fprintf outchan format arg1 ... argN formats the arguments arg1 to argN according to the format string format, and outputs the resulting string on the channel outchan.
The format string is a character string which contains two types of objects: plain characters, which are simply copied to the output channel, and conversion specifications, each of which causes conversion and printing of arguments.
Conversion specifications have the following form:
% [flags] [width] [.precision] type
In short, a conversion specification consists in the % character, followed by optional modifiers and a type which is made of one or two characters.
The types and their meanings are:
d, i: convert an integer argument to signed decimal. The flag # adds underscores to large values for readability.u, n, l, L, or N: convert an integer argument to unsigned decimal. Warning: n, l, L, and N are used for scanf, and should not be used for printf. The flag # adds underscores to large values for readability.x: convert an integer argument to unsigned hexadecimal, using lowercase letters. The flag # adds a 0x prefix to non zero values.X: convert an integer argument to unsigned hexadecimal, using uppercase letters. The flag # adds a 0X prefix to non zero values.o: convert an integer argument to unsigned octal. The flag # adds a 0 prefix to non zero values.s: insert a string argument.S: convert a string argument to OCaml syntax (double quotes, escapes).c: insert a character argument.C: convert a character argument to OCaml syntax (single quotes, escapes).f: convert a floating-point argument to decimal notation, in the style dddd.ddd.F: convert a floating-point argument to OCaml syntax (dddd. or dddd.ddd or d.ddd e+-dd). Converts to hexadecimal with the # flag (see h).e or E: convert a floating-point argument to decimal notation, in the style d.ddd e+-dd (mantissa and exponent).g or G: convert a floating-point argument to decimal notation, in style f or e, E (whichever is more compact). Moreover, any trailing zeros are removed from the fractional part of the result and the decimal-point character is removed if there is no fractional part remaining.h or H: convert a floating-point argument to hexadecimal notation, in the style 0xh.hhhh p+-dd (hexadecimal mantissa, exponent in decimal and denotes a power of 2).B: convert a boolean argument to the string true or falseb: convert a boolean argument (deprecated; do not use in new programs).ld, li, lu, lx, lX, lo: convert an int32 argument to the format specified by the second letter (decimal, hexadecimal, etc).nd, ni, nu, nx, nX, no: convert a nativeint argument to the format specified by the second letter.Ld, Li, Lu, Lx, LX, Lo: convert an int64 argument to the format specified by the second letter.a: user-defined printer. Take two arguments and apply the first one to outchan (the current output channel) and to the second argument. The first argument must therefore have type out_channel -> 'b -> unit and the second 'b. The output produced by the function is inserted in the output of fprintf at the current point.t: same as %a, but take only one argument (with type out_channel -> unit) and apply it to outchan.\{ fmt %\}: convert a format string argument to its type digest. The argument must have the same type as the internal format string fmt.( fmt %): format string substitution. Take a format string argument and substitute it to the internal format string fmt to print following arguments. The argument must have the same type as the internal format string fmt.!: take no argument and flush the output.%: take no argument and output one % character.\@: take no argument and output one \@ character.,: take no argument and output nothing: a no-op delimiter for conversion specifications.
The optional flags are:
-: left-justify the output (default is right justification).0: for numerical conversions, pad with zeroes instead of spaces.+: for signed numerical conversions, prefix number with a + sign if positive.- space: for signed numerical conversions, prefix number with a space if positive.
#: request an alternate formatting style for the integer types and the floating-point type F.
The optional width is an integer indicating the minimal width of the result. For instance, %6d prints an integer, prefixing it with spaces to fill at least 6 characters.
The optional precision is a dot . followed by an integer indicating how many digits follow the decimal point in the %f, %e, %E, %h, and %H conversions or the maximum number of significant digits to appear for the %F, %g and %G conversions. For instance, %.4f prints a float with 4 fractional digits.
The integer in a width or precision can also be specified as *, in which case an extra integer argument is taken to specify the corresponding width or precision. This integer argument precedes immediately the argument to print. For instance, %.*f prints a float with as many fractional digits as the value of the argument given before the float.