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#include <stdio.h>
#include <stdarg.h>
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 asprintf() and vasprintf() 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 snprintf() and vsnprintf() functions will write at most size, Ns, -1 of the characters printed into the output string (the size, Ns, '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 sprintf() and vsprintf() functions effectively assume a size of INT_MAX + 1.
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:
| ' #' | The value should be converted to an "alternate form". For c, d, i, n, p, s, and u conversions, this option has no effect. For o conversions, the precision of the number is increased to force the first character of the output string to a zero. For x and X conversions, a non-zero result has the string ‘0x’ (or ‘0X’ for X conversions) 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 digits follow it (normally, a decimal point appears in the results of those conversions only if a digit follows). For g and G conversions, trailing zeros are not removed from the result as they would otherwise be. |
| ' 0 '(zero) | Zero padding. For all conversions except n, the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion ( d, i, o, u, i, x, and X), the 0 flag is ignored. |
| ' -' | A negative field width flag; the converted value is to be left adjusted on the field boundary. Except for n conversions, 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. |
| ' '(space) | A blank should be left before a positive number produced by a signed conversion ( a, A, d, e, E, f, F, g, G, or i). |
| ' +' | A sign must always be placed before a number produced by a signed conversion. A + overrides a space if both are used. |
| '' '(apostrophe) | Decimal conversions ( d, u, or i) or the integral portion of a floating point conversion ( f or F) should be grouped and separated by thousands using the non-monetary separator returned by localeconv(3). |
| Modifier | d, i | o, u, x, X | n |
| hh | signed char | unsigned char | signed char * |
| h | short | unsigned short | short * |
| l (ell) | long | unsigned long | long * |
| ll (ell ell) | long long | unsigned long long | long long * |
| j | intmax_t | uintmax_t | intmax_t * |
| t | ptrdiff_t | (see note) | ptrdiff_t * |
| z | (see note) | size_t | (see note) |
| q (deprecated) | quad_t | u_quad_t | quad_t * |
Note: the t modifier, when applied to a o, u, x, or X conversion, indicates that the argument is of an unsigned type equivalent in size to a ptrdiff_t. The z modifier, when applied to a d or i conversion, indicates that the argument is of a signed type equivalent in size to a size_t. Similarly, when applied to an n conversion, it indicates that the argument is a pointer to a signed type equivalent in size to a size_t.
The following length modifier is valid for the a, A, e, E, f, F, g, or G conversion:
| Modifier | a, A, e, E, f, F, g, G |
| l (ell) | double(ignored, same behavior as without it) |
| L | long double |
The following length modifier is valid for the c or s conversion:
| Modifier | c | s |
| l (ell) | wint_t | wchar_t * |
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:
| diouxX | |
| 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. | |
| DOU | 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. |
| eE |
The
double
argument is rounded and converted in the style
[- ]damp;. ddde± ddwhere 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. |
| fF | The double argument is rounded and converted to decimal notation in the style [- ]dddamp;. 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. |
| gG | 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. |
| aA |
The
double
argument is rounded and converted to hexadecimal notation in the style
[- ]0x hamp;. 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. FreeBSD 8.0 and later always prints finite non-zero numbers using ‘1’ as the digit before the hexadecimal point. Zeroes are always represented with a mantissa of 0 (preceded by a ‘-’ if appropriate) and an exponent of +0. |
| C | Treated as c with the l (ell) modifier. |
| c |
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. |
| S | Treated as s with the l (ell) modifier. |
| s |
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. |
| 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 indicated by the int * (or variant) pointer argument. No argument is converted. |
| m | Print the string representation of the error code stored in the errno variable at the beginning of the call, as returned by strerror(3). No argument is taken. |
| % | 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.
#include <stdio.h> fprintf(stdout, "%s, %s %d, %.2d:%.2d\n", weekday, month, day, hour, min);
To print π 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 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.
| [EILSEQ] | |
| An invalid wide character code was encountered. | |
| [ENOMEM] | |
| Insufficient storage space is available. | |
| [EOVERFLOW] | |
| The size argument exceeds INT_MAX + 1, or the return value would be too large to be represented by an int. | |
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().
Always use the proper secure idiom:
snprintf(buffer, sizeof(buffer), q%sq, string);
| PRINTF (3) | May 22, 2018 |
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