buffer
may be a null pointer, however the return value (number of bytes that would be written not including the null terminator) is still calculated and returned.
5-8) Same as (1-4), except that the following errors are detected at runtime and call the currently installed constraint handler function:
- the conversion specifier
%n
is present informat
- any of the arguments corresponding to
%s
is a null pointer - stream or
format
orbuffer
is a null pointer - bufsz is zero or greater than RSIZE_MAX
- encoding errors occur in any of string and character conversion specifiers
- (for
sprintf_s
only), the string to be stored in buffer (including the trailing null) would exceed bufsz.
- the conversion specifier
- As with all bounds-checked functions,
printf_s
,fprintf_s
,sprintf_s
, andsnprintf_s
are only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdio.h>.
Contents
- 1 Parameters
- 2 Return value
- 3 Notes
- 4 Example
- 5 References
- 6 See also
[edit] Parameters
stream | - | output file stream to write to |
buffer | - | pointer to a character string to write to |
bufsz | - | up to bufsz - 1 characters may be written, plus the null terminator |
format | - | pointer to a null-terminated byte string specifying how to interpret the data |
... | - | arguments specifying data to print. If any argument after default argument promotions is not the type expected by the corresponding conversion specifier, or if there are fewer arguments than required by format, the behavior is undefined. If there are more arguments than required by format, the extraneous arguments are evaluated and ignored. |
The format string consists of ordinary byte characters (except %
), which are copied unchanged into the output stream, and conversion specifications. Each conversion specification has the following format:
- introductory
%
character.
- introductory
- (optional) one or more flags that modify the behavior of the conversion:
-
-
: the result of the conversion is left-justified within the field (by default it is right-justified). -
+
: the sign of signed conversions is always prepended to the result of the conversion (by default the result is preceded by minus only when it is negative). - space: if the result of a signed conversion does not start with a sign character, or is empty, space is prepended to the result. It is ignored if
+
flag is present. -
#
: alternative form of the conversion is performed. See the table below for exact effects otherwise the behavior is undefined. -
0
: for integer and floating-point number conversions, leading zeros are used to pad the field instead of space characters. For integer numbers it is ignored if the precision is explicitly specified. For other conversions using this flag results in undefined behavior. It is ignored if-
flag is present.
-
- (optional) integer value or
*
that specifies minimum field width. The result is padded with space characters (by default), if required, on the left when right-justified, or on the right if left-justified. In the case when*
is used, the width is specified by an additional argument of type int, which appears before the argument to be converted and the argument supplying precision if one is supplied. If the value of the argument is negative, it results with the-
flag specified and positive field width (Note: This is the minimum width: The value is never truncated.).
- (optional) integer value or
- (optional)
.
followed by integer number or*
, or neither that specifies precision of the conversion. In the case when*
is used, the precision is specified by an additional argument of type int, which appears before the argument to be converted, but after the argument supplying minimum field width if one is supplied. If the value of this argument is negative, it is ignored. If neither a number nor*
is used, the precision is taken as zero. See the table below for exact effects of precision.
- (optional)
- (optional) length modifier that specifies the size of the argument (in combination with the conversion format specifier, it specifies the type of the corresponding argument).
- conversion format specifier.
The following format specifiers are available:
Conversion Specifier | Explanation | Expected Argument Type | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Length | hh (C99) | h | (none) | l | ll (C99) | j (C99) | z (C99) | t (C99) | L | |
% | Writes literal % . The full conversion specification must be %% . | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A |
c |
The argument is first converted to unsigned char.If the l modifier is used, the argument is first converted to a character string as if by %ls with a wchar_t[2] argument. | N/A | N/A | int | wint_t | N/A | N/A | N/A | N/A | N/A |
s |
The argument must be a pointer to the initial element of an array of characters. Precision specifies the maximum number of bytes to be written. If Precision is not specified, writes every byte up to and not including the first null terminator.If the l specifier is used, the argument must be a pointer to the initial element of an array of wchar_t, which is converted to char array as if by a call to wcrtomb with zero-initialized conversion state. | N/A | N/A | char* | wchar_t* | N/A | N/A | N/A | N/A | N/A |
d i |
Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters. | signed char | short | int | long | long long | intmax_t | signed size_t | ptrdiff_t | N/A |
o |
Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters.In the alternative implementation precision is increased if necessary, to write one leading zero.In that case if both the converted value and the precision are 0, single 0 is written. | unsigned char | unsigned short | unsigned int | unsigned long | unsigned long long | uintmax_t | size_t | unsigned version of ptrdiff_t | N/A |
x X |
For the | N/A | ||||||||
u |
Precision specifies the minimum number of digits to appear. The default precision is 1. If both the converted value and the precision are 0 the conversion results in no characters. | N/A | ||||||||
f F |
Precision specifies the exact number of digits to appear after the decimal point character. The default precision is 6.In the alternative implementation decimal point character is written even if no digits follow it.For infinity and not-a-number conversion style see notes. | N/A | N/A | double | double(C99) | N/A | N/A | N/A | N/A | long double |
e E |
For the | N/A | N/A | N/A | N/A | N/A | N/A | |||
a A (C99) |
For the | N/A | N/A | N/A | N/A | N/A | N/A | |||
g G |
For the
Unless alternative representation is requested the trailing zeros are removed, also the decimal point character is removed if no fractional part is left.For infinity and not-a-number conversion style see notes. | N/A | N/A | N/A | N/A | N/A | N/A | |||
n |
The result is written to the value pointed to by the argument. The specification may not contain any flag, field width, or precision. | signed char* | short* | int* | long* | long long* | intmax_t* | signed size_t* | ptrdiff_t* | N/A |
p | Writes an implementation defined character sequence defining a pointer. | N/A | N/A | void* | N/A | N/A | N/A | N/A | N/A | N/A |
The floating-point conversion functions convert infinity to inf
or infinity
. Which one is used is implementation defined.
