curl_printf(3) Library Functions Manual curl_printf(3)
NAME
curl_maprintf, curl_mfprintf, curl_mprintf, curl_msnprintf,
curl_msprintf, curl_mvaprintf, curl_mvfprintf, curl_mvprintf, curl_mvs-
nprintf, curl_mvsprintf - formatted output conversion
SYNOPSIS
#include <curl/mprintf.h>
int curl_mprintf(const char *format, ...);
int curl_mfprintf(FILE *fd, const char *format, ...);
int curl_msprintf(char *buffer, const char *format, ...);
int curl_msnprintf(char *buffer, size_t maxlength, const char *format, ...);
int curl_mvprintf(const char *format, va_list args);
int curl_mvfprintf(FILE *fd, const char *format, va_list args);
int curl_mvsprintf(char *buffer, const char *format, va_list args);
int curl_mvsnprintf(char *buffer, size_t maxlength, const char *format,
va_list args);
char *curl_maprintf(const char *format , ...);
char *curl_mvaprintf(const char *format, va_list args);
DESCRIPTION
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.
We discourage users from using any of these functions in new applica-
tions.
Functions in the curl_mprintf() family produce output according to a
format as described below. The functions curl_mprintf() and
curl_mvprintf() write output to stdout, the standard output stream;
curl_mfprintf() and curl_mvfprintf() write output to the given output
stream; curl_msprintf(), curl_msnprintf(), curl_mvsprintf(), and
curl_mvsnprintf() write to the character string buffer.
The functions curl_msnprintf() and curl_mvsnprintf() write at most
maxlength bytes (including the terminating null byte ('0')) to buffer.
The functions curl_mvprintf(), curl_mvfprintf(), curl_mvsprintf(),
curl_mvsnprintf() are equivalent to the functions curl_mprintf(),
curl_mfprintf(), curl_msprintf(), curl_msnprintf(), respectively, except
that they are called with a va_list instead of a variable number of ar-
guments. These functions do not call the va_end macro. Because they in-
voke the va_arg macro, the value of ap is undefined after the call.
The functions curl_maprintf() and curl_mvaprintf() return the output
string as pointer to a newly allocated memory area. The returned string
must be curl_free(3)ed 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.
FORMAT STRING
The format string is composed of zero or more directives: ordinary char-
acters (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 flags, an optional minimum
field width, an optional precision and an optional length modifier.
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 '$' is used, it must be used throughout for all con-
versions taking an argument and all width and precision arguments, but
it may be mixed with "%%" formats, which do not consume an argument.
There may be no gaps in the numbers of arguments specified using '$';
for example, if arguments 1 and 3 are specified, argument 2 must also be
specified somewhere in the format string.
Flag characters
The character % is followed by zero or more of the following flags:
# The value should be converted to its "alternate form".
0 The value should be zero padded.
- 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.
(space)
(a space: ' ') A blank should be left before a positive number
(or empty string) produced by a signed conversion.
+ A sign (+ or -) should always be placed before a number produced
by a signed conversion. By default, a sign is used only for nega-
tive numbers. A '+' overrides a space if both are used.
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 gets 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$" (for some decimal integer m) to specify
that the field width is given in the next argument, or in the m-th argu-
ment, respectively, which must be of type int. A negative field width is
taken as a '-' flag followed by a positive field width. In no case does
a nonexistent or small field width cause truncation of a field; if the
result of a conversion is wider than the field width, the field is ex-
panded to contain the conversion result.
Precision
An optional precision in the form of a period ('.') followed by an op-
tional decimal digit string. Instead of a decimal digit string one may
write "*" or "*m$" (for some decimal integer m) to specify that the pre-
cision is given in the next argument, or in the m-th argument, respec-
tively, 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 d, i, o, u, x, and X conversions, the number of digits to
appear after the radix character for a, A, e, E, f, and F conversions,
the maximum number of significant digits for g and G conversions, or the
maximum number of characters to be printed from a string for s and S
conversions.
Length modifier
h A following integer conversion corresponds to a short or unsigned
short argument.
l (ell) A following integer conversion corresponds to a long or un-
signed long argument, or a following n conversion corresponds to
a pointer to a long argument
ll (ell-ell). A following integer conversion corresponds to a long
long or unsigned long long argument, or a following n conversion
corresponds to a pointer to a long long argument.
q A synonym for ll.
L A following a, A, e, E, f, F, g, or G conversion corresponds to a
long double argument.
z A following integer conversion corresponds to a size_t or ssize_t
argument.
Conversion specifiers
A character that specifies the type of conversion to be applied. The
conversion specifiers and their meanings are:
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.
o, u, x, X
The unsigned int argument is converted to unsigned octal (o), un-
signed decimal (u), or unsigned hexadecimal (x and X) notation.
The letters abcdef are used for x conversions; the letters ABCDEF
are used for X conversions. The precision, if any, gives the min-
imum 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 preci-
sion 0, the output is empty.
e, E The double argument is rounded and output in the style
"[-]d.ddde{+|-}dd"
f, F The double argument is rounded and output to decimal notation in
the style "[-]ddd.ddd".
g, G The double argument is converted in style f or e.
c The int argument is converted to an unsigned char, and the re-
sulting character is written.
s The const char * 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 null
byte. If a precision is specified, no more than the number speci-
fied 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.
p The void * pointer argument is printed in hexadecimal.
n The number of characters written so far is stored into the inte-
ger pointed to by the corresponding argument.
% A '%' symbol is written. No argument is converted.
PROTOCOLS
This functionality affects all supported protocols
EXAMPLE
const char *name = "John";
int main(void)
{
curl_mprintf("My name is %s\n", name);
curl_mprintf("Pi is almost %f\n", (double)25.0/8);
}
AVAILABILITY
Added in curl 7.1
RETURN VALUE
The curl_maprintf and curl_mvaprintf functions return a pointer to a
newly allocated string, or NULL if it failed.
All other functions return the number of characters actually printed
(excluding the null byte used to end output to strings). Note that this
sometimes differ from how the POSIX versions of these functions work.
SEE ALSO
fprintf(3), printf(3), sprintf(3), vprintf(3)
libcurl 2026-07-12 curl_printf(3)
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