| PRINTF(3) | Library Functions Manual | PRINTF(3) |
int
printf(const char * restrict format, ...);
int
fprintf(FILE * restrict stream, const char * restrict format, ...);
int
dprintf(int fd, const char * restrict format, ...);
int
sprintf(char * restrict str, const char * restrict format, ...);
int
snprintf(char * restrict str, size_t size, const char * restrict format, ...);
int
asprintf(char ** restrict ret, const char * restrict format, ...);
#include <stdarg.h>
int
vprintf(const char * restrict format, va_list ap);
int
vfprintf(FILE * restrict stream, const char * restrict format, va_list ap);
int
vsprintf(char * restrict str, const char * restrict format, va_list ap);
int
vdprintf(int fd, const char * restrict format, va_list ap);
int
vsnprintf(char * restrict str, size_t size, const char * restrict format, va_list ap);
int
vsnprintf_ss(char * restrict str, size_t size, const char * restrict format, va_list ap);
int
vasprintf(char ** restrict ret, const char * restrict format, va_list ap);
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.
vsnprintf_ss() is a signal-safe standalone version that does not handle floating point formats.
asprintf() and vasprintf() return a pointer to a buffer sufficiently large to hold the string in the ret argument. 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, these functions will return -1 and set ret to be a NULL pointer. Please note that these functions are not standardized, and not all implementations can be assumed to set the ret argument to NULL on error. It is more portable to check for a return value of -1 instead.
snprintf(), vsnprintf(), and vsnprintf_ss() will write at most size-1 of the characters printed into the output string (the size'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. If size is zero, nothing is written and str may be a NULL pointer.
sprintf() and vsprintf() effectively assume an infinite size.
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:
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.| 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.
Note: if the standard integer types described in stdint(3) are used, it is recommended that the predefined format string specifier macros are used when possible. These are further described in inttypes(3).
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:
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..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.
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.
.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.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 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.
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.
%#x' or ‘%#lx').%' 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.
\0' used to end output to strings). If an output error was encountered, these functions shall return a negative value.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 <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);
}
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 your stack, leading to a possible security hole. This holds true even if you have built the string “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:
snprintf(buffer, sizeof(buffer), "%s", string);
There is no way for printf to know the size of each argument passed. If you use positional arguments you must 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 your code is non-portable and liable to fail.
In this implementation, passing a NULL char * argument to the %s format specifier will output (null) instead of crashing. Programs that depend on this behavior are non-portable and may crash on other systems or in the future.
The printf family of functions do not correctly handle multibyte characters in the format argument.
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), "%s", string);
| December 26, 2010 | NetBSD 6.1 |