NASM 0.98bf

[nasm/avx512.git] / outelf.c

/* outelf.c     output routines for the Netwide Assembler to produce
 *              ELF32 (i386 of course) object file format
 *
 * The Netwide Assembler is copyright (C) 1996 Simon Tatham and
 * Julian Hall. All rights reserved. The software is
 * redistributable under the licence given in the file "Licence"
 * distributed in the NASM archive.
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

#include "nasm.h"
#include "nasmlib.h"
#include "outform.h"

#ifdef OF_ELF

/*
 * Relocation types.
 */
#define R_386_32 1                     /* ordinary absolute relocation */
#define R_386_PC32 2                   /* PC-relative relocation */
#define R_386_GOT32 3                  /* an offset into GOT */
#define R_386_PLT32 4                  /* a PC-relative offset into PLT */
#define R_386_GOTOFF 9                 /* an offset from GOT base */
#define R_386_GOTPC 10                 /* a PC-relative offset _to_ GOT */

struct Reloc {
    struct Reloc *next;
    long address;                      /* relative to _start_ of section */
    long symbol;                       /* ELF symbol info thingy */
    int type;                          /* type of relocation */
};

struct Symbol {
    long strpos;                       /* string table position of name */
    long section;                      /* section ID of the symbol */
    int type;                          /* symbol type */
    long value;                        /* address, or COMMON variable align */
    long size;                         /* size of symbol */
    long globnum;                      /* symbol table offset if global */
    struct Symbol *next;               /* list of globals in each section */
    struct Symbol *nextfwd;            /* list of unresolved-size symbols */
    char *name;                        /* used temporarily if in above list */
};

#define SHT_PROGBITS 1
#define SHT_NOBITS 8

#define SHF_WRITE 1
#define SHF_ALLOC 2
#define SHF_EXECINSTR 4

struct Section {
    struct SAA *data;
    unsigned long len, size, nrelocs;
    long index;
    int type;                          /* SHT_PROGBITS or SHT_NOBITS */
    int align;                         /* alignment: power of two */
    unsigned long flags;               /* section flags */
    char *name;
    struct SAA *rel;
    long rellen;
    struct Reloc *head, **tail;
    struct Symbol *gsyms;              /* global symbols in section */
};

#define SECT_DELTA 32
static struct Section **sects;
static int nsects, sectlen;

#define SHSTR_DELTA 256
static char *shstrtab;
static int shstrtablen, shstrtabsize;

static struct SAA *syms;
static unsigned long nlocals, nglobs;

static long def_seg;

static struct RAA *bsym;

static struct SAA *strs;
static unsigned long strslen;

static FILE *elffp;
static efunc error;
static evalfunc evaluate;

static struct Symbol *fwds;

static char elf_module[FILENAME_MAX];

extern struct ofmt of_elf;

#define SHN_ABS 0xFFF1
#define SHN_COMMON 0xFFF2
#define SHN_UNDEF 0

#define SYM_SECTION 0x04
#define SYM_GLOBAL 0x10
#define SYM_DATA 0x01
#define SYM_FUNCTION 0x02

#define GLOBAL_TEMP_BASE 6             /* bigger than any constant sym id */

#define SEG_ALIGN 16                   /* alignment of sections in file */
#define SEG_ALIGN_1 (SEG_ALIGN-1)

static const char align_str[SEG_ALIGN] = ""; /* ANSI will pad this with 0s */

#define ELF_MAX_SECTIONS 16            /* really 10, but let's play safe */
static struct ELF_SECTDATA {
    void *data;
    long len;
    int is_saa;
} *elf_sects;
static int elf_nsect;
static long elf_foffs;

static void elf_write(void);
static void elf_sect_write(struct Section *, unsigned char *, unsigned long);
static void elf_section_header (int, int, int, void *, int, long,
                                int, int, int, int);
static void elf_write_sections (void);
static struct SAA *elf_build_symtab (long *, long *);
static struct SAA *elf_build_reltab (long *, struct Reloc *);
static void add_sectname (char *, char *);

