curl_mprintf.3: add description

These functions have existed in the API since the dawn of time. It is
about time we describe how they work, even if we discourage users from
using them.

Closes #7010

1 files changed, 186 insertions, 36 deletions

diff --git a/docs/libcurl/curl_mprintf.3 b/docs/libcurl/curl_mprintf.3
index ae82c4dd4..99f5236b8 100644
--- a/docs/libcurl/curl_mprintf.3
+++ b/docs/libcurl/curl_mprintf.3
@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -47,57 +47,207 @@ curl_mvsprintf - formatted output conversion
 .br
 .BI "char *curl_mvaprintf(const char *" format ", va_list " args ");"
 .SH DESCRIPTION
-These are all functions that produce output according to a format string and
-given arguments. These are mostly clones of the well-known C-style functions
-and there will be no detailed explanation of all available formatting rules
-and usage here.
+These functions produce output according to the format string and given
+arguments. They are mostly clones of the well-known C-style functions but
+there are slight differences in behavior.
 
-See this table for notable exceptions.
-.RS
+We discourage users from using any of these functions in new applications.
+
+Functions in the curl_mprintf() family produce output according to a format as
+described below.  The functions \fBcurl_mprintf()\fP and \fBcurl_mvprintf()\fP
+write output to stdout, the standard output stream; \fBcurl_mfprintf()\fP and
+\fBcurl_mvfprintf()\fP write output to the given output stream;
+\fBcurl_msprintf()\fP, \fBcurl_msnprintf()\fP, \fBcurl_mvsprintf()\fP, and
+\fBcurl_mvsnprintf()\fP write to the character string \fBbuffer\fP.
+
+The functions \fBcurl_msnprintf()\fP and \fBcurl_mvsnprintf()\fP write at most
+\fImaxlength\fP bytes (including the terminating null byte ('\\0')) to
+\fIbuffer\fP.
+
+The functions \fBcurl_mvprintf()\fP, \fBcurl_mvfprintf()\fP,
+\fBcurl_mvsprintf()\fP, \fBcurl_mvsnprintf()\fP are equivalent to the
+functions \fBcurl_mprintf()\fP, \fBcurl_mfprintf()\fP, \fBcurl_msprintf()\fP,
+\fBcurl_msnprintf()\fP, respectively, except that they are called with a
+va_list instead of a variable number of arguments.  These functions do not
+call the va_end macro.  Because they invoke the va_arg macro, the value of ap
+is undefined after the call.
+
+The functions \fBcurl_maprintf()\fP and \fBcurl_mvaprintf()\fP return the
+output string as pointer to a newly allocated memory area. The returned string
+must be \fIcurl_free(3)\fPed by the receiver.
+
+All of these functions write the output under the control of a format string
+that specifies how subsequent arguments are converted for output.
+
+.SH FORMAT STRING
+The format string is composed of zero or more directives: ordinary characters
+(not %), which are copied unchanged to the output stream; and conversion
+specifications, each of which results in fetching zero or more subsequent
+arguments. Each conversion specification is introduced by the character %, and
+ends with a conversion specifier. In between there may be (in this order) zero
+or more \fIflags\fP, an optional minimum \fIfield width\fP, an optional
+\fIprecision\fP and an optional \fIlength modifier\fP.
+
+.SH "The $ modifier"
+The arguments must correspond properly with the conversion specifier. By
+default, the arguments are used in the order given, where each '*' (see Field
+width and Precision below) and each conversion specifier asks for the next
+argument (and it is an error if insufficiently many arguments are given).  One
+can also specify explicitly which argument is taken, at each place where an
+argument is required, by writing "%m$" instead of '%' and "*m$" instead
+of '*', where the decimal integer m denotes the position in the argument list
+of the desired argument, indexed starting from 1.  Thus,
+
+    curl_mprintf("%*d", width, num);
+
+and
+
+    curl_mprintf("%2$*1$d", width, num);
+
+are equivalent. The second style allows repeated references to the same
+argument.
+
+If the style using '$' is used, it must be used throughout for all conversions
+taking an argument and all width and precision arguments, but it may be mixed
+with "%%" formats, which do not consume an argument.  There may be no gaps in
+the numbers of argu‐ ments specified using '$'; for example, if arguments 1
+and 3 are specified, argument 2 must also be specified somewhere in the format
+string.
+
+.SH "Flag characters"
+The character % is followed by zero or more of the following flags:
+.TP
+.B #
+The value should be converted to its "alternate form".
+.TP
+.B 0
+The value should be zero padded.
+.TP
+.B -
+The converted value is to be left adjusted on the field boundary.  (The
+default is right justification.)  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.
+.TP
+.B ' '
+(a space) A blank should be left before a positive number (or empty string)
+produced by a signed conversion.
+.TP
+.B +
+A sign (+ or -) should always be placed before a number produced by a signed
+conversion. By default, a sign is used only for negative numbers.  A '+'
+overrides a space if both are used.
+.SH "Field width"
+An optional decimal digit string (with nonzero first digit) 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). Instead of a decimal digit string one
+may write "*" or "*m$" (for some decimal integer m) to specify that the field
