Provided by: opencl-1.2-man-doc_1.0~svn33624-5_all 
NAME
printf_Function - The OpenCL C programming language implements the printf function.
int printf(constant char * restrict format, ...);
DESCRIPTION
The printf built-in function writes output to an implementation-defined stream such as
stdout under control of the string pointed to by format that specifies how subsequent
arguments are converted for output. If there are insufficient arguments for the format,
the behavior is undefined. If the format is exhausted while arguments remain, the excess
arguments are evaluated (as always) but are otherwise ignored. The printf function returns
when the end of the format string is encountered.
NOTES
printf returns 0 if it was executed successfully and -1 otherwise. printf output
synchronization.PP When the event that is associated with a particular kernel invocation
is completed, the output of all printf() calls executed by this kernel invocation is
flushed to the implementation-defined output stream. Calling clFinish(3clc) on a command
queue flushes all pending output by printf in previously enqueued and completed commands
to the implementation-defined output stream. In the case that printf is executed from
multiple work-items concurrently, there is no guarantee of ordering with respect to
written data. For example, it is valid for the output of a work-item with a global id
(0,0,1) to appear intermixed with the output of a work-item with a global id (0,0,4) and
so on. printf format string.PP The format shall be a character sequence, beginning and
ending in its initial shift state. The format 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, converting them, if applicable, according to the corresponding conversion
specifier, and then writing the result to the output stream. As format is in the constant
address space it must be resolvable at compile time and thus cannot be dynamically created
by the executing program, itself.
Each conversion specification is introduced by the character %. After the %, the following
appear in sequence:
• Zero or more flags (in any order) that modify the meaning of the conversion
specification.
• An optional minimum field width. If the converted value has fewer characters than the
field width, it is padded with spaces (by default) on the left (or right, if the left
adjustment flag, described later, has been given) to the field width. The field width
takes the form of a nonnegative decimal integer.) Note that 0 is taken as a flag, not
as the beginning of a field width.
• An optional precision that gives the minimum number of digits to appear for the d, i,
o, u, x, and X conversions, the number of digits to appear after the decimal point
character for a, A, e, E, f, and F conversions, the maximum number of significant
digits for the g and G conversions, or the maximum number of bytes to be written for s
conversions. The precision takes the form of a period (.) followed by an optional
decimal integer; if only the period is specified, the precision is taken as zero. If a
precision appears with any other conversion specifier, the behavior is undefined.
• An optional vector specifier.
• A length modifier that specifies the size of the argument. The length modifier is
required with a vector specifier and together specifies the vector type. Implicit
conversions between vector types are disallowed (as per section 6.2.1). If the vector
specifier is not specified, the length modifier is optional.
• A conversion specifier character that specifies the type of conversion to be applied.
The flag characters and their meanings are:
┌──────┬──────────────────────────────────┐
│- │ The result of the conversion is │
│ │ left-justified within the field. │
│ │ (It is right-justified if this │
│ │ flag is not specified.) │
├──────┼──────────────────────────────────┤
│+ │ The result of a signed │
│ │ conversion always begins with a │
│ │ plus or minus sign. (It begins │
│ │ with a sign only when a negative │
│ │ value is converted if this flag │
│ │ is not specified.) The results │
│ │ of all floating conversions of a │
│ │ negative zero, and of negative │
│ │ values that round to zero, │
│ │ include a minus sign. │
├──────┼──────────────────────────────────┤
│space │ If the first character of a │
│ │ signed conversion is not a sign, │
│ │ or if a signed conversion │
│ │ results in no characters, a │
│ │ space is prefixed to the result. │
│ │ If the space and + flags both │
│ │ appear, the space flag is │
│ │ ignored. │
├──────┼──────────────────────────────────┤
│# │ The result is converted to an │
│ │ "alternative form". For o │
│ │ conversion, it increases the │
│ │ precision, if and only if │
│ │ necessary, to force the first │