/*
 * Special section numbers which are used to define ELF special
 * symbols, which can be used with WRT to provide PIC relocation
 * types.
 */
static long elf_gotpc_sect, elf_gotoff_sect;
static long elf_got_sect, elf_plt_sect;
static long elf_sym_sect;

static void elf_init(FILE *fp, efunc errfunc, ldfunc ldef, evalfunc eval) 
{
    elffp = fp;
    error = errfunc;
    evaluate = eval;
    (void) ldef;                       /* placate optimisers */
    sects = NULL;
    nsects = sectlen = 0;
    syms = saa_init((long)sizeof(struct Symbol));
    nlocals = nglobs = 0;
    bsym = raa_init();
    strs = saa_init(1L);
    saa_wbytes (strs, "\0", 1L);
    saa_wbytes (strs, elf_module, (long)(strlen(elf_module)+1));
    strslen = 2+strlen(elf_module);
    shstrtab = NULL;
    shstrtablen = shstrtabsize = 0;;
    add_sectname ("", "");

    fwds = NULL;

    elf_gotpc_sect = seg_alloc();
    ldef("..gotpc", elf_gotpc_sect+1, 0L, NULL, FALSE, FALSE, &of_elf, error);
    elf_gotoff_sect = seg_alloc();
    ldef("..gotoff", elf_gotoff_sect+1, 0L, NULL, FALSE, FALSE,&of_elf,error);
    elf_got_sect = seg_alloc();
    ldef("..got", elf_got_sect+1, 0L, NULL, FALSE, FALSE, &of_elf, error);
    elf_plt_sect = seg_alloc();
    ldef("..plt", elf_plt_sect+1, 0L, NULL, FALSE, FALSE, &of_elf, error);
    elf_sym_sect = seg_alloc();
    ldef("..sym", elf_sym_sect+1, 0L, NULL, FALSE, FALSE, &of_elf, error);

    def_seg = seg_alloc();
}

static void elf_cleanup(int debuginfo) 
{
    struct Reloc *r;
    int i;

    (void) debuginfo;

    elf_write();
    fclose (elffp);
    for (i=0; i<nsects; i++) {
        if (sects[i]->type != SHT_NOBITS)
            saa_free (sects[i]->data);
        if (sects[i]->head)
            saa_free (sects[i]->rel);
        while (sects[i]->head) {
            r = sects[i]->head;
            sects[i]->head = sects[i]->head->next;
            nasm_free (r);
        }
    }
    nasm_free (sects);
    saa_free (syms);
    raa_free (bsym);
    saa_free (strs);
}

static void add_sectname (char *firsthalf, char *secondhalf) 
{
    int len = strlen(firsthalf)+strlen(secondhalf);
    while (shstrtablen + len + 1 > shstrtabsize)
        shstrtab = nasm_realloc (shstrtab, (shstrtabsize += SHSTR_DELTA));
    strcpy (shstrtab+shstrtablen, firsthalf);
    strcat (shstrtab+shstrtablen, secondhalf);
    shstrtablen += len+1;
}

static int elf_make_section (char *name, int type, int flags, int align) 
{
    struct Section *s;

    s = nasm_malloc (sizeof(*s));

    if (type != SHT_NOBITS)
        s->data = saa_init (1L);
    s->head = NULL;
    s->tail = &s->head;
    s->len = s->size = 0;
    s->nrelocs = 0;
    if (!strcmp(name, ".text"))
        s->index = def_seg;
    else
        s->index = seg_alloc();
    add_sectname ("", name);
    s->name = nasm_malloc (1+strlen(name));
    strcpy (s->name, name);
    s->type = type;
    s->flags = flags;
    s->align = align;
    s->gsyms = NULL;

    if (nsects >= sectlen)
        sects = nasm_realloc (sects, (sectlen += SECT_DELTA)*sizeof(*sects));
    sects[nsects++] = s;

    return nsects-1;
}

static long elf_section_names (char *name, int pass, int *bits) 
{
    char *p;
    int flags_and, flags_or, type, align, i;

    /*
     * Default is 32 bits.
     */
    if (!name) {
        *bits = 32;
        return def_seg;
    }

    p = name;
    while (*p && !isspace(*p)) p++;
    if (*p) *p++ = '\0';
    flags_and = flags_or = type = align = 0;

    while (*p && isspace(*p)) p++;
    while (*p) {
        char *q = p;
        while (*p && !isspace(*p)) p++;
        if (*p) *p++ = '\0';
        while (*p && isspace(*p)) p++;
        
