feat: Filesystem::getline() (#4569)

Convenience function to safely read one line of text from an open FILE*
and return it as a string.

Signed-off-by: Larry Gritz <[email protected]>

src/include/OpenImageIO/filesystem.h

@@ -223,6 +223,16 @@ OIIO_UTIL_API int fseek (FILE *file, int64_t offset, int whence);
 /// Version of ftell that works with 64 bit offsets on all systems.
 OIIO_UTIL_API int64_t ftell (FILE *file);
 
+/// Read one line of text from an open text file file until the hitting a
+/// newline, reaching `maxlen` characters without encountering a newline, or
+/// reaching the end of the file, and return the characters read as a
+/// std::string. The newline will be included in the returned string, unless
+/// the line was too long, in which case the string returned will be the
+/// partial result with no newline at the end. If an error occurs or if the
+/// end of the file is encountered before either a newline is reached or
+/// maxlen characters are read, an empty string will be returned.
+OIIO_UTIL_API std::string getline(FILE* file, size_t maxlen = 4096);
+
 /// Return the current (".") directory path.
 ///
 OIIO_UTIL_API std::string current_path ();

src/libutil/filesystem.cpp

@@ -541,6 +541,24 @@ Filesystem::ftell(FILE* file)
 
 
 
+std::string
+Filesystem::getline(FILE* file, size_t maxlen)
+{
+    std::string result;
+    char* buf;
+    OIIO_ALLOCATE_STACK_OR_HEAP(buf, char, maxlen + 1);
+    if (fgets(buf, int(maxlen + 1), file)) {
+        buf[maxlen] = 0;  // be sure it is terminated
+        if (!feof(file))
+            result.assign(buf);
+    } else {
+        result.assign("");
+    }
+    return result;
+}
+
+
+
 void
 Filesystem::open(OIIO::ifstream& stream, string_view path,
                  std::ios_base::openmode mode)

src/libutil/filesystem_test.cpp

@@ -607,6 +607,44 @@ test_last_write_time()
 
 
 
+void
+test_getline()
+{
+    const char* contents    = "Line 1\n"
+                              "Line 2\n"
+                              "Really really really long long line line 3\n"
+                              "Line 4\n"
+                              "Line 5 no newline";
+    const char* tmpfilename = "getline.txt";
+    Filesystem::write_text_file(tmpfilename, contents);
+
+    FILE* in = Filesystem::fopen(tmpfilename, "r");
+    OIIO_CHECK_ASSERT(in != nullptr);
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "Line 1\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "Line 2\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30),
+                     "Really really really long long");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), " line line 3\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "Line 4\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "");  // EOF before \n
+    fclose(in);
+
+    // Again, with a complete line at the end to be sure we read it
+    Filesystem::write_text_file(tmpfilename, "Line 1\n"
+                                             "Line 2\n");
+    in = Filesystem::fopen(tmpfilename, "r");
+    OIIO_CHECK_ASSERT(in != nullptr);
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "Line 1\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "Line 2\n");
+    OIIO_CHECK_EQUAL(Filesystem::getline(in, 30), "");
+    OIIO_CHECK_ASSERT(feof(in));
+    fclose(in);
+
+    Filesystem::remove(tmpfilename);
+}
+
+
+
 int
 main(int /*argc*/, char* /*argv*/[])
 {
@@ -617,6 +655,7 @@ main(int /*argc*/, char* /*argv*/[])
     test_scan_sequences();
     test_mem_proxies();
     test_last_write_time();
+    test_getline();
 
     return unit_test_failures;
 }