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PRINTF(3) BSD Library Functions Manual PRINTF(3) NAME printf, fprintf, dprintf sprintf, snprintf, asprintf, vprintf, vfprintf, vsprintf, vdprintf, vsnprintf, vsnprintf_ss, vasprintf -- formatted out- put conversion LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <stdio.h> 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); DESCRIPTION The printf() family of functions produces output according to a format as described below. The printf() and vprintf() functions write output to stdout, the standard output stream; fprintf() and vfprintf() write output to the given output stream; dprintf() and vdprintf() write output to the give file descriptor fd; sprintf(), snprintf(), vsprintf(), vsnprintf(), and vsnprintf_ss() write to the character string str; and asprintf() and vasprintf() write to a dynamically allocated string that is stored in ret. 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 out- put. 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 func- tions 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 char- acters (not %), which are copied unchanged to the output stream; and con- version 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 promo- tion) with the conversion specifier. After the %, the following appear in sequence: o An optional field, consisting of a decimal digit string followed by a $, specifying the next argument to access. If this field is not pro- vided, the argument following the last argument accessed will be used. Arguments are numbered starting at 1. If unaccessed arguments in the format string are interspersed with ones that are accessed the results will be indeterminate. o Zero or more of the following flags: `#' 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 num- ber is increased to force the first character of the output string to a zero (except if a zero value is printed with an explicit precision of 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 re- sult as they would otherwise be. `0' (zero) Zero padding. For all conversions except n, the con- verted 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 ig- nored. `-' 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 ze- ros. 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. `'' 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-mone- tary separator returned by localeconv(3). o An optional decimal digit string 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-adjust- ment flag has been given) to fill out the field width. o An optional precision, in the form of a period `.' followed by an op- tional digit string. If the digit string is omitted, the precision is taken as zero. 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 decimal-point 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 con- versions. o An optional length modifier, that specifies the size of the argument. The following length modifiers are valid for the d, i, n, o, u, x, or X conversion: 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, in- dicates 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 * o A character that specifies the type of conversion to be applied. 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 adjust- ment 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 [-]d.ddde+-dd where there is one digit before the decimal-point character and the number of digits after it is equal to the pre- cision; if the precision is missing, it is taken as 6; if the precision is zero, no decimal-point character appears. An E con- version 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 [-]ddd.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 in style 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 re- moved from the fractional part of the result; a decimal point ap- pears only if it is followed by at least one digit. aA The double argument is rounded and converted to hexadecimal nota- tion in the style [-]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 ex- ponent consists of a positive or negative sign followed by a dec- imal 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. C Treated as c with the l (ell) modifier. c The int argument is converted to an unsigned char, and the re- sulting 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 charac- ter; if a precision is specified, no more than the number speci- fied 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 ex- pected to be a pointer to an array of wide characters (pointer to a wide string). For each wide character in the string, the (po- tentially 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 af- ter the string. Wide characters from the array are written up to (but not including) a terminating wide NUL character; if a preci- sion 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 representa- tion 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 inte- ger indicated by the int * (or variant) pointer argument. No ar- gument is converted. % A `%' is written. No argument is converted. The complete con- version 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. RETURN VALUES These functions return the number of characters printed, or that would be printed if there was adequate space in case of snprintf(), vsnprintf(), and vsnprintf_ss() (not including the trailing `\0' used to end output to strings). If an output error was encountered, these functions shall re- turn a negative value. EXAMPLES 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\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); } ERRORS In addition to the errors documented for the write(2) system call, the printf() family of functions may fail if: [EILSEQ] An invalid wide-character code was encountered. [ENOMEM] Insufficient storage space is available. SEE ALSO printf(1), fmtcheck(3), scanf(3), setlocale(3), wprintf(3), printf(9) STANDARDS Subject to the caveats noted in the BUGS section below, the fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions con- form to ANSI X3.159-1989 ("ANSI C89") and ISO/IEC 9899:1999 ("ISO C99"). With the same reservation, the snprintf() and vsnprintf() functions con- form to ISO/IEC 9899:1999 ("ISO C99"). HISTORY The functions snprintf() and vsnprintf() first appeared in 4.4BSD. The functions asprintf() and vasprintf() are modeled on the ones that first appeared in the GNU C library. The function vsnprintf_ss() is non-stan- dard and appeared in NetBSD 4.0. The functions dprintf() and vdprintf() are parts of IEEE Std 1003.1-2008 ("POSIX.1") and appeared in NetBSD 6.0. CAVEATS Because sprintf() and vsprintf() assume an infinitely long string, call- ers must be careful not to overflow the actual space; this is often im- possible to assure. For safety, programmers should use the snprintf() and asprintf() family of interfaces instead. Unfortunately, the snprintf() interfaces are not available on older systems and the asprintf() interfaces are not yet portable. It is important never to pass a string with user-supplied data as a for- mat 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. BUGS The conversion formats %D, %O, and %U are not standard and are provided only for backward compatibility. 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. The printf family of functions do not correctly handle multibyte charac- ters in the format argument. SECURITY CONSIDERATIONS The sprintf() and vsprintf() functions are easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack. Because sprintf() and vsprintf() assume an infinitely long string, callers must be careful not to overflow the actual space; this is often hard to assure. For safety, programmers should use the snprintf() interface instead. For example: 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 pro- gram'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 un- trusted strings as the format argument, as an attacker can put format specifiers in the string to mangle your stack, leading to a possible se- curity hole. This holds true even if the string was built using a func- tion like snprintf(), as the resulting string may still contain user-sup- plied conversion specifiers for later interpolation by printf(). Always use the proper secure idiom: snprintf(buffer, sizeof(buffer), "%s", string); BSD December 26, 2010 BSD
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | EXAMPLES | ERRORS | SEE ALSO | STANDARDS | HISTORY | CAVEATS | BUGS | SECURITY CONSIDERATIONS
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