PRINTF(3) BSD Programmer's Manual PRINTF(3) NNAAMMEE pprriinnttff, ffpprriinnttff, sspprriinnttff, ssnnpprriinnttff, vvpprriinnttff, vvffpprriinnttff,, vvsspprriinnttff, vvssnnpprriinnttff - formatted output conversion SSYYNNOOPPSSIISS ##iinncclluuddee <<ssttddiioo..hh>> _i_n_t pprriinnttff(_c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _._._.); _i_n_t ffpprriinnttff(_F_I_L_E _*_s_t_r_e_a_m, _c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _._._.); _i_n_t sspprriinnttff(_c_h_a_r _*_s_t_r, _c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _._._.); _i_n_t ssnnpprriinnttff(_c_h_a_r _*_s_t_r, _s_i_z_e___t _s_i_z_e, _c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _._._.); ##iinncclluuddee <<ssttddaarrgg..hh>> _i_n_t vvpprriinnttff(_c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _v_a___l_i_s_t _a_p); _i_n_t vvffpprriinnttff(_F_I_L_E _*_s_t_r_e_a_m, _c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _v_a___l_i_s_t _a_p); _i_n_t vvsspprriinnttff(_c_h_a_r _*_s_t_r, _c_h_a_r _*_f_o_r_m_a_t, _v_a___l_i_s_t _a_p); _i_n_t vvssnnpprriinnttff(_c_h_a_r _*_s_t_r, _s_i_z_e___t _s_i_z_e, _c_o_n_s_t _c_h_a_r _*_f_o_r_m_a_t, _v_a___l_i_s_t _a_p); DDEESSCCRRIIPPTTIIOONN The pprriinnttff() family of functions produces output according to a _f_o_r_m_a_t as described below. PPrriinnttff() and vvpprriinnttff() write output to _s_t_d_o_u_t_, the standard output stream; ffpprriinnttff() and vvffpprriinnttff() write output to the giv- en output _s_t_r_e_a_m; sspprriinnttff(), ssnnpprriinnttff(), vvsspprriinnttff(), and vvssnnpprriinnttff() write to the character string _s_t_r. These functions write the output under the control of a _f_o_r_m_a_t string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for output. These functions return the number of characters printed (not including the trailing `\0' used to end output to strings). SSnnpprriinnttff() and vvssnnpprriinnttff() will write at most _s_i_z_e-1 of the characters printed into the output string (the _s_i_z_e'th character then gets the terminating `\0'); if the return value is greater than or equal to the _s_i_z_e argument, the string was too short and some of the printed characters were discarded. SSpprriinnttff() and vvsspprriinnttff() effectively assume an infinite _s_i_z_e. 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: ++oo Zero or more of the following flags: -- A ## character specifying that the value should be converted to an ``alternate form''. For cc, dd, ii, nn, pp, ss, and uu, conversions, this option has no effect. For oo conversions, the precision of the number is increased to force the first character of the out- put string to a zero (except if a zero value is printed with an explicit precision of zero). For xx and XX conversions, a non-zero result has the string `0x' (or `0X' for XX conversions) prepended to it. For ee, EE, ff, gg, and GG, conversions, the result will al- ways contain a decimal point, even if no digits follow it (nor- mally, a decimal point appears in the results of those conver- sions only if a digit follows). For gg and GG conversions, trail- ing zeros are not removed from the result as they would otherwise be. -- A zero `00' character specifying zero padding. For all conver- sions except nn, the converted value is padded on the left with zeros rather than blanks. If a precision is given with a numeric conversion (Mc d, ii, oo, uu, ii, xx, and XX), the `00' flag is ignored. -- A negative field width flag `--' indicates the converted value is to be left adjusted on the field boundary. Except for nn conver- sions, the converted value is padded on the right with blanks, rather than on the left with blanks or zeros. A `--' overrides a `00' if both are given. -- A space, specifying that a blank should be left before a positive number produced by a signed conversion (dd, ee, EE, ff, gg, GG, or ii). -- 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. ++oo 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. ++oo 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 dd, ii, oo, uu, xx, and XX conversions, the number of digits to appear after the decimal-point for ee, EE, and ff conversions, the maximum num- ber of significant digits for gg and GG conversions, or the maximum number of characters to be printed from a string for ss conversions. ++oo The