--- name: "xprintf" section: "5" platform: "macos" package: "Libc" version: "1752.120.2" release: "macOS 26.5" platforms: ["macos"] url: "https://new.carta.tech/xprintf/5" --- # xprintf(5) extensible printf ## NAME **xprintf** – extensible printf ## SYNOPSIS **#include \** *typedef int*\ **printf\_arginfo\_function**(*const struct printf\_info \*info*, *size\_t n*, *int \*argtypes*); *typedef int*\ **printf\_function**(*FILE \*stream*, *const struct printf\_info \*info*, *const void \*const \*args*); ## DESCRIPTION The standard [printf(3)](/printf/3.md) family of routines provides a convenient way to convert one or more arguments to various forms for output, under the control of a format string. The format string may contain any number of conversion specifications, which start with the ‘%’ character and end with a conversion specifier character (like ‘d’ or ‘f’), with conversion flag characters in-between. Extensible printf is an enhancement that allows adding new (user-defined) conversion specifiers, or modifying/removing existing ones. The implementation of extensible printf in Mac OS X is derived from the FreeBSD version, which is based on the one in GNU libc (GLIBC). Documentation for the GLIBC version is available at: http://www.gnu.org/software/libc/manual/html\_node/Customizing-Printf.html The main problem with the usual forms of extensible printf is that changes to [printf(3)](/printf/3.md) are program-wide. But this is unsafe, since frameworks, libraries or some other thread could change printf behavior in ways unexpected by the main program, or the latter could unexpectedly affect the former. So instead, the implementation used in Mac OS X makes changes to conversion specifiers within printf domains, which are independent structures containing the specifier definitions. These domains are created as described in [xprintf\_domain(3)](/xprintf_domain/3.md), and once set up, it can be passed to a [xprintf(3)](/xprintf/3.md) variant along with the format string and arguments to generate output. The standard [printf(3)](/printf/3.md) behavior is never affected. To define a new conversion specifier, two function typedefs are defined, and the user must provide two functions based on these typedefs. These functions will get called from extensible printf while processing the corresponding conversion specification. During the first of three phases of extensible printf processing, the format string is parsed, and for each conversion specification, a *struct* *printf\_info* is created, containing the option flags specified in the conversion specification as well as other settings. Important fields in *struct* *printf\_info* are: - *alt* Boolean value whether the ‘#’ flag was specified. - *context* A *void* *\** pointer to arbitrary data specified in the original call to register\_printf\_domain\_function(3). - *group* Boolean value whether the ‘'’ flag was specified. - *is\_char* Boolean value whether the ‘hh’ flag was specified. - *is\_intmax* Boolean value whether the ‘j’ flag was specified. - *is\_long* Boolean value whether the ‘l’ flag was specified. - *is\_long\_double* Boolean value whether the ‘L’ or ‘ll’ flags were specified. - *is\_ptrdiff* Boolean value whether the ‘t’ flag was specified. - *is\_quad* Boolean value whether the ‘q’ flag was specified. - *is\_short* Boolean value whether the ‘h’ flag was specified. - *is\_size* Boolean value whether the ‘z’ flag was specified. - *is\_vec* Boolean value whether the ‘v’ flag was specified. - *left* Boolean value whether the ‘-’ flag was specified. - *loc* The extended locale (see [xlocale(3)](/xlocale/3.md)) specified by the extensible printf caller (never NULL). - *pad* The padding character; either ‘0’ or space. - *prec* The value of the optional precision. -1 means the precision was unspecified. - *showsign* Boolean value whether the ‘+’ flag was specified. - *signchar* The sign character, either ‘+’, space or zero if none. - *space* Boolean value whether the space flag was specified. - *spec* The specifier character itself. - *vsep* The separator character between vector items (using the ‘v’ flag). Can be any one of the four characters “,:;\_” or ‘X’ if no separator character was specified (meaning that a space is used as the separator, unless the specifier is ‘c’, in which case no separator is used). - *width* The value of the minimum field width (defaults to zero). All other structure fields are either unused or private (and shouldn't be used). This *struct* *printf\_info* structure is then passed to the corresponding **printf\_arginfo\_function** callback function. The callback function should return the number of consecutive arguments the specifier