        if (!nasm_strnicmp(q, "align=", 6)) {
            align = atoi(q+6);
            if (align == 0)
                align = 1;
            if ( (align-1) & align ) {   /* means it's not a power of two */
                error (ERR_NONFATAL, "section alignment %d is not"
                       " a power of two", align);
                align = 1;
            }
        } else if (!nasm_stricmp(q, "alloc")) {
            flags_and |= SHF_ALLOC;
            flags_or |= SHF_ALLOC;
        } else if (!nasm_stricmp(q, "noalloc")) {
            flags_and |= SHF_ALLOC;
            flags_or &= ~SHF_ALLOC;
        } else if (!nasm_stricmp(q, "exec")) {
            flags_and |= SHF_EXECINSTR;
            flags_or |= SHF_EXECINSTR;
        } else if (!nasm_stricmp(q, "noexec")) {
            flags_and |= SHF_EXECINSTR;
            flags_or &= ~SHF_EXECINSTR;
        } else if (!nasm_stricmp(q, "write")) {
            flags_and |= SHF_WRITE;
            flags_or |= SHF_WRITE;
        } else if (!nasm_stricmp(q, "nowrite")) {
            flags_and |= SHF_WRITE;
            flags_or &= ~SHF_WRITE;
        } else if (!nasm_stricmp(q, "progbits")) {
            type = SHT_PROGBITS;
        } else if (!nasm_stricmp(q, "nobits")) {
            type = SHT_NOBITS;
        }
    }

    if (!strcmp(name, ".comment") ||
        !strcmp(name, ".shstrtab") ||
        !strcmp(name, ".symtab") ||
        !strcmp(name, ".strtab")) {
        error (ERR_NONFATAL, "attempt to redefine reserved section"
               "name `%s'", name);
        return NO_SEG;
    }

    for (i=0; i<nsects; i++)
        if (!strcmp(name, sects[i]->name))
            break;
    if (i == nsects) {
        if (!strcmp(name, ".text"))
            i = elf_make_section (name, SHT_PROGBITS,
                                  SHF_ALLOC | SHF_EXECINSTR, 16);
        else if (!strcmp(name, ".data"))
            i = elf_make_section (name, SHT_PROGBITS,
                                  SHF_ALLOC | SHF_WRITE, 4);
        else if (!strcmp(name, ".bss"))
            i = elf_make_section (name, SHT_NOBITS,
                                  SHF_ALLOC | SHF_WRITE, 4);
        else
            i = elf_make_section (name, SHT_PROGBITS, SHF_ALLOC, 1);
        if (type)
            sects[i]->type = type;
        if (align)
            sects[i]->align = align;
        sects[i]->flags &= ~flags_and;
        sects[i]->flags |= flags_or;
    } else if (pass == 1) {
        if (type || align || flags_and)
            error (ERR_WARNING, "section attributes ignored on"
                   " redeclaration of section `%s'", name);
    }

    return sects[i]->index;
}

static void elf_deflabel (char *name, long segment, long offset,
                           int is_global, char *special) 
{
    int pos = strslen;
    struct Symbol *sym;
    int special_used = FALSE;

    if (name[0] == '.' && name[1] == '.' && name[2] != '@') {
        /*
         * This is a NASM special symbol. We never allow it into
         * the ELF symbol table, even if it's a valid one. If it
         * _isn't_ a valid one, we should barf immediately.
         */
        if (strcmp(name, "..gotpc") && strcmp(name, "..gotoff") &&
            strcmp(name, "..got") && strcmp(name, "..plt") &&
            strcmp(name, "..sym"))
            error (ERR_NONFATAL, "unrecognised special symbol `%s'", name);
        return;
    }

    if (is_global == 3) {
        struct Symbol **s;
        /*
         * Fix up a forward-reference symbol size from the first
         * pass.
         */
        for (s = &fwds; *s; s = &(*s)->nextfwd)
            if (!strcmp((*s)->name, name)) {
                struct tokenval tokval;
                expr *e;
                char *p = special;

                while (*p && !isspace(*p)) p++;
                while (*p && isspace(*p)) p++;
                stdscan_reset();
                stdscan_bufptr = p;
                tokval.t_type = TOKEN_INVALID;
                e = evaluate(stdscan, NULL, &tokval, NULL, 1, error, NULL);
                if (e) {
                    if (!is_simple(e))
                        error (ERR_NONFATAL, "cannot use relocatable"
                               " expression as symbol size");
                    else
                        (*s)->size = reloc_value(e);
                }

