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PRINTF(3) BSD Library Functions Manual PRINTF(3) NAME printf, fprintf, sprintf, snprintf, asprintf, vprintf, vfprintf, vsprintf, vsnprintf, vasprintf -- formatted output conversion LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <stdio.h> int printf(const char *format, ...); int fprintf(FILE *stream, const char *format, ...); int sprintf(char *str, const char *format, ...); int snprintf(char *str, size_t size, const char *format, ...); int asprintf(char **ret, const char *format, ...); #include <stdarg.h> int vprintf(const char *format, va_list ap); int vfprintf(FILE *stream, const char *format, va_list ap); int vsprintf(char *str, const char *format, va_list ap); int vsnprintf(char *str, size_t size, const char *format, va_list ap); int vasprintf(char **ret, const char *format, va_list ap); DESCRIPTION The printf() family of functions produces output according to a format as described below. Printf() and vprintf() write output to stdout, the standard output stream; fprintf() and vfprintf() write output to the given output stream; sprintf(), snprintf(), vsprintf(), and vsnprintf() write to the character string str; and asprintf() and vasprintf() dynami- cally allocate a new string with malloc(3). These functions write the output under the control of a format string that specifies how subsequent arguments (or arguments accessed via the variable-length argument facilities of stdarg(3)) are converted for out- put. These functions return the number of characters printed (not including the trailing `\0' used to end output to strings), except for snprintf() and vsnprintf(), which return the number of characters that would have been printed if the size were unlimited (again, not including the final `\0'). Asprintf() and vasprintf() set *ret to be a pointer to a buffer suffi- ciently large to hold the formatted string. This pointer should be passed to free(3) to release the allocated storage when it is no longer needed. If sufficient space cannot be allocated, asprintf() and vasprintf() will return -1 and set ret to be a NULL pointer. Snprintf() and vsnprintf() will write at most size-1 of the characters printed into the output string (the size'th character then gets the ter- minating `\0'); 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 infinite size. 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: o An optional field, consisting of a decimal digit string followed by a $, specifying the next argument to access. If this field is not pro- vided, the argument following the last argument accessed will be used. Arguments are numbered starting at 1. If unaccessed arguments in the format string are interspersed with ones that are accessed the results will be indeterminate. o Zero or more of the following flags: - 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 ex- plicit 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. - A 0 (zero) character specifying zero padding. For all conver- sions 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. - A negative field width flag - indicates the converted value is to be left adjusted on the field boundary. Except for n conver- sions, 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. - 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). - 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. o 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-adjust- ment flag has been given) to fill out the field width. o 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 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 num- ber of significant digits for g and G conversions, or the maximum number of characters to be printed from a string for s conversions. o The optional character h, specifying that a following d, i, o, u, x, or X conversion corresponds to a short int or unsigned short int ar- gument, or that a following n conversion corresponds to a pointer to a short int argument. o The optional character l (ell) specifying that a following d, i, o, u, x, or X conversion applies to a pointer to a long int or unsigned long int argument, or that a following n conversion corresponds to a pointer to a long int argument. o The optional characters ll (ell ell) specifying that a following d, i, o, u, x, or X conversion applies to a pointer to a long long int or unsigned long long int argument, or that a following n conversion corresponds to a pointer to a long long int argument. o The optional character q, specifying that a following d, i, o, u, x, or X conversion corresponds to a quad int or unsigned quad int argu- ment, or that a following n conversion corresponds to a pointer to a quad int argument. o The character L specifying that a following e, E, f, g, or G conver- sion corresponds to a long double argument. o A character that specifies the type of conversion to be applied. A field width or precision, or both, may be indicated by an asterisk `*' or an asterisk followed by one or more decimal digits and a `$' instead of a digit string. In this case, an int argument supplies the field width or precision. A negative field width is treated as a left adjust- ment 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. The conversion specifiers and their meanings are: 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. The letters abcdef are used for x conversions; the letters ABCDEF are used for X 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. 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. eE The double argument is rounded and converted in the style [-]d.ddde+-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 E con- version uses the letter E (rather than e) to introduce the expo- nent. The exponent always contains at least two digits; if the value is zero, the exponent is 00. 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 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 double argument is converted in style f or e (or E for G 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 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. c The int argument is converted to an unsigned char, and the re- sulting character is written. s The 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 NUL charac- ter; if a precision is specified, no more than the number speci- fied 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. p The void * pointer argument is printed in hexadecimal (as if by `%#x' or `%#lx'). n The number of characters written so far is stored into the inte- ger indicated by the int * (or variant) pointer argument. No ar- gument is converted. % A `%' is written. No argument is converted. The complete con- version 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. EXAMPLES To print a date and time in the form "Sunday, July 3, 10:02", where weekday and month 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); } SEE ALSO printf(1), scanf(3) STANDARDS The fprintf(), printf(), sprintf(), vprintf(), vfprintf(), and vsprintf() functions conform to ISO/IEC 9899:1990 ("ISO C90"). HISTORY The functions asprintf() and vasprintf() first appeared in the GNU C li- brary. These were implemented by Peter Wemm <peter@FreeBSD.org> in FreeBSD 2.2, but were later replaced with a different implementation from Todd C. Miller <Todd.Miller@courtesan.com> for OpenBSD 2.3. BUGS The conversion formats %D, %O, and are not standard and are provided only for backward compatibility. The effect of padding the format with zeros (either by the 0 flag or by specifying a precision), and the benign ef- fect (i.e., none) of the # flag on %n and %p conversions, as well as other nonsensical combinations such as %Ld, are not standard; such combi- nations should be avoided. Because sprintf() and vsprintf() assume an infinitely long string, call- ers must be careful not to overflow the actual space; this is often hard to assure. For safety, programmers should use the snprintf() interface instead. Unfortunately, this interface is not portable. BSD June 4, 1993 BSD
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | EXAMPLES | SEE ALSO | STANDARDS | HISTORY | BUGS
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