printf(S) 6 January 1993 printf(S) Name printf, fprintf, sprintf - print formatted output Syntax cc . . . -lc #include <stdio.h> int printf(format, arg . . . const char *format; int fprintf(stream, format, arg . . . FILE *stream; const char *format; int sprintf(s, format [, arg ] . . . const char *s; char *format; Description The printf function places output on the standard output stream stdout. fprintf places output on the named output stream. sprintf places ``out- put,'' followed by the null character (\0), in consecutive bytes starting at *s; it is the user's responsibility to ensure that enough storage is available. 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. Each of these functions converts, formats, and prints its args under con- trol of the format. The format is a character string that contains three types of objects: plain characters, which are simply copied to the output stream; escape sequences that represent non-graphic characters; and conversion specifications, each of which results in fetching of zero or more args. The results are undefined if there are insufficient args for the format. If the format is exhausted 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 speci- fication. + An optional, decimal digit string specifying a minimum field width. If the converted value has fewer characters than the field width, it is padded on the left (or right, if the left-adjustment flag `-', described below, has been given) to the field width. The padding is with blanks unless the field width digit string starts with a zero, in which case the padding is with zeros. + A precision that gives the minimum number of digits to appear for the d, i, o, u, x, or X conversions, the number of digits to appear after the decimal point for the e, E, and f conversions, the maximum number of significant digits for the g and G conversion, or the maximum num- ber of characters to be printed from a string in s conversion. The precision takes the form of a period (``.'') followed by a decimal digit string; a null digit string is treated as zero. Padding speci- fied by the precision overrides the padding specified by the field width. + An optional l (ell) specifying that a following d, i, o, u, x, or X conversion character applies to a long integer 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 or both 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 actually converted is not fetched until the conversion letter is seen, so the args specifying field width or precision must appear before the arg (if any) to be converted. A negative field width argument is taken as a `-' flag followed by a positive field width. If the precision argument is negative, it is changed to zero. The flag characters and their meanings are: - The result of the conversion is left-justified within the field. + The result of a signed conversion always begins with a sign (+ or -). <Space> If the first character of a signed conversion is not a sign, a blank is prefixed to the result. This implies that if the blank and + flags both appear, the blank flag is ignored. # This flag specifies that the value is to be converted to an ``alternate form.'' For c, d, i, 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 or X conversion, a non-zero result has 0x or 0X prefixed to it. For e, E, f, g, and G conversions, the result always contains a decimal 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 are not removed from the result (which they normally are). The conversion characters and their meanings are: d, i, o, u, x, X The integer arg is converted to signed decimal (d or i) , unsigned octal, (o) , decimal (u) , or hexadecimal notation (x or 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 is expanded with leading zeroes. 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, six 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, six digits are produced; if the precision is zero, no decimal point appears. The E format code produces a number with E instead of e introducing 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 significant digits. The style used depends on the value converted: style e is used only if the exponent resulting from the conversion is less than -4 or greater than the precision. Trailing zeroes are removed from the result; a decimal point appears only if it is fol- lowed 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 printed until a null charac- ter (\0) is encountered or the number of characters indi- cated by the precision 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 results. % Print a %; no argument is converted. In printing floating point types (float and double), if the exponent is 0x7FF and the mantissa is not equal to zero, then the output is [-]NaN0xdddddddd where 0xdddddddd is the hexadecimal representation of the leftmost 32 bits of the mantissa. If the mantissa is zero, the output is [+-]inf In no case does a nonexistent or small field width cause truncation of a field; if the result of a conversion is wider 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(S) had been called. Examples 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 %i, %d:%.2d", weekday, month, day, hour, min); To print pi to 5 decimal places: printf("pi = %.5f", 4 * atan(1.0)); See also ecvt(S), putc(S), scanf(S), stdio(S) Standards conformance fprintf, printf and sprintf are conformant with: AT&T SVID Issue 2; X/Open Portability Guide, Issue 3, 1989; ANSI X3.159-1989 Programming Language -- C; Intel386 Binary Compatibility Specification, Edition 2 (iBCSe2); IEEE POSIX Std 1003.1-1990 System Application Program Interface (API) [C Language] (ISO/IEC 9945-1); and NIST FIPS 151-1.