│ │ digit of the result to be a zero │
│ │ (if the value and precision are │
│ │ both 0, a single 0 is printed). │
│ │ For x (or X) conversion, a │
│ │ nonzero result has 0x (or 0X) │
│ │ prefixed to it. For a, A, e, E, │
│ │ f, F, g, and G conversions, the │
│ │ result of converting a │
│ │ floating-point number always │
│ │ contains a decimal-point │
│ │ character, even if no digits │
│ │ follow it. (Normally, a │
│ │ decimal-point character appears │
│ │ in the result of these │
│ │ conversions only if a digit │
│ │ follows it.) For g and G │
│ │ conversions, trailing zeros are │
│ │ not removed from the result. For │
│ │ other conversions, the behavior │
│ │ is undefined. │
├──────┼──────────────────────────────────┤
│0 │ For d, i, o, u, x, X, a, A, e, │
│ │ E, f, F, g, and G conversions, │
│ │ leading zeros (following any │
│ │ indication of sign or base) are │
│ │ used to pad to the field width │
│ │ rather than performing space │
│ │ padding, except when converting │
│ │ an infinity or NaN. If the 0 │
│ │ and - flags both appear, the 0 │
│ │ flag is ignored. For d, i, o, u, │
│ │ x, and X conversions, if a │
│ │ precision is specified, the 0 │
│ │ flag is ignored. For other │
│ │ conversions, the behavior is │
│ │ undefined. │
└──────┴──────────────────────────────────┘
The vector specifier and its meaning is:
┌───┬──────────────────────────────────┐
│vn │ Specifies that a following a, A, │
│ │ e, E, f, F, g, G, d, i, o, u, x, │
│ │ or X conversion specifier │
│ │ applies to a vector argument, │
│ │ where n is the size of the │
│ │ vector and must be 2, 3, 4, 8 or │
│ │ 16. The vector value is │
│ │ displayed in the following │
│ │ general form: │
│ │ │
│ │ │
│ │ value1 C value2 C ... C │
│ │ valuen │
│ │ │
│ │ │
│ │ │
│ │ where C is a separator │
│ │ character. The value for │
│ │ this separator character │
│ │ is a comma. │
└───┴──────────────────────────────────┘
If the vector specifier is not used, the length modifiers and their meanings are:
┌────────┬──────────────────────────────────┐
│hh │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a char or │
│ │ uchar argument (the argument │
│ │ will have been promoted │
│ │ according to the integer │
│ │ promotions, but its value shall │
│ │ be converted to char or uchar │
│ │ before printing). │
├────────┼──────────────────────────────────┤
│h │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a short or │
│ │ ushort argument (the argument │
│ │ will have been promoted │
│ │ according to the integer │
│ │ promotions, but its value shall │
│ │ be converted to short or │
│ │ unsigned short before printing). │
├────────┼──────────────────────────────────┤
│l (ell) │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a long or │
│ │ ulong argument. The l modifier │
│ │ is supported by the full │
│ │ profile. For the embedded │
│ │ profile, the l modifier is │
│ │ supported only if 64-bit │
│ │ integers are supported by the │
│ │ device. │
└────────┴──────────────────────────────────┘
If the vector specifier is used, the length modifiers and their meanings are:
┌────────┬──────────────────────────────────┐
│hh │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a charn or │
│ │ ucharn argument (the argument │
│ │ will not be promoted). │
├────────┼──────────────────────────────────┤
│h │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a shortn or │
│ │ ushortn argument (the argument │
│ │ will not be promoted); that a │
│ │ following a, A, e, E, f, F, g, │
│ │ or G conversion specifier │
│ │ applies to a halfn argument if │
│ │ the cl_khr_fp16(3clc) extension │
│ │ is supported. │
├────────┼──────────────────────────────────┤
│hl │ This modifier can only be used │
│ │ with the vector specifier. │
│ │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a intn or │
│ │ uintn argument; that a following │
│ │ a, A, e, E, f, F, g, or G │
│ │ conversion specifier applies to │
│ │ a floatn argument. │
├────────┼──────────────────────────────────┤
│l (ell) │ Specifies that a following d, i, │
│ │ o, u, x, or X conversion │
│ │ specifier applies to a longn or │
│ │ ulongn argument; that a │
│ │ following a, A, e, E, f, F, g, │
│ │ or G conversion specifier │
│ │ applies to a doublen argument. │
│ │ The l modifier is supported by │
│ │ the full profile. For the │
│ │ embedded profile, the l modifier │
│ │ is supported only if 64-bit │
│ │ integers or double-precision │
│ │ floating-point are supported by │
│ │ the device. │
└────────┴──────────────────────────────────┘
If a vector specifier appears without a length modifier, the behavior is undefined. The
vector data type described by the vector specifier and length modifier must match the data
type of the argument; otherwise the behavior is undefined.
