Show source line contents in error messages v1 PROPOSED

Lassi Pulkkinen: 1
 Show source line contents in error messages

 1 files changed, 32 insertions(+), 0 deletions(-)

[RFC PATCH] Show source line contents in error messages Export this patch

---
Obtaining line contents in a very stupid way, but also probably the best
we can do given preprocessor shenanigans. Apart from the lack of special
handling for stdin / fake file names. _That said_,

$ gcc -xc -
x
<stdin>:1:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ at end of input
$ echo x > '<stdin>'
$ gcc -xc -
x
<stdin>:1:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ at end of input
    1 | x
      | ^

Oh, and NUL bytes throw off the line counter, because fgets.

It would make sense to have an option for disabling this. The equivalent
flags for GCC and Clang respectively are -fdiagnostics-plain-output and
-fno-caret-diagnostics. Neither is understood by the other, and both are
rather long.

 token.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/token.c b/token.c
index b67baa6..223f6a9 100644
--- a/token.c
+++ b/token.c
@@ -4,6 +4,7 @@
 #include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include "util.h"
 #include "cc.h"
 
@@ -193,11 +194,42 @@ tokencheck(const struct token *t, enum tokenkind kind, const char *msg)
 void error(const struct location *loc, const char *fmt, ...)
 {
 	va_list ap;
+	FILE *file;
+	static char line[1024];
+	size_t len, i;
+	int w;
 
 	fprintf(stderr, "%s:%zu:%zu: error: ", loc->file, loc->line, loc->col);
 	va_start(ap, fmt);
 	vfprintf(stderr, fmt, ap);
 	va_end(ap);
 	putc('\n', stderr);
+
+	file = fopen(loc->file, "r");
+	if (!file)
+		goto exit;
+	for (i = 1;;) {
+		if (!fgets(line, sizeof(line), file))
+			goto exit;
+		len = strlen(line);
+		if (i == loc->line)
+			break;
+		if (line[len - 1] == '\n')
+			++i;
+	}
+	if (line[len - 1] == '\n')
+		line[len - 1] = '\0', --len;
+
+	w = snprintf(NULL, 0, "%zu", loc->line);
+	if (w < 4)
+		w = 4;
+	fprintf(stderr, " %*s |\n", w, "");
+	fprintf(stderr, " %*zu |\t%s\n", w, loc->line, line);
+	fprintf(stderr, " %*s |\t", w, "");
+	for (i = 0; i <= len && i + 1 < loc->col; ++i)
+		putc(isspace(line[i]) ? line[i] : ' ', stderr);
+	fputs("^\n", stderr);
+
+exit:
 	exit(1);
 }
-- 
2.45.0