Provided by: opencl-1.2-man-doc_1.0~svn22836-1.1_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