All rights reserved.
This code is derived from software contributed to Berkeley by
Chris Torek.
%sccs.include.redist.man%
@(#)printf.3 6.9 (Berkeley) 03/05/91
printf(format [ , arg ... ] )
char *format;
fprintf(stream, format [ , arg ... ] )
FILE *stream;
char *format;
sprintf(str, format [ , arg ... ] )
char *str, *format;
snprintf(str, size, format [ , arg ... ] )
char *str;
size_t size;
char *format; .PP
.B char *smprintf(format
[ , arg ... ]
.B )
#include <varargs.h>
vprintf(format, ap)
char *format;
va_list ap;
vfprintf(stream, format, ap)
FILE *stream;
char *format;
va_list ap;
vsprintf(str, format, ap)
char *str, *format;
va_list ap;
vsnprintf(str, size, format, ap)
char *str;
size_t size;
char *format;
va_list ap; .PP
.B char *vsmprintf(format, ap)
.br
.B char *format;
.br
.B va_list ap;
and
.I smprintf
and
.I vsmprintf
dynamically allocate a new string with
.IR malloc .
Each function converts, formats, and prints the arguments following the format (or accessed via the variable-length argument facilities of varargs (3)) under control of the format argument. Except for
.I smprintf
and
.IR vsmprintf ,
all of these functions return
Each function returns the number of characters printed (not including the trailing `\e0' used to end output to strings). .I Smprintf
and
.I vsmprintf
return a pointer to a string of an appropriate length;
this pointer should be passed to
.I free
to release the associated storage
when it is no longer needed.
If sufficient space is not avaliable,
.I smprintf
and
.I vsmprintf
will return
.SM
.BR NULL .
Snprintf and vsnprintf will write at most size -1 of the characters printed into the output string (the size 'th character then gets the terminating `\e0'); 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. Sprintf and vsprintf effectively assume an infinte size .
The format is a string comprised of two types of objects: plain characters, which are simply copied to the output stream, and conversion specifications, each of which causes conversion and printing of the next successive arg . These arguments must correspond properly (after type promotion) with the conversion specifier. Each conversion is introduced by the % (percent sign) character. The rest of the conversion includes, in the following order,
4 \(bu Zero or more of the following flags:
4 \(bu a `#' character specifying that 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 (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 e , E , 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.
4 \(bu A zero `0' character specifying 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.
4 \(bu A minus sign (`-') specifying left adjustment of the converted value in the indicated field. 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.
4 \(bu A space, specifying that a blank should be left before a positive number produced by a signed conversion ( d , e , E , f , g , G , or i ).
4 \(bu a `+' character specifying that a sign always be placed before a number produced by a signed conversion. A `+' overrides a space if both are used.
4 \(bu An optional digit string specifying a field width. If the converted value has fewer characters than the field width, it will be padded on the left (or right, if the left-adjustment flag is used) to make up the field width.
4 \(bu An optional precision, in the form of a period (`.') 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 d , i , o , u , x , and X conversions, the number of digits to appear after the decimal point for e , E , 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 conversions.
4 \(bu The character h , specifying that a following d , i , o , u , x , or X conversion corresponds to a "short int" or "unsigned short int" argument, or that a following n conversion corresponds to a pointer to a "short int" argument.
4 \(bu the character l (ell) specifying that a following d , i , o , u , x , or X conversion corresponds to a "long int" or "unsigned long int" argument, or that a following n conversion corresponds to a pointer to a "long int" argument.
4 \(bu The character L specifying that a following e , E , f , g , or G conversion corresponds to a "long double" argument (but note that long double values are not currently supported by the \s-2VAX\s0 and Tahoe compilers).
4 \(bu A character which indicates the type of conversion to be applied.
A field width or precision may be an asterisk (`*') instead of a digit string. In this case an int argument supplies the value. 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.
The conversion characters and their meanings are:
4 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 respectively. 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.
4 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.
4 eE The double argument is rounded and converted in the style `[-]d.ddde\(+-dd' where there is one digit before the decimal point and the number after is equal to the precision specification for the argument. If the precision is missing, 6 digits are given; if the precision is explicitly zero, no decimal point 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.
4 f The double argument is rounded and converted to decimal notation in the style `[-]ddd.ddd' where the number of digits after the decimal point is equal to the precision. If the precision is missing, 6 digits are given; if the precision is explicitly 0, no digits and no decimal point are printed. If a decimal point appears, at least one digit appears before it.
4 g The double argument is printed in style f or e (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.
4 c The int argument is converted to an "unsigned char", and the resulting character is printed.
4 s The "char *" argument is taken to be a string (character pointer). Characters from the string are printed until a null character is reached, or until the number of characters indicated by the precision have been printed, whichever occurs first; if a precision is given, no null character need be present.
4 p The "void *" pointer argument is printed in hexadecimal (as if by `%#x' or `%#lx').
4 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.
4 % Prints a `%'; no argument is converted.
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 it. Similarly, padding takes place only if the specified field width exceeds the actual width.
To print a date and time in the form `Sunday, July 3, 10:02', where weekday and month are pointers to null-terminated strings:
printf("%s, %s %d, %02d:%.2d", weekday, month, day, hour, min);
To print to 5 decimals:
printf("pi = %.5f", 4*atan(1.0));To allocate a 128 byte string and print into it:
#include <stdio.h> #include <varargs.h> char *newfmt(va_alist) va_dcl { char *p, *malloc(), *fmt; va_list ap; if ((p = malloc(128)) == NULL) return (NULL); va_start(ap); fmt = va_arg(ap, char *); (void) vsprintf(p, fmt, ap); va_end(ap); return (p); }
Because sprintf and 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 snprintf interface instead. Unfortunately, this interface is not portable.