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Name

printf, fprintf, sprintf - print formatted output

Synopsis

#include <stdio.h>

int printf(const char *format, /* args */ ... );

int fprintf(FILE *strm, const char *format, /* args */ ... );

int sprintf(char *s, const char *format, /* args */ ... );

MT-Level

See the NOTES section of this page.

Description


printf() places output on the standard output stream stdout.

fprintf() places output on strm.

sprintf() places output, 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 control of the format. The format is a character string that contains three types of objects defined below:

    .
  1. plain characters that are simply copied to the output stream;
    2. .
  2. escape sequences that represent non-graphic characters;
    3. .
  3. conversion specifications.

The following escape sequences produce the associated action on display devices capable of the action:

\a
Alert. Ring the bell.
\b
Backspace. Move the printing position to one character before the current position, unless the current position is the start of a line.
\f
Form feed. Move the printing position to the initial printing position of the next logical page.
\n
Newline. Move the printing position to the start of the next line.
\r
Carriage return. Move the printing position to the start of the current line.
\t
Horizontal tab. Move the printing position to the next implementation-defined horizontal tab position on the current line.
\v
Vertical tab. Move the printing position to the start of the next implementation-defined vertical tab position.

All forms of the printf() functions allow for the insertion of a language-dependent decimal-point character. The decimal-point character is defined by the program’s locale (category LC_NUMERIC ). In the C locale, or in a locale where the decimal-point character is not defined, the decimal-point character defaults to a period (.).

Each conversion specification is introduced by the character %. After the character %, the following appear in sequence:

An optional field, consisting of a decimal digit string followed by a $, specifying the next args to be converted. If this field is not provided, the args following the last args converted will be used.

Zero or more flags, which modify the meaning of the conversion specification.

An optional string of decimal digits to specify a minimum 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 (-), described below, has been given) to the field width. If the format is %s or %ws (wide-character string), then the field width should be interpreted as the minimum columns of screen display. E.g. %10s means if the converted value has a screen width of 7 columns, then 3 spaces would be padded on the right.

An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, or X conversions (the field is padded with leading zeros), the number of digits to appear after the decimal-point character for the e, E, and f conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of characters to be printed from a string in s or ws conversions. The precision takes the form of a period (.) followed by a decimal digit string; a null digit string is treated as zero. Padding specified by the precision overrides the padding specified by the field width.

An optional h specifies that a following d, i, o, u, x, or X conversion specifier applies to a short int or unsigned short int argument (the argument will be promoted according to the integral promotions and its value converted to short int or unsigned short int before printing); an optional h specifies that a following n conversion specifier applies to a pointer to a short int argument. An optional l (ell) specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; an optional l (ell) specifies that a following n conversion specifier applies to a pointer to a long int argument. An optional ll (ell ell) specifies that a following d, i, o, u, x, or X conversion specifier applies to a long long int or unsigned long long int argument; an optional ll (ell ell) specifies that a following n conversion specifier applies to a pointer to a long long int argument. An optional L specifies that a following e, E, f, g, or G conversion specifier applies to a long double argument. If an h, l, or L appears before any other conversion specifier, the behavior is undefined.

A conversion character (see below) 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 args supplies the field width or precision. The args 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 args (if any) to be converted. If the precision argument is negative, it will be changed to zero. A negative field width argument is taken as a - flag, followed by a positive field width.

In format strings containing the *digits$ form of a conversion specification, a field width or precision may also be indicated by the sequence *digits$, giving the position in the argument list of an integer args containing the field width or precision.

When numbered argument specifications are used, specifying the Nth argument requires that all the leading arguments, from the first to the (N-1)th, be specified in the format string.

The flag characters and their meanings are:

-
The result of the conversion will be left-justified within the field. (It will be right-justified if this flag is not specified.)
+
The result of a signed conversion will always begin with a sign (+ or -). (It will begin with a sign only when a negative value is converted if this flag is not specified.)
space
If the first character of a signed conversion is not a sign, a space will be placed before the result. This means that if the space and + flags both appear, the space flag will be ignored.
#
The value is to be converted to an alternate form. For c, d, i, s, and u conversions, the flag has no effect. For an 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 will have 0x (or 0X) prepended to it. For e, E, f, g, and G conversions, the result will always contain a decimal-point character, 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 zeros will not be removed from the result as they normally are.
  • For d, i, o, u, x, X, e, E, f, g, and G conversions, leading zeros (following any indication of sign or base) are used to pad to the field width; no space padding is performed. If the 0 and en flags both appear, the 0 flag will be ignored. For d, i, o, u, x, and X conversions, if a precision is specified, the 0 flag will be ignored. For other conversions, the behavior is undefined.