If a length modifier appears with any conversion specifier other than as specified above,
the behavior is undefined
The conversion specifiers and their meanings are:
┌────────┬──────────────────────────────────┐
│d,i │ The int, charn, shortn, intn or │
│ │ longn argument is converted to │
│ │ signed decimal in the style │
│ │ [−]dddd. The precision specifies │
│ │ the minimum number of digits to │
│ │ appear; if the value being │
│ │ converted can be represented in │
│ │ fewer digits, it is expanded │
│ │ with leading zeros. The default │
│ │ precision is 1. The result of │
│ │ converting a zero value with a │
│ │ precision of zero is no │
│ │ characters. │
├────────┼──────────────────────────────────┤
│o,u,x,X │ The unsigned int, ucharn, │
│ │ ushortn, uintn or ulongn │
│ │ argument is converted to │
│ │ unsigned octal (o), unsigned │
│ │ decimal (u), or unsigned │
│ │ hexadecimal notation (x or X) in │
│ │ the style dddd; the letters │
│ │ abcdef are used for x conversion │
│ │ and the letters ABCDEF for X │
│ │ conversion. The precision │
│ │ specifies the minimum number of │
│ │ digits to appear; if the value │
│ │ being converted can be │
│ │ represented in fewer digits, it │
│ │ is 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,F │ A double, halfn, floatn or │
│ │ doublen argument representing a │
│ │ floating-point number is │
│ │ converted to decimal notation in │
│ │ the style [−]ddd.ddd, where the │
│ │ number of digits after the │
│ │ decimal-point character is equal │
│ │ to the precision specification. │
│ │ If the precision is missing, it │
│ │ is taken as 6; if the precision │
│ │ is zero and the # flag is not │
│ │ specified, no decimal-point │
│ │ character appears. If a │
│ │ decimal-point character appears, │
│ │ at least one digit appears │
│ │ before it. The value is rounded │
│ │ to the appropriate number of │
│ │ digits. A double, halfn, floatn │
│ │ or doublen argument representing │
│ │ an infinity is converted in one │
│ │ of the styles [-]inf or │
│ │ [-]infinity — which style is │
│ │ implementation-defined. A │
│ │ double, halfn, floatn or doublen │
│ │ argument representing a NaN is │
│ │ converted in one of the styles │
│ │ [- ]nan or │
│ │ [-]nan(n-char-sequence) — which │
│ │ style, and the meaning of any │
│ │ n-charsequence, is │
│ │ implementation-defined. The F │
│ │ conversion specifier produces │
│ │ INF, INFINITY, or NAN instead of │
│ │ inf, infinity, or nan, │
│ │ respectively. When applied to │
│ │ infinite and NaN values, the -, │
│ │ +, and space flag characters │
│ │ have their usual meaning; the # │
│ │ and 0 flag characters have no │
│ │ effect. │
├────────┼──────────────────────────────────┤
│e,E │ A double, halfn, floatn or │
│ │ doublen argument representing a │
│ │ floating-point number is │
│ │ converted in the style [−]d.ddd │
│ │ e±dd, where there is one digit │
│ │ (which is nonzero if the │
│ │ argument is nonzero) before the │
│ │ decimal-point character and the │
│ │ number of digits after it is │
│ │ equal to the precision; if the │
│ │ precision is missing, it is │
│ │ taken as 6; if the precision is │
│ │ zero and the # flag is not │
│ │ specified, no decimal-point │
│ │ character appears. The value is │
│ │ rounded to the appropriate │
│ │ number of digits. The E │
│ │ conversion specifier produces a │
│ │ number with E instead of e │