Not-a-number is converted to nan
or nan(char_sequence)
. Which one is used is implementation defined.
The conversions F
, E
, G
, A
output INF
, INFINITY
, NAN
instead.
Even though %c
expects int
argument, it is safe to pass a char
because of the integer promotion that takes place when a variadic function is called.
The correct conversion specifications for the fixed-width character types (int8_t, etc) are defined in the header <inttypes.h> (although PRIdMAX, PRIuMAX, etc is synonymous with %jd
, %ju
, etc).
The memory-writing conversion specifier %n is a common target of security exploits where format strings depend on user input and is not supported by the bounds-checked printf_s
family of functions.
There is a sequence point after the action of each conversion specifier; this permits storing multiple %n results in the same variable or, as an edge case, printing a string modified by an earlier %n within the same call.
If a conversion specification is invalid, the behavior is undefined.
[edit] Return value
1,2) number of characters transmitted to the output stream or negative value if an output error or an encoding error (for string and character conversion specifiers) occurred.
3) number of characters written to buffer (not counting the terminating null character), or a negative value if an encoding error (for string and character conversion specifiers) occurred.
4) number of characters (not including the terminating null character) which would have been written to buffer if bufsz was ignored, or a negative value if an encoding error (for string and character conversion specifiers) occurred.
5,6) number of characters transmitted to the output stream or negative value if an output error, a runtime constraints violation error, or an encoding error occurred.
7) number of characters written to buffer, not counting the null character (which is always written as long as buffer
is not a null pointer and bufsz is not zero and not greater than RSIZE_MAX), or zero on runtime constraint violations, and negative value on encoding errors.
8) number of characters not including the terminating null character (which is always written as long as buffer is not a null pointer and bufsz is not zero and not greater than RSIZE_MAX), which would have been written to buffer if bufsz was ignored, or a negative value if a runtime constraints violation or an encoding error occurred.
[edit] Notes
The C standard and POSIX specify that the behavior of sprintf
and its variants is undefined when an argument overlaps with the destination buffer. Example:
sprintf(dst, "%s and%s", dst, t); // <- broken: undefined behavior
POSIX specifies that errno is set on error. It also specifies additional conversion specifications, most notably support for argument reordering (n$ immediately after % indicates n
th argument).
Calling snprintf
with zero bufsz and null pointer for buffer is useful to determine the necessary buffer size to contain the output:
const char fmt[] = "sqrt(2) =%f";int sz = snprintf(NULL, 0, fmt, sqrt(2));char buf[sz + 1]; // note +1 for terminating null bytesnprintf(buf, sizeof buf, fmt, sqrt(2));
snprintf_s
, just like snprintf
, but unlike sprintf_s
, will truncate the output to fit in bufsz - 1.