                /*
                 * Remove it from the list of unresolved sizes.
                 */
                nasm_free ((*s)->name);
                *s = (*s)->nextfwd;
                return;
            }
        return;                        /* it wasn't an important one */
    }

    saa_wbytes (strs, name, (long)(1+strlen(name)));
    strslen += 1+strlen(name);

    sym = saa_wstruct (syms);

    sym->strpos = pos;
    sym->type = is_global ? SYM_GLOBAL : 0;
    sym->size = 0;
    if (segment == NO_SEG)
        sym->section = SHN_ABS;
    else {
        int i;
        sym->section = SHN_UNDEF;
        if (nsects == 0 && segment == def_seg) {
            int tempint;
            if (segment != elf_section_names (".text", 2, &tempint))
                error (ERR_PANIC, "strange segment conditions in ELF driver");
            sym->section = nsects;
        } else {
            for (i=0; i<nsects; i++)
                if (segment == sects[i]->index) {
                    sym->section = i+1;
                    break;
                }
        }
    }

    if (is_global == 2) {
        sym->size = offset;
        sym->value = 0;
        sym->section = SHN_COMMON;
        /*
         * We have a common variable. Check the special text to see
         * if it's a valid number and power of two; if so, store it
         * as the alignment for the common variable.
         */
        if (special) {
            int err;
            sym->value = readnum (special, &err);
            if (err)
                error(ERR_NONFATAL, "alignment constraint `%s' is not a"
                      " valid number", special);
            else if ( (sym->value | (sym->value-1)) != 2*sym->value - 1)
                error(ERR_NONFATAL, "alignment constraint `%s' is not a"
                      " power of two", special);
        }
        special_used = TRUE;
    } else
        sym->value = (sym->section == SHN_UNDEF ? 0 : offset);

    if (sym->type == SYM_GLOBAL) {
        /*
         * If sym->section == SHN_ABS, then the first line of the else
         * section would cause a core dump, because it's a reference
         * beyond the end of the section array.
         * This case happens with code such as this:
         *     GLOBAL foo
         *     foo equ 0
         *
         * We avoid the crash by skipping over the else section. It's
         * a bit unclear whether or not this is the right thing to do,
         * but it appears to work correctly.
         */
        if (sym->section == SHN_UNDEF || sym->section == SHN_COMMON)
        {
            bsym = raa_write (bsym, segment, nglobs);
        }
        else if (sym->section != SHN_ABS) 
        {
            /*
             * This is a global symbol; so we must add it to the linked
             * list of global symbols in its section. We'll push it on
             * the beginning of the list, because it doesn't matter
             * much which end we put it on and it's easier like this.
             *
             * In addition, we check the special text for symbol
             * type and size information.
             */
            sym->next = sects[sym->section-1]->gsyms;
            sects[sym->section-1]->gsyms = sym;

            if (special) {
                int n = strcspn(special, " ");

                if (!nasm_strnicmp(special, "function", n))
                    sym->type |= SYM_FUNCTION;
                else if (!nasm_strnicmp(special, "data", n) ||
                         !nasm_strnicmp(special, "object", n))
                    sym->type |= SYM_DATA;
                else
                    error(ERR_NONFATAL, "unrecognised symbol type `%.*s'",
                          n, special);
                if (special[n]) {
                    struct tokenval tokval;
                    expr *e;
                    int fwd = FALSE;
                    char *saveme=stdscan_bufptr;   /*bf*/

                    while (special[n] && isspace(special[n]))
                        n++;
                    /*
                     * We have a size expression; attempt to
                     * evaluate it.
                     */
                    stdscan_reset();
                    stdscan_bufptr = special+n;
                    tokval.t_type = TOKEN_INVALID;
                    e = evaluate(stdscan, NULL, &tokval, &fwd, 0, error, NULL);
                    if (fwd) {
                        sym->nextfwd = fwds;
                        fwds = sym;
                        sym->name = nasm_strdup(name);
                    } else if (e) {
                        if (!is_simple(e))
                            error (ERR_NONFATAL, "cannot use relocatable"
                                   " expression as symbol size");
                        else
                            sym->size = reloc_value(e);
                    }
                    stdscan_bufptr=saveme;    /*bf*/
                }
                special_used = TRUE;
            }
        }
        sym->globnum = nglobs;
        nglobs++;
    } else
        nlocals++;

    if (special && !special_used)
        error(ERR_NONFATAL, "no special symbol features supported here");
}

static void elf_add_reloc (struct Section *sect, long segment,
                           int type) 
{
    struct Reloc *r;

    r = *sect->tail = nasm_malloc(sizeof(struct Reloc));
    sect->tail = &r->next;
    r->next = NULL;

    r->address = sect->len;
    if (segment == NO_SEG)
        r->symbol = 2;
    else {
        int i;
        r->symbol = 0;
        for (i=0; i<nsects; i++)
            if (segment == sects[i]->index)
                r->symbol = i+3;
        if (!r->symbol)
            r->symbol = GLOBAL_TEMP_BASE + raa_read(bsym, segment);
    }
    r->type = type;