│ │ introducing the exponent. The │
│ │ exponent always contains at │
│ │ least two digits, and only as │
│ │ many more digits as necessary to │
│ │ represent the exponent. If the │
│ │ value is zero, the exponent is │
│ │ zero. A double, halfn, floatn or │
│ │ doublen argument representing an │
│ │ infinity or NaN is converted in │
│ │ the style of an f or F │
│ │ conversion specifier │
├────────┼──────────────────────────────────┤
│g,G │ A double, halfn, floatn or │
│ │ doublen argument representing a │
│ │ floating-point number is │
│ │ converted in style f or e (or in │
│ │ style F or E in the case of a G │
│ │ conversion specifier), depending │
│ │ on the value converted and the │
│ │ precision. Let P equal the │
│ │ precision if nonzero, 6 if the │
│ │ precision is omitted, or 1 if │
│ │ the precision is zero. Then, if │
│ │ a conversion with style E would │
│ │ have an exponent of X: — if P > │
│ │ X ≥ −4, the conversion is with │
│ │ style f (or F) and precision P − │
│ │ (X + 1). — otherwise, the │
│ │ conversion is with style e (or │
│ │ E) and precision P − 1. │
│ │ Finally, unless the # flag is │
│ │ used, any trailing zeros are │
│ │ removed from the fractional │
│ │ portion of the result and the │
│ │ decimal-point character is │
│ │ removed if there is no │
│ │ fractional portion remaining. A │
│ │ double, halfn, floatn or doublen │
│ │ e argument representing an │
│ │ infinity or NaN is converted in │
│ │ the style of an f or F │
│ │ conversion specifier. │
├────────┼──────────────────────────────────┤
│a,A │ A double, halfn, floatn or │
│ │ doublen argument representing a │
│ │ floating-point number is │
│ │ converted in the style │
│ │ [−]0xh.hhhh p±d, where there is │
│ │ one hexadecimal digit (which is │
│ │ nonzero if the argument is a │
│ │ normalized floating-point number │
│ │ and is otherwise unspecified) │
│ │ before the decimal-point │
│ │ character) and the number of │
│ │ hexadecimal digits after it is │
│ │ equal to the precision; if the │
│ │ precision is missing, then the │
│ │ precision is sufficient for an │
│ │ exact representation of the │
│ │ value; if the precision is zero │
│ │ and the # flag is not specified, │
│ │ no decimal point character │
│ │ appears. The letters abcdef are │
│ │ used for a conversion and the │
│ │ letters ABCDEF for A conversion. │
│ │ The A conversion specifier │
│ │ produces a number with X and P │
│ │ instead of x and p. The exponent │
│ │ always contains at least one │
│ │ digit, and only as many more │
│ │ digits as necessary to represent │
│ │ the decimal exponent of 2. If │
│ │ the value is zero, the exponent │
│ │ is zero. A double, halfn, │
│ │ floatn or doublen argument │
│ │ representing an infinity or NaN │
│ │ is converted in the style of an │
│ │ f or F conversion specifier. │
│ │ Binary implementations can │
│ │ choose the hexadecimal digit to │
│ │ the left of the decimal-point │
│ │ character so that subsequent │
│ │ digits align to nibble (4-bit) │
│ │ boundaries. │
└────────┴──────────────────────────────────┘
NOTE: The conversion specifiers e,E,g,G,a,A convert a float or half argument that is a
scalar type to a double only if the double data type is supported. If the double data type
is not supported, the argument will be a float instead of a double and the half type will
be converted to a float.
