add mmap_readat

mmap.h

@@ -12,6 +12,9 @@ extern "C" {
  * map in filesize and return pointer to map. */
 const char* mmap_read(const char *filename,size_t* filesize);
 
+/* like mmap_read but use openat instead of open */
+const char* mmap_readat(const char *filename,size_t* filesize,int dirfd);
+
 /* open file for writing, mmap whole file privately (copy on write),
  * close file, write length of map in filesize and return pointer to
  * map. */

mmap/mmap_readat.3

@@ -0,0 +1,23 @@
+.TH mmap_readat 3
+.SH NAME
+mmap_readat \- memory map a file for reading
+.SH SYNTAX
+.B #include <mmap.h>
+
+char* \fBmmap_readat\fP(const char* \fIfilename\fR,size_t* \fIfilesize\fR,int \fIdirfd\fR);
+.SH DESCRIPTION
+mmap_readat opens \fIfilename\fR for reading, maps the whole file into
+memory, closes the file, writes the length of the file to \fIfilesize\fR
+and returns a pointer to the mapped file.
+
+If \fIfilename\fR is a relative path and \fIdirfd\fR is not -1,
+\fIfilename\fR is interpreted relative to \fIdirfd\fR, which must be an
+open directory.
+
+The file is unmapped by the operating system if the process terminates.
+It can also be manually unmapped by calling \fBmunmap\fR from
+<sys/mman.h>.
+
+If the file could not be opened or mapped, (void*)0 is returned.
+.SH "SEE ALSO"
+mmap_unmap(3)

mmap/mmap_readat.c

@@ -0,0 +1,36 @@
+#define _POSIX_C_SOURCE 200809
+#define _ATFILE_SOURCE
+
+#include <sys/types.h>
+#include <unistd.h>
+#ifndef __MINGW32__
+#include <sys/stat.h>
+#include <unistd.h>
+#include <fcntl.h>
+#include <sys/mman.h>
+#endif
+#include "open.h"
+#include "mmap.h"
+
+extern const char* mmap_readat(const char* filename,size_t * filesize,int dirfd) {
+#ifdef __MINGW32__
+  return 0;
+#else
+  int fd=openat(dirfd,filename,O_RDONLY);
+  char *map;
+  if (fd>=0) {
+    register off_t o=lseek(fd,0,SEEK_END);
+    if (sizeof(off_t)!=sizeof(size_t) && o > (off_t)(size_t)-1) { close(fd); return 0; }
+    *filesize=(size_t)o;
+    if (o>0) {
+      map=mmap(0,*filesize,PROT_READ,MAP_SHARED,fd,0);
+      if (map==(char*)-1)
+	map=0;
+    } else
+      map="";
+    close(fd);
+    return map;
+  }
+  return 0;
+#endif
+}