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Format Specifier <\/strong><\/th>	Data Type<\/strong><\/th>	Description<\/strong><\/th><\/tr><\/thead>
`%d<\/code> or %i<\/code><\/td>`	`int<\/code> (signed)<\/td>`	Prints a signed decimal integer.<\/td><\/tr>
`%f<\/code> or %F<\/code><\/td>`	`float<\/code>, double<\/code><\/td>`	Prints a floating-point number in scientific (exponential) notation.<\/td><\/tr>
`%s<\/code><\/td>`	`char*<\/code>(string)<\/td>`	Prints a sequence of characters until a null terminator (`\\0<\/code>) is found.<\/td><\/tr>`
`%p<\/code><\/td>`	Pointer (`void*<\/code>)<\/td>`	Prints a memory address, typically in hexadecimal format.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n Let's look at a more advanced format string using different format specifiers. To begin we will examine the following C code to be translated to Panama (FFI & FFM) code.<\/p>\n\n\n\n int charCount = printf(\"%s is %d years old and is %.1f feet tall.\\n\", \"Fred\", 60, 5.9d);<\/pre>\n\n\n\n Above you'll notice `%s<\/code>, %d<\/code> and %.1f<\/code> which tells you there are three<\/strong> format specifiers: C string, int, and double(floating point number). Now we can call the makeInvoker()<\/code> method with the appropriate data types generated from jextract. The types would be the following: C_POINTER<\/code>, C_INT<\/code>, and C_DOUBLE<\/code>.<\/p>\n\n\n\n` void main() {\n try (Arena arena = Arena.ofConfined()) { \n \/\/ ******************************************************************************\n \/\/ How to call printf() with format string such as\n \/\/ * printf(\"%s is %d years old and is %.1f feet tall.\\\", \"Fred\", 60, 5.9f);\n \/\/ ******************************************************************************\n MemorySegment formatCStr = arena.allocateFrom(\"%s is %d years old and is %.1f feet tall.\\n\");\n MemorySegment nameCStr = arena.allocateFrom(\"Fred\");\n\n \/\/ Create an instance of a generated org.unix.printf class from jextract. (FYI, class is named all lowercase)\n printf printfAlpha = printf.makeInvoker(\n stdio_h.C_POINTER, \/ 2nd param - C string %s. A pointer to character array address \/\n stdio_h.C_INT, \/ 3rd param - C int %d. Integer value (32bit whole number) \/\n stdio_h.C_DOUBLE); \/ 4th param - C double %f. Floating point decimal. floats promote to a double\/\n\n int charCount = printfAlpha.apply(\n formatCStr,\n nameCStr,\n 60,\n 5.9f);\n }\n }<\/pre>\n\n\n\nThis time I separated the calls makeInvoker()<\/code> and apply()<\/code> instead of the earlier example of method chaining. By separating the calls you can see makeInvoker()<\/code> returns an instance of a printf<\/code> object capable of receiving 3 variadic arguments (\"Fred\", 60, 5.9d<\/code>). The first parameter will be the format string<\/strong> \"%s is %d years old and is %.1f feet tall.\\n\"<\/code>.<\/p>\n\n\n\n The output is:<\/p>\n\n\n\n Fred is 60 years old and is 5.9 feet tall.<\/pre>\n\n\n\nCongratulations for getting this far! While it's nice to create and output C strings. Let's look at how to create primitive data types off of the Java's memory heap. <\/p>\n\n\n\n Let's look at how to create C primitive data types and later how to create arrays of primitives.