[edit] Example
Run this code
#include <inttypes.h>#include <stdint.h>#include <stdio.h>int main(void){ const char* s = "Hello"; printf("Strings:\n"); // same as puts("Strings"); printf(" padding:\n"); printf("\t[%10s]\n", s); printf("\t[%-10s]\n", s); printf("\t[%*s]\n", 10, s); printf(" truncating:\n"); printf("\t%.4s\n", s); printf("\t%.*s\n", 3, s); printf("Characters:\t%c%%\n", 'A'); printf("Integers:\n"); printf("\tDecimal:\t%i%d%.6i%i%.0i%+i%i\n", 1, 2, 3, 0, 0, 4,-4); printf("\tHexadecimal:\t%x%x%X%#x\n", 5, 10, 10, 6); printf("\tOctal:\t\t%o%#o%#o\n", 10, 10, 4); printf("Floating-point:\n"); printf("\tRounding:\t%f%.0f%.32f\n", 1.5, 1.5, 1.3); printf("\tPadding:\t%05.2f%.2f%5.2f\n", 1.5, 1.5, 1.5); printf("\tScientific:\t%E%e\n", 1.5, 1.5); printf("\tHexadecimal:\t%a%A\n", 1.5, 1.5); printf("\tSpecial values:\t0/0=%g 1/0=%g\n", 0.0 / 0.0, 1.0 / 0.0); printf("Fixed-width types:\n"); printf("\tLargest 32-bit value is%" PRIu32 " or%#" PRIx32 "\n", UINT32_MAX, UINT32_MAX );}
Possible output:
Strings: padding: [ Hello] [Hello ] [ Hello] truncating: Hell HelCharacters: A%Integers: Decimal: 1 2 000003 0 +4 -4 Hexadecimal: 5 a A 0x6 Octal: 12 012 04Floating-point: Rounding: 1.500000 2 1.30000000000000004440892098500626 Padding: 01.50 1.50 1.50 Scientific: 1.500000E+00 1.500000e+00 Hexadecimal: 0x1.8p+0 0X1.8P+0 Special values: 0/0=-nan 1/0=infFixed-width types: Largest 32-bit value is 4294967295 or 0xffffffff
[edit] References
- C23 standard (ISO/IEC 9899:2024):
- 7.21.6.1 The fprintf function (p: TBD)
- 7.21.6.3 The printf function (p: TBD)
- 7.21.6.5 The snprintf function (p: TBD)
- 7.21.6.6 The sprintf function (p: TBD)
- K.3.5.3.1 The fprintf_s function (p: TBD)
- K.3.5.3.3 The printf_s function (p: TBD)
- K.3.5.3.5 The snprintf_s function (p: TBD)
- K.3.5.3.6 The sprintf_s function (p: TBD)
- C17 standard (ISO/IEC 9899:2018):
- 7.21.6.1 The fprintf function (p: 225-230)
- 7.21.6.3 The printf function (p: 236)
- 7.21.6.5 The snprintf function (p: 237)
- 7.21.6.6 The sprintf function (p: 237)
- K.3.5.3.1 The fprintf_s function (p: 430)
- K.3.5.3.3 The printf_s function (p: 432)
- K.3.5.3.5 The snprintf_s function (p: 432-433)
- K.3.5.3.6 The sprintf_s function (p: 433)
- C11 standard (ISO/IEC 9899:2011):
- 7.21.6.1 The fprintf function (p: 309-316)
- 7.21.6.3 The printf function (p: 324)
- 7.21.6.5 The snprintf function (p: 325)
- 7.21.6.6 The sprintf function (p: 325-326)
- K.3.5.3.1 The fprintf_s function (p: 591)
- K.3.5.3.3 The printf_s function (p: 593-594)
- K.3.5.3.5 The snprintf_s function (p: 594-595)
- K.3.5.3.6 The sprintf_s function (p: 595-596)
- C99 standard (ISO/IEC 9899:1999):
- 7.19.6.1 The fprintf function (p: 274-282)
- 7.19.6.3 The printf function (p: 290)
- 7.19.6.5 The snprintf function (p: 290-291)
- 7.19.6.6 The sprintf function (p: 291)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.9.6.1 The fprintf function
- 4.9.6.3 The printf function
- 4.9.6.5 The sprintf function
[edit] See also
wprintffwprintfswprintfwprintf_sfwprintf_sswprintf_ssnwprintf_s (C95)(C95)(C95)(C11)(C11)(C11)(C11) | prints formatted wide character output to stdout, a file stream or a buffer (function) [edit] |
vprintfvfprintfvsprintfvsnprintfvprintf_svfprintf_svsprintf_svsnprintf_s (C99)(C11)(C11)(C11)(C11) | prints formatted output to stdout, a file stream or a buffer using variable argument list (function) [edit] |
fputs | writes a character string to a file stream (function) [edit] |
scanffscanfsscanfscanf_sfscanf_ssscanf_s (C11)(C11)(C11) | reads formatted input from stdin, a file stream or a buffer (function) [edit] |
C++ documentation for printf, fprintf, sprintf, snprintf |