typedef struct {
  union {
    unsigned long l_symndx;  /* function name symbol index */
    unsigned long l_paddr;   /* address of line number     */
  } l_addr;
  unsigned short l_lnno;     /* line number                */
} LINENO;

Warning: This structure's size is not a multiple of four. When reading from file, it is strongly recommended that either (1) you read each entry in a loop, reading LINESZ bytes each time, or allocate a block of memory and calculate a pointer to each entry you need by multiplying by LINESZ. In no case should you assume that array addressing or sizeof(LINENO) will be useful.

Each executable section has its own line number table. Each function in that section is numbered independently, with the start of the function (the line with the opening brace) numbered as line one for that function. Each function in the line number table will have one entry where l_lnno is zero and the symbol table entry for the function is in l_symndx. This entry is followed by entries for each line of the function, with l_lnno set to the function-relative line number (1..N) and l_paddr set to the address of the first assembler codes for that line.

To figure out absolute line numbers, you must look in the symbol table for the function, find the "beginning of function" symbol (type C_FCN, usually right after the function's C_EXT or C_STAT symbol) where the absolute line number for the function (equivalent to line one in the line number table's scheme) is stored (in AUXENT.x_sym.x_misc.x_lnsz.x_lnno), and add that to the relative line numbers in the table.

The trick to getting line numbers right is to remember that the lines of the source file start at one (the first line in the file is line one) and functions are numbered starting at one also. When you add them up, you get one too many ones, so you must then subtract one to get the right line number.