<\/p>\n\n\n\n Creating\/Getting\/Setting C primitive data types from Java<\/h3>\n\n\n\nTo make this easy to remember think of AllocatorFrom -> MemorySegment -> get\/set<\/code> (aka MemSeg<\/code> pattern). When creating C primitive data (variables) remember that they are like objects where space is allocated and has a memory addresses to change or access the value (dereferenced).<\/p>\n\n\n\n Creating a C String or C primitive call an Arena instance's allocateFrom()<\/code> method.<\/strong><\/p>\n\n\n\n \nString<\/code> - allocateFrom(String str)<\/code>, allocateFrom(String str, CharSet cs)<\/code> e.g.<\/strong> arena.allocateFrom(\"hello\", UTF_8)<\/code><\/li>\n\n\n\n byte<\/code> - allocateFrom(ValueLayout.OfByte layout, byte value)<\/code><\/li>\n\n\n\n boolean<\/code><\/code> - allocateFrom(ValueLayout.OfBoolean layout, boolean<\/code> value)<\/code><\/li>\n\n\n\n char<\/code> - allocateFrom(ValueLayout.OfChar layout, char value)<\/code><\/li>\n\n\n\n double<\/code><\/code> - allocateFrom(ValueLayout.OfDouble<\/code> layout, double<\/code> value)<\/code><\/li>\n\n\n\n float<\/code><\/code> - allocateFrom(ValueLayout.OfFloat layout, float<\/code> value)<\/code><\/li>\n\n\n\n int<\/code><\/code> - allocateFrom(ValueLayout.OfInt layout, int<\/code> value)<\/code><\/li>\n\n\n\n long<\/code><\/code> - allocateFrom(ValueLayout.OfLong layout, long<\/code> value)<\/code><\/li>\n\n\n\n short<\/code><\/code> - allocateFrom(ValueLayout.OfShort layout, short<\/code> value)<\/code><\/li>\n<\/ul>\n\n\n\nNote:<\/strong> I've only listed out the common methods used. There are many other overloaded methods to choose from that will allow you to create arrays, structs and custom memory layouts.<\/p>\n\n\n\n Getting primitive data from a MemorySegment<\/code> object the following are special get() methods:<\/strong><\/p>\n\n\n\n \nbyte - get(ValueLayout.OfByte, long offset)<\/code><\/li>\n\n\n\n boolean - get(ValueLayout.OfBoolean, long offset)<\/code><\/li>\n\n\n\n char - get(ValueLayout.OfChar, long offset)<\/code><\/li>\n\n\n\n double - get(ValueLayout.OfDouble, long offset)<\/code><\/li>\n\n\n\n float - get(ValueLayout.OfFloat, long offset)<\/code><\/li>\n\n\n\n int - get(ValueLayout.OfInt, long offset)<\/code><\/li>\n\n\n\n long - get(ValueLayout.OfLong, long offset)<\/code><\/li>\n\n\n\n short - get(ValueLayout.OfShort, long offset)<\/code><\/li>\n<\/ul>\n\n\n\nNote: there are other overloaded methods to create custom memory layouts<\/p>\n\n\n\n Setting primitive data to a MemorySegment<\/code> object:<\/strong><\/p>\n\n\n\n \nbyte - set(ValueLayout.OfByte, long offset, byte value)<\/code><\/li>\n\n\n\n boolean - set(ValueLayout.OfBoolean, long offset, boolean<\/code> value<\/code>)<\/code><\/li>\n\n\n\n char - set(ValueLayout.OfChar, long offset, char value<\/code>)<\/code><\/li>\n\n\n\n double - set(ValueLayout.OfDouble, long offset, double value<\/code>)<\/code><\/li>\n\n\n\n float - set(ValueLayout.OfFloat, long offset, float value<\/code>)<\/code><\/li>\n\n\n\n int - set(ValueLayout.OfInt, long offset, int value<\/code>)<\/code><\/li>\n\n\n\n long - set(ValueLayout.OfLong, long offset, long value<\/code>)<\/code><\/li>\n\n\n\n short - set(ValueLayout.OfShort, long offset, short value<\/code>)<\/code><\/li>\n<\/ul>\n\n\n\nMemorySegment formatCStr = arena.allocateFrom(\"A slice of %f \\n\");\n\n\/\/ Create an off heap double containing the value of Pi.