Note: Calling assembly functions from C

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+---
+title: "Calling assembly functions from C"
+url: calling-assembly-functions-from-c.html
+date: 2024-06-17T16:12:13+02:00
+type: note
+draft: false
+---
+
+This is using the portable GNU assembler and TinyCC compiler but GCC or Clang
+can be used as well.
+
+First lets define a simple function in assembly.
+
+```asm
+# sum.s
+.intel_syntax noprefix
+
+.global sum
+
+.text
+
+sum:
+    add rdi, rsi
+    mov rax, rdi
+    ret
+```
+
+Lets compile this with GNU assembler `as sum.s -o sum.o`.
+
+Now we need a C program that calls this function.
+
+```c
+// main.c
+#include <stdio.h>
+
+// We need to define the signature of the function.
+int sum(int a, int b);
+
+int main() {
+    for(int i=0; i<10; ++i) {
+        printf("SUM of 5+%d is %d\n", i, sum(5, i));
+    }
+    return 0;
+}
+```
+
+Now lets compile and link into final program with `tcc main.c sum.o -o main`.
+