    sect->nrelocs++;
}

/*
 * This routine deals with ..got and ..sym relocations: the more
 * complicated kinds. In shared-library writing, some relocations
 * with respect to global symbols must refer to the precise symbol
 * rather than referring to an offset from the base of the section
 * _containing_ the symbol. Such relocations call to this routine,
 * which searches the symbol list for the symbol in question.
 *
 * R_386_GOT32 references require the _exact_ symbol address to be
 * used; R_386_32 references can be at an offset from the symbol.
 * The boolean argument `exact' tells us this.
 *
 * Return value is the adjusted value of `addr', having become an
 * offset from the symbol rather than the section. Should always be
 * zero when returning from an exact call.
 *
 * Limitation: if you define two symbols at the same place,
 * confusion will occur.
 *
 * Inefficiency: we search, currently, using a linked list which
 * isn't even necessarily sorted.
 */
static long elf_add_gsym_reloc (struct Section *sect,
                                long segment, long offset,
                                int type, int exact) 
{
    struct Reloc *r;
    struct Section *s;
    struct Symbol *sym, *sm;
    int i;

    /*
     * First look up the segment/offset pair and find a global
     * symbol corresponding to it. If it's not one of our segments,
     * then it must be an external symbol, in which case we're fine
     * doing a normal elf_add_reloc after first sanity-checking
     * that the offset from the symbol is zero.
     */
    s = NULL;
    for (i=0; i<nsects; i++)
        if (segment == sects[i]->index) {
            s = sects[i];
            break;
        }
    if (!s) {
        if (exact && offset != 0)
            error (ERR_NONFATAL, "unable to find a suitable global symbol"
                   " for this reference");
        else
            elf_add_reloc (sect, segment, type);
        return offset;
    }

    if (exact) {
        /*
         * Find a symbol pointing _exactly_ at this one.
         */
        for (sym = s->gsyms; sym; sym = sym->next)
            if (sym->value == offset)
                break;
    } else {
        /*
         * Find the nearest symbol below this one.
         */
        sym = NULL;
        for (sm = s->gsyms; sm; sm = sm->next)
            if (sm->value <= offset && (!sym || sm->value > sym->value))
                sym = sm;
    }
    if (!sym && exact) {
        error (ERR_NONFATAL, "unable to find a suitable global symbol"
               " for this reference");
        return 0;
    }

    r = *sect->tail = nasm_malloc(sizeof(struct Reloc));
    sect->tail = &r->next;
    r->next = NULL;

    r->address = sect->len;
    r->symbol = GLOBAL_TEMP_BASE + sym->globnum;
    r->type = type;

    sect->nrelocs++;

    return offset - sym->value;
}

static void elf_out (long segto, void *data, unsigned long type,
                      long segment, long wrt) 
{
    struct Section *s;
    long realbytes = type & OUT_SIZMASK;
    long addr;
    unsigned char mydata[4], *p;
    int i;

    type &= OUT_TYPMASK;

    /*
     * handle absolute-assembly (structure definitions)
     */
    if (segto == NO_SEG) {
        if (type != OUT_RESERVE)
            error (ERR_NONFATAL, "attempt to assemble code in [ABSOLUTE]"
                   " space");
        return;
    }

    s = NULL;
    for (i=0; i<nsects; i++)
        if (segto == sects[i]->index) {
            s = sects[i];
            break;
        }
    if (!s) {
        int tempint;                   /* ignored */
        if (segto != elf_section_names (".text", 2, &tempint))
            error (ERR_PANIC, "strange segment conditions in ELF driver");
        else
            s = sects[nsects-1];
    }

    if (s->type == SHT_NOBITS && type != OUT_RESERVE) {
        error(ERR_WARNING, "attempt to initialise memory in"
              " BSS section `%s': ignored", s->name);
        if (type == OUT_REL2ADR)
            realbytes = 2;
        else if (type == OUT_REL4ADR)
            realbytes = 4;
        s->len += realbytes;
        return;
    }