\nMemorySegment cDouble = arena.allocateFrom(C_DOUBLE, Math.PI);\n\n\/\/ Create an instance of a printf object via makeInvoker() that accepts a double (%f floating point number)\nprintf printfFun = printf.makeInvoker(C_DOUBLE); \n\n\/\/ Invoke printf function such as printf(\"A slice of %f \\n\", 3.141593d); \nprintfFun.apply(formatCStr, cDouble.get(C_DOUBLE, 0)); \/\/ A slice of 3.141593<\/pre>\n\n\n\nOutput:\nA slice of 3.141593<\/code><\/pre>\n\n\n\nOf course you can also call by passing in Java's Math.PI<\/code> (primitive double) into the function.<\/p>\n\n\n\n printfFun.apply(cFormatStr, Math.PI); \/\/ A slice of 3.141593<\/code><\/p>\n\n\n\n Note:<\/strong> It's better to use the jextract<\/code> generated stdio_h.C_DOUBLE<\/code> (as opposed to ValueLayout.JAVA_DOUBLE<\/code>) because it takes on the underlying OS' bit width. It might not occupy the same number of bits if you are assuming it's 64 bits (8 bytes). The following is the C_DOUBLE<\/code> variable of type OfDouble<\/code> that gets generated by jextract<\/code> as follows.<\/p>\n\n\n\n \/\/ jextract generated C_DOUBLE\npublic static final ValueLayout.OfDouble C_DOUBLE = (ValueLayout.OfDouble) Linker.nativeLinker().canonicalLayouts().get(\"double\");<\/pre>\n\n\n\nCreating C primitive arrays<\/h3>\n\n\n\nNow that you know how to create primitive data types let's create C primitive arrays. Shown below is allocating space to hold a single dimensional array off of the Java heap. Then we just re-access the C array using getAtIndex(ValueLayout, index)<\/code> and then displaying the contents.<\/p>\n\n\n\n out.println(\"An array of data\");\nMemorySegment cDoubleArray = arena.allocateFrom(C_DOUBLE, \n 1.0, 2.0, 3.0, 4.0,\n 1.0, 1.0, 1.0, 1.0,\n 3.0, 4.0, 5.0, 6.0,\n 5.0, 6.0, 7.0, 8.0\n);\n\nfor (long i = 0; i < (44); i++) {\n if (i>0 && i % 4 == 0) {\n System.out.println();\n }\n out.printf(\" %f \", cDoubleArray.getAtIndex(C_DOUBLE, i));\n}<\/pre>\n\n\n\nThe output:<\/p>\n\n\n\n An array of data\n 1.000000 2.000000 3.000000 4.000000 \n 1.000000 1.000000 1.000000 1.000000 \n 3.000000 4.000000 5.000000 6.000000 \n 5.000000 6.000000 7.000000 8.000000 <\/pre>\n\n\n\nAs demonstrated above, you can use the MemorySegment.getAtIndex(ValueLayout, index)<\/code> method to extract and display a specific value from the array. <\/p>\n\n\n\n To change the array's values, you'll need to call the MemorySegment.setAtIndex()<\/code> method. The provided example below shows how to fetch each value and multiply it by 3.<\/p>\n\n\n\n for (long i = 0; i < 16; i++) {\n double newVal = cDoubleArray.getAtIndex(C_DOUBLE, i) * 3;\n cDoubleArray.setAtIndex(C_DOUBLE, i, newVal);\n}<\/pre>\n\n\n\nOutput is shown below:<\/p>\n\n\n\n 3.000000 6.000000 9.000000 12.000000 \n 3.000000 3.000000 3.000000 3.000000 \n 9.000000 12.000000 15.000000 18.000000 \n 15.000000 18.000000 21.000000 24.000000 <\/pre>\n\n\n\nConclusion<\/h1>\n\n\n\nIn Part 1, we learned about the what, where, and whys regarding project Panama. Next, we examined the anotomy of a typical Hello World C program. After learning how to use jextract<\/code> to generate Java code from stdio.h<\/code>, we were able to create a Java Hello World to access the C function printf()<\/code>. Lastly, we learned how to create C primitive data types including arrays.<\/p>\n\n\n\n If you're still interested, in Part 2 we will continue our journey into Project Panama<\/a>, by looking at C's concept of structs<\/strong> and pointers<\/strong>.