optional character hh, specifying that a following dd, ii, oo, uu, xx, or XX conversion corresponds to a _s_h_o_r_t _i_n_t or _u_n_s_i_g_n_e_d _s_h_o_r_t _i_n_t ar- gument, or that a following nn conversion corresponds to a pointer to a _s_h_o_r_t _i_n_t argument. ++oo The optional character ll (ell) specifying that a following dd, ii, oo, uu, xx, or XX conversion applies to a pointer to a _l_o_n_g _i_n_t or _u_n_s_i_g_n_e_d _l_o_n_g _i_n_t argument, or that a following nn conversion corresponds to a pointer to a _l_o_n_g _i_n_t argument. ++oo The optional character qq, specifying that a following dd, ii, oo, uu, xx, or XX conversion corresponds to a _q_u_a_d _i_n_t or _u_n_s_i_g_n_e_d _q_u_a_d _i_n_t argu- ment, or that a following nn conversion corresponds to a pointer to a _q_u_a_d _i_n_t argument. ++oo The character LL specifying that a following ee, EE, ff, gg, or GG conver- sion corresponds to a _l_o_n_g _d_o_u_b_l_e argument (but note that long double values are not currently supported by the VAX and Tahoe compilers). ++oo A character that specifies 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 _i_n_t 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. The conversion specifiers and their meanings are: ddiioouuxxXX The _i_n_t (or appropriate variant) argument is converted to signed decimal (dd and ii), unsigned octal (oo), unsigned decimal (uu), or unsigned hexadecimal (xx and XX) notation. The letters aabbccddeeff are used for xx conversions; the letters AABBCCDDEEFF are used for conver- sions. 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. DDOOUU The _l_o_n_g _i_n_t argument is converted to signed decimal, unsigned octal, or unsigned decimal, as if the format had been lldd, lloo, or lluu respectively. These conversion characters are deprecated, and will eventually disappear. eeEE The _d_o_u_b_l_e argument is rounded and converted in the style [-]d..dddee+-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 EE con- version uses the letter EE (rather than ee) to introduce the expo- nent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. ff The _d_o_u_b_l_e 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 _d_o_u_b_l_e argument is converted in style ff or ee (or EE for GG con- versions). 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 ee 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. cc The _i_n_t argument is converted to an _u_n_s_i_g_n_e_d _c_h_a_r, and the re- sulting character is written. ss The ``_c_h_a_r _*'' argument is expected to be a pointer to an array of character type (pointer to a string). Characters from the ar- ray are written up to (but not including) a terminating NUL char- acter; if a precision is specified, no more than the number spec- ified 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. pp The ``_v_o_i_d _*'' pointer argument is printed in hexadecimal (as if by `%#x' or `%#lx'). nn The number of characters written so far is stored into the inte- ger indicated by the ``_i_n_t _*'' (or variant) pointer argument. No argument is converted. %% A `%' is written. No argument is converted. The complete conver- sion specification is `%%'. 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. EEXXAAMMPPLLEESS To print a date and time in the form `Sunday, July 3, 10:02', where _w_e_e_k_d_a_y and _m_o_n_t_h 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); } SSEEEE AALLSSOO printf(1), scanf(3) SSTTAANNDDAARRDDSS The ffpprriinnttff(), pprriinnttff(), sspprriinnttff(), vvpprriinnttff(), vvffpprriinnttff(), and vvsspprriinnttff() functions conform to ANSI C X3.159-1989 (``ANSI C ''). HHIISSTTOORRYY The functions ssnnpprriinnttff() and vvssnnpprriinnttff() are new to this release. BBUUGGSS The conversion formats %%DD, %%OO, and %%UU are not standard and are provided only for backward compatibility. The effect of padding the %%pp format with zeros (either by the `00' flag or by specifying a precision), and the benign effect (i.e., none) of the `##' flag on %%nn and %%pp conversions, as well as other nonsensical combinations such as %%LLdd, are not standard; such combinations should be avoided. Because sspprriinnttff() and vvsspprriinnttff() 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 ssnnpprriinnttff() interface instead. Unfortunately, this interface is not portable. 4.4BSD June 4, 1993 4