    if (type == OUT_RESERVE) {
        if (s->type == SHT_PROGBITS) {
            error(ERR_WARNING, "uninitialised space declared in"
                  " non-BSS section `%s': zeroing", s->name);
            elf_sect_write (s, NULL, realbytes);
        } else
            s->len += realbytes;
    } else if (type == OUT_RAWDATA) {
        if (segment != NO_SEG)
            error(ERR_PANIC, "OUT_RAWDATA with other than NO_SEG");
        elf_sect_write (s, data, realbytes);
    } else if (type == OUT_ADDRESS) {
        addr = *(long *)data;
        if (segment != NO_SEG) {
            if (segment % 2) {
                error(ERR_NONFATAL, "ELF format does not support"
                      " segment base references");
            } else {
                if (wrt == NO_SEG) {
                    elf_add_reloc (s, segment, R_386_32);
                } else if (wrt == elf_gotpc_sect+1) {
                    /*
                     * The user will supply GOT relative to $$. ELF
                     * will let us have GOT relative to $. So we
                     * need to fix up the data item by $-$$.
                     */
                    addr += s->len;
                    elf_add_reloc (s, segment, R_386_GOTPC);
                } else if (wrt == elf_gotoff_sect+1) {
                    elf_add_reloc (s, segment, R_386_GOTOFF);
                } else if (wrt == elf_got_sect+1) {
                    addr = elf_add_gsym_reloc (s, segment, addr,
                                               R_386_GOT32, TRUE);
                } else if (wrt == elf_sym_sect+1) {
                    addr = elf_add_gsym_reloc (s, segment, addr,
                                               R_386_32, FALSE);
                } else if (wrt == elf_plt_sect+1) {
                    error(ERR_NONFATAL, "ELF format cannot produce non-PC-"
                          "relative PLT references");
                } else {
                    error (ERR_NONFATAL, "ELF format does not support this"
                           " use of WRT");
                    wrt = NO_SEG;      /* we can at least _try_ to continue */
                }
            }
        }
        p = mydata;
        if (realbytes != 4 && segment != NO_SEG)
            error (ERR_NONFATAL, "ELF format does not support non-32-bit"
                   " relocations");
        WRITELONG (p, addr);
        elf_sect_write (s, mydata, realbytes);
    } else if (type == OUT_REL2ADR) {
        error (ERR_NONFATAL, "ELF format does not support 16-bit"
               " relocations");
    } else if (type == OUT_REL4ADR) {
        if (segment == segto)
            error(ERR_PANIC, "intra-segment OUT_REL4ADR");
        if (segment != NO_SEG && segment % 2) {
            error(ERR_NONFATAL, "ELF format does not support"
                  " segment base references");
        } else {
            if (wrt == NO_SEG) {
                elf_add_reloc (s, segment, R_386_PC32);
            } else if (wrt == elf_plt_sect+1) {
                elf_add_reloc (s, segment, R_386_PLT32);
            } else if (wrt == elf_gotpc_sect+1 ||
                       wrt == elf_gotoff_sect+1 ||
                       wrt == elf_got_sect+1) {
                error(ERR_NONFATAL, "ELF format cannot produce PC-"
                      "relative GOT references");
            } else {
                error (ERR_NONFATAL, "ELF format does not support this"
                       " use of WRT");
                wrt = NO_SEG;      /* we can at least _try_ to continue */
            }
        }
        p = mydata;
        WRITELONG (p, *(long*)data - realbytes);
        elf_sect_write (s, mydata, 4L);
    }
}

static void elf_write(void) 
{
    int nsections, align;
    char *p;
    int commlen;
    char comment[64];
    int i;

    struct SAA *symtab;
    long symtablen, symtablocal;

    /*
     * Work out how many sections we will have. We have SHN_UNDEF,
     * then the flexible user sections, then the four fixed
     * sections `.comment', `.shstrtab', `.symtab' and `.strtab',
     * then optionally relocation sections for the user sections.
     */
    nsections = 5;                     /* SHN_UNDEF and the fixed ones */
    add_sectname ("", ".comment");
    add_sectname ("", ".shstrtab");
    add_sectname ("", ".symtab");
    add_sectname ("", ".strtab");
    for (i=0; i<nsects; i++) {
        nsections++;                   /* for the section itself */
        if (sects[i]->head) {
            nsections++;               /* for its relocations */
            add_sectname (".rel", sects[i]->name);
        }
    }

    /*
     * Do the comment.
     */
    *comment = '\0';
    commlen = 2+sprintf(comment+1, "The Netwide Assembler %s", NASM_VER);