<\/p>\n\n\n\n Resources<\/h1>\n\n\n\n\nPanama4Newbies: https:\/\/github.com\/carldea\/panama4newbies<\/a><\/li>\n\n\n\n Panama4Newbies part 1 source: https:\/\/github.com\/carldea\/panama4newbies\/tree\/main\/part01<\/a><\/li>\n\n\n\n Interconnecting Java and Native Code with the FFM API - JavaOne 2025 : https:\/\/www.youtube.com\/watch?v=Bhh07Rz5enw<\/a><\/li>\n\n\n\n OpenJDK's Project Panama - https:\/\/openjdk.java.net\/projects\/panama\/<\/a><\/li>\n\n\n\n jextract releases - http:\/\/jdk.java.net\/panama\/<\/a><\/li>\n\n\n\n Mailing list: - https:\/\/mail.openjdk.java.net\/mailman\/listinfo\/panama-dev<\/a><\/li>\n\n\n\n Fosdem 2022 Project Panama talk Native Language Access: Project Panama for Newbies (Carl Dea): https:\/\/youtu.be\/REyl4cOsItE<\/a><\/li>\n\n\n\n Panama Examples: https:\/\/hg.openjdk.java.net\/panama\/dev\/raw-file\/4810a7de75cb\/doc\/panama_foreign.html#using-panama-foreign-jdk<\/a><\/li>\n\n\n\n Minecraft - https:\/\/www.minecraft.net\/en-us<\/a><\/li>\n\n\n\n LWJGL and JNI - https:\/\/github.com\/LWJGL\/lwjgl3\/blob\/master\/config\/macos\/build.xml<\/a><\/li>\n\n\n\n JNI: https:\/\/docs.oracle.com\/javase\/8\/docs\/technotes\/guides\/jni\/<\/a><\/li>\n<\/ul>\n<\/body><\/html>\n","protected":false},"excerpt":{"rendered":"In this series of articles, we will explore the APIs from OpenJDK\u2019s Project Panama. <\/p>\n My intent is to show you how to be proficient in using the Foreign Linker APIs (module \u2018jdk.incubator.foreign\u2019) as it relates to Java interoperability with native code.<\/p>\n While this article is for newbies, I assume you know the basics of the Java language, a little bash scripting, and a familiarity with C programming concepts. If you are new to C language don\u2019t worry I will go over the concepts later.<\/p>\n","protected":false},"author":32,"featured_media":46303,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[398,537],"tags":[542,548,549,33,545,546,508,539,541,547,543,35,540,544],"class_list":["post-46300","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-jeps","category-project-panama","tag-c-c","tag-coding","tag-hacking","tag-java","tag-java-17","tag-jep389","tag-jni","tag-minecraft","tag-native","tag-newbies","tag-opengl","tag-openjdk","tag-panama","tag-tensorflow"],"yoast_head":"\nProject Panama for Newbies (Part 1) \| Foojay Today<\/title>\n<meta name=\"description\" content=\"Learn how the Java programming language has new ways to access native libraries that are written in native languages like C, C++, and Fortran.\" \/>\n<meta name=\"robots\" content=\"index, follow, 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What is Project Panama?<\/a><\/span>Why Use Project Panama?<\/a><\/span>Where do I get the jextract tool?<\/a><\/span>Do I need a C compiler?<\/a><\/span>Let\u2019s Do It!<\/a><\/span>Anatomy of a Hello World in C<\/a><\/span>Panama Hello World Example<\/a><\/span>Let\u2019s jextract STDIO please!<\/a><\/span>How does it work?<\/a><\/span>

What about 3rd party C libraries?<\/a><\/li>
Creating\/Getting\/Setting C primitive data types from Java<\/a><\/li>

Creating C primitive arrays<\/a><\/li><\/ul><\/div><\/div>\n

Updated<\/mark> December 28, 2025 (originally published August 10<\/strong>, 2021, republished January 1, 2026):<\/strong> This article now features Java 25 and the Foreign Function & Memory (FFM) API, which has been a standard feature since JDK 22.<\/p>\n\n\n\n