.\" $OpenBSD: printf.3,v 1.35 2001/06/23 15:00:25 provos Exp $ .\" .\" Copyright (c) 1990, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" Chris Torek and the American National Standards Committee X3, .\" on Information Processing Systems. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)printf.3 8.1 (Berkeley) 6/4/93 .\" .Dd June 4, 1993 .Dt PRINTF 3 .Os .Sh NAME .Nm printf , .Nm fprintf , .Nm sprintf , .Nm snprintf , .Nm asprintf , .Nm vprintf , .Nm vfprintf , .Nm vsprintf , .Nm vsnprintf , .Nm vasprintf .Nd formatted output conversion .Sh SYNOPSIS .Fd #include .Ft int .Fn printf "const char *format" ... .Ft int .Fn fprintf "FILE *stream" "const char *format" ... .Ft int .Fn sprintf "char *str" "const char *format" ... .Ft int .Fn snprintf "char *str" "size_t size" "const char *format" ... .Ft int .Fn asprintf "char **ret" "const char *format" ... .Fd #include .Ft int .Fn vprintf "const char *format" "va_list ap" .Ft int .Fn vfprintf "FILE *stream" "const char *format" "va_list ap" .Ft int .Fn vsprintf "char *str" "const char *format" "va_list ap" .Ft int .Fn vsnprintf "char *str" "size_t size" "const char *format" "va_list ap" .Ft int .Fn vasprintf "char **ret" "const char *format" "va_list ap" .Sh DESCRIPTION The .Fn printf family of functions produce output according to the given .Fa format as described below. This format may contain .Dq conversion specifiers ; the results of such conversions, if any, depend on the arguments following the .Fa format string. .Pp The .Fn printf and .Fn vprintf functions write output to the standard output stream, .Em stdout ; .Fn fprintf and .Fn vfprintf write output to the supplied stream pointer .Fa stream ; .Fn sprintf , .Fn snprintf , .Fn vsprintf , and .Fn vsnprintf write to the character string .Fa str ; .Fn asprintf and .Fn vasprintf write to a dynamically allocated string that is stored in .Fa ret . .Pp These functions write the output under the control of a .Fa format string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of .Xr stdarg 3 ) are converted for output. .Pp These functions return the number of characters printed (not including the trailing .Ql \e0 used to end output to strings), except for .Fn snprintf and .Fn vsnprintf , which return the number of characters that would have been printed if the .Fa size were unlimited .Po again, not including the final .Ql \e0 .Pc . .Pp .Fn asprintf and .Fn vasprintf return a pointer to a buffer sufficiently large to hold the string in the .Fa ret argument. This pointer should be passed to .Xr 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 .Fa ret to be a null pointer. .Pp .Fn snprintf and .Fn vsnprintf will write at most .Fa size Ns \-1 of the characters printed into the output string (the .Fa size Ns 'th character then gets the terminating .Ql \e0 ) ; if the return value is greater than or equal to the .Fa size argument, the string was too short and some of the printed characters were discarded. .Pp .Fn sprintf and .Fn vsprintf effectively assume an infinite .Fa size . .Pp The format string is composed of zero or more directives: ordinary .\" multibyte characters (not .Cm % ) , 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 .Cm % . The arguments must correspond properly (after type promotion) with the conversion specifier. After the .Cm % , the following appear in sequence: .Bl -bullet .It An optional field, consisting of a decimal digit string followed by a .Cm $ specifying the next argument to access. If this field is not provided, the argument following the last argument accessed will be used. Arguments are numbered starting at .Cm 1 . .It Zero or more of the following flags: .Bl -hyphen .It A hash .Ql Cm # character specifying that the value should be converted to an ``alternate form''. For .Cm c , .Cm d , .Cm i , .Cm n , .Cm p , .Cm s , and .Cm u , conversions, this option has no effect. For .Cm o conversions, the precision of the number 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 .Cm x and .Cm X conversions, a non-zero result has the string .Ql 0x (or .Ql 0X for .Cm X conversions) prepended to it. For .Cm e , .Cm E , .Cm f , .Cm g , and .Cm 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 .Cm g and .Cm G conversions, trailing zeros are not removed from the result as they would otherwise be. .It A zero .Sq Cm \&0 character specifying zero padding. For all conversions except .Cm n , the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion .Pf ( Cm d , .Cm i , .Cm o , .Cm u , .Cm x , and .Cm X ) , the .Sq Cm \&0 flag is ignored. .It A negative field width flag .Sq Cm \- indicates the converted value is to be left adjusted on the field boundary. Except for .Cm n conversions, the converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A .Sq Cm \- overrides a .Sq Cm \&0 if both are given. .It A space, specifying that a blank should be left before a positive number produced by a signed conversion .Pf ( Cm d , .Cm e , .Cm E , .Cm f , .Cm g , .Cm G , or .Cm i ) . .It A .Sq Cm + character specifying that a sign always be placed before a number produced by a signed conversion. A .Sq Cm + overrides a space if both are used. .El .It 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-adjustment flag has been given) to fill out the field width. .It An optional precision, in the form of a period .Sq Cm \&. followed by an optional digit string. If the digit string is omitted, the precision is taken as zero. This gives the minimum number of digits to appear for .Cm d , .Cm i , .Cm o , .Cm u , .Cm x , and .Cm X conversions, the number of digits to appear after the decimal-point for .Cm e , .Cm E , and .Cm f conversions, the maximum number of significant digits for .Cm g and .Cm G conversions, or the maximum number of characters to be printed from a string for .Cm s conversions. .It The optional character .Cm h , specifying that a following .Cm d , .Cm i , .Cm o , .Cm u , .Cm x , or .Cm X conversion corresponds to a .Li short int or .Li unsigned short int argument, or that a following .Cm n conversion corresponds to a pointer to a .Li short int argument. .It The optional character .Cm l (ell) specifying that a following .Cm d , .Cm i , .Cm o , .Cm u , .Cm x , or .Cm X conversion corresponds to a .Li long int or .Li unsigned long int argument, or that a following .Cm n conversion corresponds to a pointer to a .Li long int argument. .It The optional character sequence .Cm \&ll , specifying that a following .Cm d , .Cm i , .Cm o , .Cm u , .Cm x , or .Cm X conversion corresponds to a .Li quad int or .Li unsigned quad int argument, or that a following .Cm n conversion corresponds to a pointer to a .Li quad int argument. The use of .Cm q has been depreciated as conversion character. .It The character .Cm L specifying that a following .Cm e , .Cm E , .Cm f , .Cm g , or .Cm G conversion corresponds to a .Li long double argument (but note that long double values are not currently supported by the .Tn VAX and .Tn Tahoe compilers). .It A character that specifies the type of conversion to be applied. .El .Pp A field width or precision, or both, may be indicated by an asterisk .Ql * or an asterisk followed by one or more decimal digits and a .Ql $ instead of a digit string. In this case, an .Li 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. .Pp The conversion specifiers and their meanings are: .Bl -tag -width "diouxX" .It Cm diouxX The .Li int (or appropriate variant) argument is converted to signed decimal .Pf ( Cm d and .Cm i ) , unsigned octal .Pq Cm o , unsigned decimal .Pq Cm u , or unsigned hexadecimal .Pf ( Cm x and .Cm X ) notation. The letters .Cm abcdef are used for .Cm x conversions; the letters .Cm ABCDEF are used for .Cm 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. .It Cm DOU The .Li long int argument is converted to signed decimal, unsigned octal, or unsigned decimal, as if the format had been .Cm ld , .Cm lo , or .Cm lu respectively. These conversion characters are deprecated, and will eventually disappear. .It Cm eE The .Li double argument is rounded and converted in the style .Sm off .Pf [\-]d Cm \&. No ddd Cm e No \\*(Pmdd .Sm on 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 .Cm E conversion uses the letter .Cm E (rather than .Cm e ) to introduce the exponent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. .It Cm f The .Li double argument is rounded and converted to decimal notation in the style .Sm off .Pf [-]ddd Cm \&. No ddd , .Sm on 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. .It Cm gG The .Li double argument is converted in style .Cm f or .Cm e (or .Cm E for .Cm 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 .Cm 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. .It Cm c The .Li int argument is converted to an .Li unsigned char , and the resulting character is written. .It Cm s The .Li 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 .Tn 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 .Tn NUL character. .It Cm p The .Li void * pointer argument is printed in hexadecimal (as if by .Ql %#x or .Ql %#lx ) . .It Cm n The number of characters written so far is stored into the integer indicated by the .Li int * (or variant) pointer argument. No argument is converted. .It Cm % A .Ql % is written. No argument is converted. The complete conversion specification is .Ql %% . .El .Pp In no case does a non-existent or small field width cause truncation of a field; if the result of a conversion is wider than the field width, the field is expanded to contain the conversion result. .Sh EXAMPLES To print a date and time in the form `Sunday, July 3, 10:02', where .Va weekday and .Va month are pointers to strings: .Bd -literal -offset indent #include fprintf(stdout, "%s, %s %d, %.2d:%.2d\en", weekday, month, day, hour, min); .Ed .Pp To print \*(Pi to five decimal places: .Bd -literal -offset indent #include #include fprintf(stdout, "pi = %.5f\en", 4 * atan(1.0)); .Ed .Pp To allocate a 128 byte string and print into it: .Bd -literal -offset indent #include #include #include 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); } .Ed .Sh SEE ALSO .Xr printf 1 , .Xr scanf 3 .Sh STANDARDS The .Fn fprintf , .Fn printf , .Fn sprintf , .Fn vprintf , .Fn vfprintf , and .Fn vsprintf functions conform to .St -ansiC . .Sh HISTORY The functions .Fn snprintf and .Fn vsnprintf first appeared in .Bx 4.4 . .Pp The functions .Fn asprintf and .Fn vasprintf first appeared in the GNU C library. This implementation first appeared in .Ox 2.3 . .Sh CAVEATS The conversion formats .Cm \&%D , .Cm \&%O , and .Cm %U are not standard and are provided only for backward compatibility. The effect of padding the .Cm %p format with zeros (either by the .Sq Cm 0 flag or by specifying a precision), and the benign effect (i.e., none) of the .Sq Cm # flag on .Cm %n and .Cm %p conversions, as well as other nonsensical combinations such as .Cm %Ld , are not standard; such combinations should be avoided. .Pp Because .Fn sprintf and .Fn vsprintf assume an infinitely long string, callers must be careful not to overflow the actual space; this is often impossible to assure. For safety, programmers should use the .Fn snprintf and .Fn asprintf family of interfaces instead. Unfortunately, the .Fn snprintf interface is not available on older systems and the .Fn asprintf interface is not portable. .Pp It is important never to pass a string with user-supplied data as a format without using .Ql %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 .Dq by hand using a function like .Fn snprintf , as the resulting string may still contain user-supplied conversion specifiers for later interpolation by .Fn printf . .Pp Be sure to use the proper secure idiom: .Bd -literal -offset indent snprintf(buffer, sizeof(buffer), "%s", string); .Ed .Pp 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.