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Avoid potentially implementation-defined behavior when using a pipe as input

--- a/src/asm/lexer.c

+++ b/src/asm/lexer.c

@@ -156,13 +156,32 @@

 	if (pBuffer == NULL)

 		fatalerror("%s: Out of memory!", __func__);

-	size_t size = 0, capacity;

+	size_t size = 0, capacity = -1;

 	char *buf = NULL;

-	fseek(f, 0, SEEK_END);

-	capacity = ftell(f);

-	rewind(f);

+	/*

+	 * Check if we can get the file size without implementation-defined

+	 * behavior:

+	 *

+	 * From ftell(3p):

+	 * [On error], ftell() and ftello() shall return −1, and set errno to

+	 * indicate the error.

+	 *

+	 * The ftell() and ftello() functions shall fail if: [...]

+	 * ESPIPE The file descriptor underlying stream is associated with a

+	 * pipe, FIFO, or socket.

+	 *

+	 * From fseek(3p):

+	 * The behavior of fseek() on devices which are incapable of seeking

+	 * is implementation-defined.

+	 */

+	if (ftell(f) != -1) {

+		fseek(f, 0, SEEK_END);

+		capacity = ftell(f);

+		rewind(f);

+	}

+	// If ftell errored or the block above wasn't executed

 	if (capacity == -1)

 		capacity = 4096;