    /*
     * Output the ELF header.
     */
    fwrite ("\177ELF\1\1\1\0\0\0\0\0\0\0\0\0", 16, 1, elffp);
    fwriteshort (1, elffp);            /* ET_REL relocatable file */
    fwriteshort (3, elffp);            /* EM_386 processor ID */
    fwritelong (1L, elffp);            /* EV_CURRENT file format version */
    fwritelong (0L, elffp);            /* no entry point */
    fwritelong (0L, elffp);            /* no program header table */
    fwritelong (0x40L, elffp);         /* section headers straight after
                                        * ELF header plus alignment */
    fwritelong (0L, elffp);            /* 386 defines no special flags */
    fwriteshort (0x34, elffp);         /* size of ELF header */
    fwriteshort (0, elffp);            /* no program header table, again */
    fwriteshort (0, elffp);            /* still no program header table */
    fwriteshort (0x28, elffp);         /* size of section header */
    fwriteshort (nsections, elffp);    /* number of sections */
    fwriteshort (nsects+2, elffp);     /* string table section index for
                                        * section header table */
    fwritelong (0L, elffp);            /* align to 0x40 bytes */
    fwritelong (0L, elffp);
    fwritelong (0L, elffp);

    /*
     * Build the symbol table and relocation tables.
     */
    symtab = elf_build_symtab (&symtablen, &symtablocal);
    for (i=0; i<nsects; i++)
        if (sects[i]->head)
            sects[i]->rel = elf_build_reltab (&sects[i]->rellen,
                                              sects[i]->head);

    /*
     * Now output the section header table.
     */

    elf_foffs = 0x40 + 0x28 * nsections;
    align = ((elf_foffs+SEG_ALIGN_1) & ~SEG_ALIGN_1) - elf_foffs;
    elf_foffs += align;
    elf_nsect = 0;
    elf_sects = nasm_malloc(sizeof(*elf_sects) * (2 * nsects + 10));

    elf_section_header (0, 0, 0, NULL, FALSE, 0L, 0, 0, 0, 0); /* SHN_UNDEF */
    p = shstrtab+1;
    for (i=0; i<nsects; i++) {
        elf_section_header (p - shstrtab, sects[i]->type, sects[i]->flags,
                            (sects[i]->type == SHT_PROGBITS ?
                             sects[i]->data : NULL), TRUE,
                            sects[i]->len, 0, 0, sects[i]->align, 0);
        p += strlen(p)+1;
    }
    elf_section_header (p - shstrtab, 1, 0, comment, FALSE,
                        (long)commlen, 0, 0, 1, 0);/* .comment */
    p += strlen(p)+1;
    elf_section_header (p - shstrtab, 3, 0, shstrtab, FALSE,
                        (long)shstrtablen, 0, 0, 1, 0);/* .shstrtab */
    p += strlen(p)+1;
    elf_section_header (p - shstrtab, 2, 0, symtab, TRUE,
                        symtablen, nsects+4, symtablocal, 4, 16);/* .symtab */
    p += strlen(p)+1;
    elf_section_header (p - shstrtab, 3, 0, strs, TRUE,
                        strslen, 0, 0, 1, 0);       /* .strtab */
    for (i=0; i<nsects; i++) if (sects[i]->head) {
        p += strlen(p)+1;
        elf_section_header (p - shstrtab, 9, 0, sects[i]->rel, TRUE,
                            sects[i]->rellen, nsects+3, i+1, 4, 8);
    }

    fwrite (align_str, align, 1, elffp);

    /*
     * Now output the sections.
     */
    elf_write_sections();

    nasm_free (elf_sects);
    saa_free (symtab);
}

static struct SAA *elf_build_symtab (long *len, long *local) 
{
    struct SAA *s = saa_init(1L);
    struct Symbol *sym;
    unsigned char entry[16], *p;
    int i;

    *len = *local = 0;

    /*
     * First, an all-zeros entry, required by the ELF spec.
     */
    saa_wbytes (s, NULL, 16L);         /* null symbol table entry */
    *len += 16;
    (*local)++;

    /*
     * Next, an entry for the file name.
     */
    p = entry;
    WRITELONG (p, 1);                  /* we know it's 1st thing in strtab */
    WRITELONG (p, 0);                  /* no value */
    WRITELONG (p, 0);                  /* no size either */
    WRITESHORT (p, 4);                 /* type FILE */
    WRITESHORT (p, SHN_ABS);
    saa_wbytes (s, entry, 16L);
    *len += 16;
    (*local)++;

    /*
     * Now some standard symbols defining the segments, for relocation
     * purposes.
     */
    for (i = 1; i <= nsects+1; i++) {
        p = entry;
        WRITELONG (p, 0);              /* no symbol name */
        WRITELONG (p, 0);              /* offset zero */
        WRITELONG (p, 0);              /* size zero */
        WRITESHORT (p, 3);             /* local section-type thing */
        WRITESHORT (p, (i==1 ? SHN_ABS : i-1));   /* the section id */
        saa_wbytes (s, entry, 16L);
        *len += 16;
        (*local)++;
    }