handles, including zero (the maximum number of consecutive arguments a single specifier can handle is \_\_PRINTFMAXARG, which is currently set to 2, but could be increased in the future if there is need). The callback function is also passed an integer array and the length of that array; the length will typically be \_\_PRINTFMAXARG. The function should fill out the array up to the number of arguments it expects, using the following values: - PA\_CHAR The argument type is an *int* cast to a *char*. - PA\_DOUBLE The argument type is a *double*. OR-ing PA\_DOUBLE with PA\_FLAG\_LONG\_DOUBLE specifies a *long double* type. - PA\_FLOAT (Defined but unused; best to avoid, since *float* is automatically promoted to *double* anyways.) - PA\_INT The argument type is *int* (either signed or unsigned). The size can be adjusted by OR-ing the following values to PA\_INT: - PA\_FLAG\_INTMAX The integer is the size of a *intmax\_t*. - PA\_FLAG\_LONG The integer is the size of a *long*. - PA\_FLAG\_LONG\_LONG The integer is the size of a *long long*. - PA\_FLAG\_PTRDIFF The integer is the size of a *ptrdiff\_t*. - PA\_FLAG\_QUAD The integer is the size of a *quad\_t* (deprecated). - PA\_FLAG\_SHORT The integer is the size of a *short*. - PA\_FLAG\_SIZE The integer is the size of a *size\_t*. - PA\_POINTER The argument type is a pointer type, cast to a *void \**. - PA\_STRING The argument type is a null-terminated character string (*char \**). - PA\_VECTOR The argument type is an AltiVec or SSE vector (16 bytes). - PA\_WCHAR The argument type is a *wchar\_t*. - PA\_WSTRING The argument type is a null-terminated wide character string (*wchar\_t \**). After the **printf\_arginfo\_function** returns, phase 2 of extensible printf processing involves converting the argument according to the types specified by the returned type array. Note that positional arguments are dealt with here as well. Then in phase 3, output is generated, either from the text in-between the conversion specifications, or by calling the so-called rendering functions associated with each conversion specifier (with typedef **printf\_function**). The rendering function is passed the same *struct* *printf\_info* structure, as well as an array of pointers to each of the arguments converted in phase 2 that it is responsible for. The callback should write its output to the provided output stdio stream, and then return the number of characters written. ## EXAMPLE Here is an example that demonstrates many of the features of extensible printf: ``` #include #include #include #include #include #include /* The Coordinate type */ typedef struct { double x; double y; } Coordinate; #define L (1 << 0) #define P (1 << 1) /* The renderer callback for Coordinate */ static int print_coordinate (FILE *stream, const struct printf_info *info, const void *const *args) { const Coordinate *c; int width, ret, which = 0; char fmt[32]; char *bp, *cp, *ep; /* The optional coordinate labels */ const char **labels = (const char **)info->context; /* Get the argument pointer to a Coordinate */ c = *((const Coordinate **) (args[0])); /* Set up the format string */ cp = fmt; if(info->alt) *cp++ = '('; bp = cp; if(labels) { which |= L; *cp++ = '%'; *cp++ = 's'; } *cp++ = '%'; if(info->group) *cp++ = '\''; *cp++ = '*'; if(info->prec >= 0) { which |= P; *cp++ = '.'; *cp++ = '*'; } *cp++ = 'l'; *cp++ = 'f'; ep = cp; if(info->alt) *cp++ = ','; *cp++ = ' '; while(bp < ep) *cp++ = *bp++; if(info->alt) *cp++ = ')'; *cp = 0; width = info->left ? -info->width : info->width; /* Output to the given stream */ switch(which) { case 0: ret = fprintf_l(stream, info->loc, fmt, width, c->x, width, c->y); break; case L: ret = fprintf_l(stream, info->loc, fmt, labels[0], width, c->x, labels[1], width, c->y); break; case P: ret = fprintf_l(stream, info->loc, fmt, width, info->prec, c->x, width, info->prec, c->y); break; case (L | P): ret = fprintf_l(stream, info->loc, fmt, labels[0], width, info->prec, c->x, labels[1], width, info->prec, c->y); break; } return ret; } /* The arginfo callback for Coordinate */ static int coordinate_arginfo (const struct printf_info *info, size_t n, int *argtypes) { /* We always take exactly one argument and this is a pointer to the structure.. */ if (n > 0) argtypes[0] = PA_POINTER; return 1; } int main (void) { Coordinate mycoordinate = {12345.6789, 3.141593}; printf_domain_t domain; locale_t loc; const char *labels[] = {"x=", "y="}; /* Set up a domain to add support for Coordinate conversion */ domain = new_printf_domain(); if(!domain) err(1, "new_printf_domain"); /* Set up an extended locale to test locale support */ loc = newlocale(LC_ALL_MASK, "uk_UA.UTF-8", NULL); if(!loc) err(1, "newlocale"); /* Register