┌──┬──────────────────────────────────┐
│c │ The int argument is converted to │
│ │ an unsigned char, and the │
│ │ resulting character is written. │
├──┼──────────────────────────────────┤
│s │ The argument shall be a literal │
│ │ string. No special provisions │
│ │ are made for multibyte │
│ │ characters. The behavior of │
│ │ printf with the s conversion │
│ │ specifier is undefined if the │
│ │ argument value is not a pointer │
│ │ to a literal string. Characters │
│ │ from the literal string array │
│ │ are written up to (but not │
│ │ including) the terminating null │
│ │ character. If the precision is │
│ │ specified, no more than that │
│ │ many bytes are written. If the │
│ │ precision is not specified or is │
│ │ greater than the size of the │
│ │ array, the array shall contain a │
│ │ null character. │
├──┼──────────────────────────────────┤
│p │ The argument shall be a pointer │
│ │ to void. The pointer can refer │
│ │ to a memory region in the │
│ │ global, constant, local or │
│ │ private address space. The value │
│ │ of the pointer is converted to a │
│ │ sequence of printing characters │
│ │ in an implementation-defined │
│ │ manner. │
├──┼──────────────────────────────────┤
│% │ A % character is written. No │
│ │ argument is converted. The │
│ │ complete conversion │
│ │ specification shall be %%. │
└──┴──────────────────────────────────┘
If a conversion specification is invalid, the behavior is undefined. If any argument is
not the correct type for the corresponding conversion specification, the behavior is
undefined.
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.
For a and A conversions, the value is correctly rounded to a hexadecimal floating number
with the given precision. Differences between OpenCL C and C99 printf
• The l modifier followed by a c conversion specifier or s conversion specifier is not
supported by OpenCL C.
• The ll, j, z, t, and L length modifiers are not supported by OpenCL C.
• The n conversion specifier is not supported by OpenCL C but is reserved.
• OpenCL C adds the optional vn vector specifier to support printing of vector types.
• The conversion specifiers f, F, e, E, g, G, a, A convert a float argument to a double
only if the double data type is supported. Refer to the description of
CL_DEVICE_DOUBLE_FP_CONFIG. If the double data type is not supported, the argument
will be a float instead of a double.
• For the embedded profile, the l length modifier is supported only if 64-bit integers
are supported.
• In OpenCL C, printf returns 0 if it was executed successfully and -1 otherwise vs.
C99 where printf returns the number of characters printed or a negative value if an
output or encoding error occurred.
• In OpenCL C, the conversion specifier s can only be used for arguments that are
literal strings.
Examples
float4 f = (float4)(1.0f, 2.0f, 3.0f, 4.0f);
uchar4 uc = (uchar4)(0xFA, 0xFB, 0xFC, 0xFD);
printf("f4 = %2.2v4hlf\n", f); printf("uc =
%#v4hhx\n", uc);
// The above two calls print the following:
f4 = 1.00,2.00,3.00,4.00 uc =
0xfa,0xfb,0xfc,0xfd
A few examples of valid use cases of printf for the conversion specifier s are given
below. The argument value must be a pointer to a literal string.
kernel void my_kernel( ... ) { printf("%s\n",
"this is a test string\n"); }
A few examples of invalid use cases of printf for the conversion specifier s are given
below:
kernel void my_kernel(global char *s, ... ) {
printf("%s\n", s);
constant char *p = "this is a test string\n";
printf("%s\n", p); printf("%s\n", &p[3]); }
A few examples of invalid use cases of printf where data types given by the vector
specifier and length modifier do not match the argument type are given below:
kernel void my_kernel(global char *s, ... ) {
uint2 ui = (uint2)(0x12345678, 0x87654321);
printf("unsigned short value = (%#v2hx)\n", ui)
printf("unsigned char value = (%#v2hhx)\n", ui)
}
SPECIFICATION
OpenCL Specification[1]
AUTHORS
The Khronos Group
COPYRIGHT
Copyright © 2007-2011 The Khronos Group Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy of this
software and/or associated documentation files (the "Materials"), to deal in the Materials
without restriction, including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Materials, and to permit
persons to whom the Materials are furnished to do so, subject to the condition that this
copyright notice and permission notice shall be included in all copies or substantial
portions of the Materials.
NOTES
1. OpenCL Specification
page 284, section 6.12.13 - printf