    • Each conversion character results in fetching 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 ignored.

    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) , unsigned decimal (u), or unsigned hexadecimal notation (x and X). The x conversion uses the letters abcdef and the X conversion uses the letters ABCDEF . The precision specifies the minimum number of digits to appear. If the value being converted can be represented in fewer digits than the specified minimum, it will be expanded with leading zeros. The default precision is 1. The result of converting a zero value with a precision of zero is no characters.
    f
    The double args is converted to decimal notation in the style [-]ddd.ddd, where the number of digits after the decimal-point character (see setlocale(3C) ) is equal to the precision specification. If the precision is omitted from arg, six digits are output; if the precision is explicitly zero and the # flag is not specified, no decimal-point character appears. If a decimal-point character appears, at least 1 digit appears before it. The value is rounded to the appropriate number of digits.
    e,E
    The double args is converted to the style [-]d.ddde±dd, where there is one digit before the decimal-point character (which is non-zero if the argument is non-zero) 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 and the # flag is not specified, no decimal-point character appears. The E conversion character will produce a number with E instead of e introducing the exponent. The exponent always contains at least two digits. The value is rounded to the appropriate number of digits.
    g,G
    The double args is printed in style f or e (or in style E in the case of a G conversion character), with the precision specifying the number of significant digits. If the precision is zero, it is taken as one. The style used depends on the value converted: style e (or E) will be used only if the exponent resulting from the 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 character appears only if it is followed by a digit.
    c
    The int args is converted to an unsigned char, and the resulting character is printed.
    wc
    The int args is converted to a wide character (wchar_t), and the resulting wide character is printed.
    s
    The args is taken to be a string (character pointer) and characters from the string are written up to (but not including) a terminating null character; if the precision is specified, no more than that many characters are written. If the precision is not specified, it is taken to be infinite, so all characters up to the first null character are printed. A null value for args will yield undefined results.
    ws
    The args is taken to be a wide character string (wide character pointer) and wide characters from the string are written up to (but not including) a terminating null character; if the precision is specified, no more than that many wide characters are written. If the precision is not specified, it is taken to be infinite, so all wide characters up to the first null character are printed. A null value for args will yield undefined results.
    p
    The args should be a pointer to void. The value of the pointer is converted to an implementation-defined set of sequences of printable characters, which should be the same as the set of sequences that are matched by the %p conversion of the scanf function.
    n
    The argument should be a pointer to an integer into which is written the number of characters written to the output standard I/O stream so far by this call to printf(), fprintf(), or sprintf(). No argument is converted.
    %
    Print a %; no argument is converted.

    If the character after the % or %digits$ sequence is not a valid conversion character, the results of the conversion are undefined.

    If a floating-point value is the internal representation for infinity, the output is [±]Infinity, where Infinity is either Infinity or Inf, depending on the desired output string length. Printing of the sign follows the rules described above.

    If a floating-point value is the internal representation for ‘‘not-a-number,’’ the output is [±]NaN. Printing of the sign follows the rules described above.

    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 simply expanded to contain the conversion result. Characters generated by printf and fprintf are printed as if the putc() routine had been called.

    LC_NUMERIC
    Determines how numeric formats are handled. In the "C" locale, numeric handling follows the U.S. rules.

    Return Values

    printf(), fprintf(), and sprintf() return the number of characters transmitted, or return a negative value if an error was encountered.

    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 *p to 5 decimal places: 


    printf("pi = %.5f", 4 * atan(1.0));

    To print a list of names in columns which are 20 characters wide: 


    printf("%20s%20s%20s", lastname, firstname, middlename );

    Files

    /usr/lib/locale/locale/LC_NUMERIC/numeric
    LC_NUMERIC database for locale

    See Also

    exit(2) , lseek(2) , write(2) , abort(3C) , ecvt(3C) , putc(3S) , scanf(3S) , setlocale(3C) , stdio(3S)

    Notes

    sprintf() is MT-S afe in multi-thread applications. printf and fprintf can be used safely in a multi-thread application, as long as setlocale(3C) is not being called to change the locale.

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