the callbacks for Coordinates in the domain */ register_printf_domain_function (domain, 'C', print_coordinate, coordinate_arginfo, NULL); /* Print the coordinate using the current locale (C). */ xprintf(domain, NULL, "|%'C|\n", &mycoordinate); xprintf(domain, NULL, "|%'14C|\n", &mycoordinate); xprintf(domain, NULL, "|%'-14.2C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#14C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#-14.2C|\n", &mycoordinate); printf("-------------\n"); /* Reregister the callbacks, specifying coordinate labels * and setting the global locale (notice thousands separator) */ register_printf_domain_function (domain, 'C', print_coordinate, coordinate_arginfo, labels); if(setlocale(LC_ALL, "en_US.UTF-8") == NULL) errx(1, "setlocale"); /* Reprint with labels */ xprintf(domain, NULL, "|%'C|\n", &mycoordinate); xprintf(domain, NULL, "|%'14C|\n", &mycoordinate); xprintf(domain, NULL, "|%'-14.2C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#14C|\n", &mycoordinate); xprintf(domain, NULL, "|%'#-14.2C|\n", &mycoordinate); printf("-------------\n"); /* Now print with the test locale (notice decimal point and * thousands separator) */ xprintf(domain, loc, "|%'C|\n", &mycoordinate); xprintf(domain, loc, "|%'14C|\n", &mycoordinate); xprintf(domain, loc, "|%'-14.2C|\n", &mycoordinate); xprintf(domain, loc, "|%'#C|\n", &mycoordinate); xprintf(domain, loc, "|%'#14C|\n", &mycoordinate); xprintf(domain, loc, "|%'#-14.2C|\n", &mycoordinate); return 0; } ``` This example defines a Coordinate type, that consists of a pair of doubles. We create a conversion specifier that displays a Coordinate type, either just as two floating point numbers, or with the ‘#’ (alternate form) flag, as parenthesized numbers separated by a comma. Note the use of **printf\_l** to do the actual output; this is using regular printf from within an extensible printf renderer callback. The use of **printf\_l** also insures correct handling of extended locales. The output of the programs looks like: ``` |12345.678900 3.141593| | 12345.678900 3.141593| |12345.68 3.14 | |(12345.678900, 3.141593)| |( 12345.678900, 3.141593)| |(12345.68 , 3.14 )| ------------- |x=12,345.678900 y=3.141593| |x= 12,345.678900 y= 3.141593| |x=12,345.68 y=3.14 | |(x=12,345.678900, y=3.141593)| |(x= 12,345.678900, y= 3.141593)| |(x=12,345.68 , y=3.14 )| ------------- |x=12 345,678900 y=3,141593| |x= 12 345,678900 y= 3,141593| |x=12 345,68 y=3,14 | |(x=12 345,678900, y=3,141593)| |(x= 12 345,678900, y= 3,141593)| |(x=12 345,68 , y=3,14 )| ``` Notice: - Field width, precision and left adjustment are applied to each of the numbers. - The alternate form, using parenthesized numbers separated by a comma. - In the second group of six, the thousands separator corresponds to the global locale setting (en\_US.UTF-8). - The second and third group have a label for each number, provide through the user-defined context argument. - The third group has the decimal point and thousands separator of the extended locale argument (uk\_UA.UTF-8). ## PERFORMANCE Because of the three phase processing of extensible printf, as well as the use of two callbacks for each conversion specifier, performance is considerably slower than the one pass, highly optimized regular [printf(3)](/printf/3.md). Recursive use of [printf(3)](/printf/3.md) from within an extensible printf renderer callback (as in the EXAMPLE above) adds additional overhead. To ameliorate some of this slowness, the concept of separate compilation and execution phases has be added to extensible printf. The functions in [xprintf\_comp(3)](/xprintf_comp/3.md) allow the creation of pre-compiled extensible printf structures (performing phase one of extensible printf processing). These pre-compiled structures can then be passed to the printf variants in [xprintf\_exec(3)](/xprintf_exec/3.md) to produce the actual output (performing phases 2 and 3). The compilation phase need only be done once, while execution can be performed any number of times. A simple example of use is: ``` printf_comp_t pc = new_printf_comp(domain, loc, "%d: %C\n"); for(i = 0; i = sizeof(coords) / sizeof(*coords); i++) { xprintf_exec(pc, i, &coords[i]); } free_printf_comp(pc); ``` Here, *coords* is a array containing *Coordinate* structures that are to be printed and the *domain* and *loc* variables are as from EXAMPLE above. (Error checking on the return value from **new\_printf\_comp**() is not shown). ## SEE ALSO [printf(3)](/printf/3.md), [xlocale(3)](/xlocale/3.md), [xprintf(3)](/xprintf/3.md), [xprintf\_comp(3)](/xprintf_comp/3.md), [xprintf\_domain(3)](/xprintf_domain/3.md), [xprintf\_exec(3)](/xprintf_exec/3.md)