+width is given in the next argument, or in the m-th argument, respec‐ tively,
+which must be of type int.  A negative field width is taken as a '-' flag
+followed by a positive field width.  In no case does a nonexistent or small
+field width cause truncation of a field; if the result of a conversion is
+wider than the field width, the field is expanded to contain the conversion
+result.
+.SH "Precision"
+An optional precision in the form of a period ('.') followed by an optional
+decimal digit string. Instead of a decimal digit string one may write "*" or
+"*m$" (for some decimal integer m) to specify that the precision is given in
+the next argument, or in the m-th argument, respectively, which must be of
+type int. If the precision is given as just '.', the precision is taken to be
+zero. A negative precision is taken as if the precision were omitted. This
+gives the minimum number of digits to appear for \fBd\fP, \fBi\fP, \fBo\fP,
+\fBu\fP, \fBx\fP, and \fBX\fP conversions, the number of digits to appear
+after the radix character for \fBa\fP, \fBA\fP, \fBe\fP, \fBE\fP, \fBf\fP, and
+\fBF\fP conversions, the maximum number of significant digits for \fBg\fP and
+\fBG\fP conversions, or the maximum number of characters to be printed from a
+string for \fBs\fP and \fBS\fP conversions.
+.SH "Length modifier"
+.TP
+.B h
+A following integer conversion corresponds to a \fIshort\fP or \fIunsigned
+short\fP argument.
+.TP
+.B l
+(ell) A following integer conversion corresponds to a \fIlong\fP or
+\fIunsigned long\fP argument, or a following n conversion corresponds to a
+pointer to a long argument
+.TP
+.B ll
+(ell-ell).  A following integer conversion corresponds to a \fIlong long\fP or
+\fIunsigned long long\fP argument, or a following n conversion corresponds to
+a pointer to a long long argument.
+.TP
+.B q
+A synonym for \fBll\fP.
+.TP
+.B L
+A following a, A, e, E, f, F, g, or G conversion corresponds to a long double
+argument.
+.TP
+.B z
+A following integer conversion corresponds to a \fIsize_t\fP or \fIssize_t\fP
+argument.
+.SH "Conversion specifiers"
+A character that specifies the type of conversion to be applied. The
+conversion specifiers and their meanings are:
 .TP
-.B curl_mprintf()
-Normal printf() clone.
+.B d, i
+The int argument is converted to signed decimal notation.  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. The default
+precision is 1. When 0 is printed with an explicit precision 0, the output is
+empty.
 .TP
-.B curl_mfprintf()
-Normal fprintf() clone.
+.B o, u, x, X
+The unsigned int argument is converted to unsigned octal (o), unsigned decimal
+(u), or unsigned hexadecimal (\fBx\fP and \fBX\fP) notation.  The letters
+abcdef are used for \fBx\fP conversions; the letters ABCDEF are used for
+\fBX\fP conversions. 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.  The default precision is 1.  When 0 is printed with
+an explicit precision 0, the output is empty.
 .TP
-.B curl_msprintf()
-Normal sprintf() clone.
+.B e, E
+The double argument is rounded and output in the style "[-]d.ddde±dd"
 .TP
-.B curl_msnprintf()
-snprintf() clone. Many systems don't have this. It is just like \fBsprintf\fP
-but with an extra argument after the buffer that specifies the length of the
-target buffer.
+.B f, F
+The double argument is rounded and output to decimal notiation in the style
+[-]ddd.ddd.
 .TP
-.B curl_mvprintf()
-Normal vprintf() clone.
+.B g, G
+The double argument is converted in style f or e.
 .TP
-.B curl_mvfprintf()
-Normal vfprintf() clone.
+.B c
+The int argument is converted to an unsigned char, and the resulting character
+is written.
 .TP
-.B curl_mvsprintf()
-Normal vsprintf() clone.
+.B s
+The const 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 null byte. If a precision is specified, no more
+than the number specified are written. If a precision is given, no null byte
+need be present; if the precision is not specified, or is greater than the
+size of the array, the array must contain a terminating null byte.
 .TP
-.B curl_mvsnprintf()
-vsnprintf() clone.  Many systems don't have this. It is just like
-\fBvsprintf\fP but with an extra argument after the buffer that specifies the
-length of the target buffer.
+.B p
+The \fIvoid *\fP pointer argument is printed in hexadecimal.
 .TP
-.B curl_maprintf()
-Like printf() but returns the output string as a malloc()ed string. The
-returned string must be free()ed by the receiver.
+.B n
+The number of characters written so far is stored into the integer pointed to
+by the corresponding argument.
 .TP
-.B curl_mvaprintf()
-Like curl_maprintf() but takes a va_list pointer argument instead of a
-variable amount of arguments.
-.RE
+.B %
+A '%' is written. No argument is converted.
 .SH AVAILABILITY
 These functions will be removed from the public libcurl API in the future. Do
-not use them in any new programs or projects.
+not use them in new programs or projects.
 .SH RETURN VALUE
 The \fBcurl_maprintf\fP and \fBcurl_mvaprintf\fP functions return a pointer to
 a newly allocated string, or NULL if it failed.
 
-All other functions return the number of characters they actually outputted.
+All other functions return the number of characters they actually
+outputted. Note that this differs from how the POSIX versions of these
+functions work.
 .SH "SEE ALSO"
 .BR printf "(3), " sprintf "(3), " fprintf "(3), " vprintf "(3) "