printf(3S) printf(3S)NAME printf, fprintf, sprintf - format and output string and numeric data SYNOPSIS #include <stdio.h> int printf(format[,arg]...) char *format; int fprintf(stream, format[,arg]...) FILE *stream; char *format; int sprintf(s, format[,arg]...) char *s, format; DESCRIPTION printf places output on the standard output stream stdout. fprintf places output on the named output stream. sprintf places output, followed by the null character (\0) in con- secutive bytes starting at *s; it is the user's responsibil- ity to ensure that enough storage is available. Each of these functions converts, formats, and prints its args under control of the format. The format is a character string that contains two types of objects: plain charac- ters, which are simply copied to the output stream, and conversion specifications, each of which results in fetching zero or more args. The results are undefined if there are insufficient args for the format. If the format is exhaust- ed while args remain, the excess args are simply ignored. Each conversion specification is introduced by the character %. After the %, the following appear in sequence: Zero or more flags, which modify the meaning of the conversion specification. An optional decimal digit string specifying a minimum field width. If the converted value has fewer charac- ters than the field width, it will be padded to the field width on the left (default) or right (if the left-adjustment flag - has been given); see below for flag specification. If the field width for an s conversion is preceded by a 0, the string is right ad- justed with zero padding on the left. A precision that gives the minimum number of digits to appear for the d, o, u, x, or X conversions, the number of digits to appear after the decimal point for the e and f conversions, the maximum number of significant April, 1990 1
printf(3S) printf(3S)digits for the g conversion, or the maximum number of characters to be printed from a string in the s conver- sion. The format of the precision is a period (.) followed by a decimal digit string; a null digit string is treated as zero. An optional l (ell) specifying that a following d, o, u, x, or X conversion character applies to a long in- teger arg. An l before any other conversion character is ignored. A character that indicates the type of conversion to be applied. A field width or precision may be indicated by an asterisk (*) instead of a digit string. In this case, an integer arg supplies the field width or precision. The arg that is ac- tually converted is not fetched until the conversion letter is seen; therefore, the args specifying field width or pre- cision must appear before the arg (if any) to be converted. The flag characters and their meanings are: - The result of the conversion will be left- justified within the field. + The result of a signed conversion will always be- gin with a sign (+ or -). blank If the first character of a signed conversion is not a sign, a blank will be prefixed to the result. This implies that if the blank and + flags both appear, the blank flag will be ignored. # This flag specifies that the value is to be con- verted to an ``alternate form.'' For c, d, s, and u conversions, the flag has no effect. For o conversion, it increases the precision to force the first digit of the result to be a zero. For x (X) conversion, a non-zero result will have 0x (0X) prefixed to it. For e, E, f, g, and G conversions, the result will always contain a de- cimal point, even if no digits follow the point (normally, a decimal point appears in the result of these conversions only if a digit follows it). For g and G conversions, trailing zeroes will not be removed from the result (which they normally are). The conversion characters and their meanings are: 2 April, 1990
printf(3S) printf(3S)d,o,u,x,X The integer arg is converted to signed decimal, unsigned octal, decimal, or hexadecimal notation (x and X), respectively; the letters abcdef are used for x conversion and the letters ABCDEF for X conversion. The precision specifies the minimum number of digits to appear; if the value being converted can be represented in fewer digits, it will be expanded with leading zeroes. (For compa- tibility with older versions, padding with leading zeroes may alternatively be specified by prefixing a zero to the field width.) This does not imply an octal value for the field width. The default precision is 1. The result of converting a zero value with a precision of zero is a null string. f The float or double arg is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal point is equal to the precision specification. If the precision is missing, 6 digits are output; if the precision is explicitly 0, no decimal point appears. e,E The float or double arg is converted in the style [-]d.ddde+dd, where there is one digit before the decimal point and the number of digits after it is equal to the precision; when the precision is missing, 6 digits are produced; if the precision is zero, no decimal point appears. The E format code produces a number with E instead of e intro- ducing the exponent. The exponent always contains at least two digits. g,G The float or double arg is printed in style f or e (or in style E in the case of a G format code), with the precision specifying the number of signi- ficant digits. The style used depends on the value converted: style e is used only if the ex- ponent resulting from the conversion is less than -4 or greater than the precision. Trailing zeroes are removed from the result; a decimal point ap- pears only if it is followed by a digit. c The character arg is printed. s The arg is taken to be a string (character pointer) and characters from the string are print- ed until a null character (\0) is encountered or the number of characters indicated by the preci- sion specification is reached. If the precision is missing, it is taken to be infinite, so all characters up to the first null character are printed. A NULL value for arg yields undefined April, 1990 3
printf(3S) printf(3S)results. % Print 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 wid- er than the field width, the field is simply expanded to contain the conversion result. Characters generated by printf and fprintf are printed as if putc(3S) had been called. RETURN VALUE Each function returns the number of characters transmitted (not including the \0 in the case of sprintf), or a negative value if an output error was encountered. EXAMPLES To print a date and time in the form ``Sunday, July 3, 10:02'', where wkday and mnth are pointers to null- terminated strings: printf("%s, %s %d, %.2d:%.2d", wkday, mnth, day, hr, min); To print pi to 5 decimal places: printf("pi=%.5f", 4*atan(1.0)); SEE ALSO ecvt(3C), intro(3), putc(3S), scanf(3S). 4 April, 1990