    /*
     * Now the other local symbols.
     */
    saa_rewind (syms);
    while ( (sym = saa_rstruct (syms)) != NULL ) {
        if (sym->type & SYM_GLOBAL)
            continue;
        p = entry;
        WRITELONG (p, sym->strpos);
        WRITELONG (p, sym->value);
        WRITELONG (p, sym->size);
        WRITESHORT (p, sym->type);     /* local non-typed thing */
        WRITESHORT (p, sym->section);
        saa_wbytes (s, entry, 16L);
        *len += 16;
        (*local)++;
    }

    /*
     * Now the global symbols.
     */
    saa_rewind (syms);
    while ( (sym = saa_rstruct (syms)) != NULL ) {
        if (!(sym->type & SYM_GLOBAL))
            continue;
        p = entry;
        WRITELONG (p, sym->strpos);
        WRITELONG (p, sym->value);
        WRITELONG (p, sym->size);
        WRITESHORT (p, sym->type);     /* global non-typed thing */
        WRITESHORT (p, sym->section);
        saa_wbytes (s, entry, 16L);
        *len += 16;
    }

    return s;
}

static struct SAA *elf_build_reltab (long *len, struct Reloc *r) {
    struct SAA *s;
    unsigned char *p, entry[8];

    if (!r)
        return NULL;

    s = saa_init(1L);
    *len = 0;

    while (r) {
        long sym = r->symbol;

        if (sym >= GLOBAL_TEMP_BASE)
            sym += -GLOBAL_TEMP_BASE + (nsects+3) + nlocals;

        p = entry;
        WRITELONG (p, r->address);
        WRITELONG (p, (sym << 8) + r->type);
        saa_wbytes (s, entry, 8L);
        *len += 8;

        r = r->next;
    }

    return s;
}

static void elf_section_header (int name, int type, int flags,
                                void *data, int is_saa, long datalen,
                                int link, int info, int align, int eltsize) 
{
    elf_sects[elf_nsect].data = data;
    elf_sects[elf_nsect].len = datalen;
    elf_sects[elf_nsect].is_saa = is_saa;
    elf_nsect++;

    fwritelong ((long)name, elffp);
    fwritelong ((long)type, elffp);
    fwritelong ((long)flags, elffp);
    fwritelong (0L, elffp);            /* no address, ever, in object files */
    fwritelong (type == 0 ? 0L : elf_foffs, elffp);
    fwritelong (datalen, elffp);
    if (data)
        elf_foffs += (datalen+SEG_ALIGN_1) & ~SEG_ALIGN_1;
    fwritelong ((long)link, elffp);
    fwritelong ((long)info, elffp);
    fwritelong ((long)align, elffp);
    fwritelong ((long)eltsize, elffp);
}

static void elf_write_sections (void) 
{
    int i;
    for (i = 0; i < elf_nsect; i++)
        if (elf_sects[i].data) {
            long len = elf_sects[i].len;
            long reallen = (len+SEG_ALIGN_1) & ~SEG_ALIGN_1;
            long align = reallen - len;
            if (elf_sects[i].is_saa)
                saa_fpwrite (elf_sects[i].data, elffp);
            else
                fwrite (elf_sects[i].data, len, 1, elffp);
            fwrite (align_str, align, 1, elffp);
        }
}

static void elf_sect_write (struct Section *sect,
                             unsigned char *data, unsigned long len) 
{
    saa_wbytes (sect->data, data, len);
    sect->len += len;
}

static long elf_segbase (long segment) 
{
    return segment;
}

static int elf_directive (char *directive, char *value, int pass) 
{
    return 0;
}

static void elf_filename (char *inname, char *outname, efunc error) 
{
    strcpy(elf_module, inname);
    standard_extension (inname, outname, ".o", error);
}

static char *elf_stdmac[] = {
    "%define __SECT__ [section .text]",
    "%macro __NASM_CDecl__ 1",
    "%define $_%1 $%1",
    "%endmacro",
    NULL
};
static int elf_set_info(enum geninfo type, char **val)
{
    return 0;
}

struct ofmt of_elf = {
    "ELF32 (i386) object files (e.g. Linux)",
    "elf",
    NULL,
    null_debug_arr,
    &null_debug_form,
    elf_stdmac,
    elf_init,
    elf_set_info,
    elf_out,
    elf_deflabel,
    elf_section_names,
    elf_segbase,
    elf_directive,
    elf_filename,
    elf_cleanup
};

#endif /* OF_ELF */