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+---
+title: "Libc/POSIX Function \"Extensions\""
+date: 2025-03-04T13:40:33+08:00
+lastmod: 2025-03-04T13:40:33+08:00
+categories: coding
+tags:
+  - c
+  - posix
+---
+
+(I've given up on this, at least for linux pam.)
+
+Recently, I’ve been working on porting some libraries to GNU/Hurd. Many (old)
+libraries use [`*_MAX` constants on POSIX system
+interfaces](https://pubs.opengroup.org/onlinepubs/9699919799.2008edition/nframe.html)
+to calculate buffer sizes. However, the GNU/Hurd maintainers urge against the
+blind use of them and refuse to define them in system headers. When old APIs are
+gone, compatibility problems come. To make my life easier, I'll put some
+reusable code snippets here to help *fix `*_MAX` bugs*.
+
+<!--more-->
+
+```c
+#include <stdlib.h>
+#include <stdarg.h>
+#include <unistd.h>
+#include <stdio.h>
+#include <errno.h>
+
+static inline char *xreadlink(const char *restrict path) {
+  char *buffer;
+  size_t allocated = 128;
+  ssize_t len;
+
+  while (1) {
+    buffer = (char*) malloc(allocated);
+    if (!buffer) { return NULL; }
+    len = readlink(path, buffer, allocated);
+    if (len < (ssize_t) allocated) { return buffer; }
+    free(buffer);
+    if (len >= (ssize_t) allocated) { allocated *= 2; continue; }
+    return NULL;
+  }
+ }
+
+
+static inline char *xgethostname(void) {
+  long max_host_name;
+  char *buffer;
+
+  max_host_name = sysconf(_SC_HOST_NAME_MAX);
+  buffer = malloc(max_host_name + 1);
+
+  if (gethostname(buffer, max_host_name + 1)) {
+    free(buffer);
+    return NULL;
+  }
+
+  buffer[max_host_name] = '\0';
+  return buffer;
+}
+
+static inline char *xgetcwd(void) {
+  char *buffer;
+  size_t allocated = 128;
+
+  while (1) {
+    buffer = (char*) malloc(allocated);
+    if (!buffer) { return NULL; }
+    getcwd(buffer, allocated);
+    if (buffer) return buffer;
+    free(buffer);
+    if (errno == ERANGE) { allocated *= 2; continue; }
+    return NULL;
+  }
+}
+
+static inline __attribute__((__format__(__printf__, 2, 3))) int
+xsprintf(char **buf_ptr, const char *restrict format, ...) {
+  char *buffer;
+  int ret;
+
+  va_list args;
+  va_start(args, format);
+
+  ret = snprintf(NULL, 0,  format, args);
+  if (ret < 0) { goto out; }
+
+  buffer = malloc(ret + 1);
+  if (!buffer) { ret = -1; goto out; }
+
+  ret = snprintf(NULL, 0,  format, args);
+  if (ret < 0) { free(buffer); goto out; }
+
+  *buf_ptr = buffer;
+
+out:
+  va_end(args);
+  return ret;
+}
+```