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[lib/scc] Add magic number to myro header

It is better to include this field in the header because it makes
the code more orthogonal.

--- a/as/myro.c

+++ b/as/myro.c

@@ -118,14 +118,11 @@

 writeout(char *name)

 {

 	FILE *fp;

-	long hdrpos;

-	struct myrohdr hdr = {0};

+	struct myrohdr hdr = { .magic = MYROMAGIC };

 	if ((fp = fopen(name, "wb")) == NULL)

 		die("error opening output file '%s'\n", name);

-	fputs("uobj", fp);

-	hdrpos = ftell(fp);

 	writehdr(fp, &hdr);

 	hdr.strsize = writestrings(fp);

 	hdr.secsize = writesections(fp);

@@ -133,7 +130,7 @@

 	hdr.relsize = writerelocs(fp);

 	writedata(fp);

-	fseek(fp, hdrpos, SEEK_SET);

+	fseek(fp, 0, SEEK_SET);

 	writehdr(fp, &hdr);

 	if (fclose(fp))

--- a/inc/myro.h

+++ b/inc/myro.h

@@ -1,10 +1,14 @@

-#define MYROHDR_SIZ    48

+#define MYROHDR_SIZ    52

 #define MYROSECT_SIZ   24

 #define MYROSYM_SIZ    24

 #define MYROREL_SIZ    20

+#define MYROMAGIC_SIZ   4

+#define MYROMAGIC      "uobj"

+

 struct myrohdr {

+	char magic[4];

 	unsigned long format;

 	unsigned long long entry;

 	unsigned long long strsize;

--- a/lib/scc/rmyro.c

+++ b/lib/scc/rmyro.c

@@ -2,6 +2,7 @@

 #include <assert.h>

 #include <stdio.h>

+#include <string.h>

 #include "../../inc/scc.h"

 #include "../../inc/myro.h"

@@ -15,7 +16,8 @@

 	fread(buf, sizeof(buf), 1, fp);

 	if (ferror(fp))

 		return EOF;

-	len = lunpack(buf, "lqqqqq",

+	strncpy(hdr->magic, buf, MYROMAGIC_SIZ);

+	len = lunpack(buf + MYROMAGIC_SIZ, "lqqqqq",

 	              &hdr->format,

 	              &hdr->entry,

 	              &hdr->strsize,

@@ -22,6 +24,7 @@

 	              &hdr->secsize,

 	              &hdr->symsize,

 	              &hdr->relsize);

+	len += MYROMAGIC_SIZ;

 	assert(len == MYROHDR_SIZ);

 	return len;

--- a/lib/scc/wmyro.c

+++ b/lib/scc/wmyro.c

@@ -2,6 +2,7 @@

 #include <assert.h>

 #include <stdio.h>

+#include <string.h>

 #include "../../inc/scc.h"

 #include "../../inc/myro.h"

@@ -12,7 +13,8 @@

 	unsigned char buf[MYROHDR_SIZ];

 	int len;

-	len = lpack(buf, "lqqqqq",

+	strncpy(buf, hdr->magic, MYROMAGIC_SIZ);

+	len = lpack(buf + MYROMAGIC_SIZ, "lqqqqq",

 	            hdr->format,

 	            hdr->entry,

 	            hdr->strsize,

@@ -19,6 +21,7 @@

 	            hdr->secsize,

 	            hdr->symsize,

 	            hdr->relsize);

+	len += MYROMAGIC_SIZ;

 	assert(MYROHDR_SIZ == len);

 	fwrite(buf, len, 1, fp);