printf, fprintf, sprintf, snprintf, printf_s, fprintf_s, sprintf_s, snprintf_s (2024)

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.

[edit] Parameters

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.

• (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) . 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) 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:

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

#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 );}

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

[edit] See also