leavitt@mordor.hw.stratus.com (Will Leavitt) (05/04/91)
Hi-
I'd like to use lex & yacc to parse strings within an application, but they
seem to be hardwired to take their input from stdin. What is the cannonical
way to get them to work on a string? Thanks!
-will
leavitt@mordor.hw.stratus.com
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leavitt@mordor.hw.stratus.com leavitt@mordor.hw.stratus.com (Will Leavitt) (05/06/91)
Thank you for all the suggestions. I had come up with various kludges using
freopen, but this is much cleaner:
from: Bradley White <bww+@K.GP.CS.CMU.EDU>
%%
char *instr;
#undef input
#define input() (*instr ? *instr++ : *instr)
#undef unput
#define unput(c) ((c), --instr)
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leavitt@mordor.hw.stratus.com shap@shasta.Stanford.EDU (shap) (05/07/91)
In article <5384@lectroid.sw.stratus.com> leavitt@mordor.hw.stratus.com (Will Leavitt) writes: > > I'd like to use lex & yacc to parse strings within an application, but they >seem to be hardwired to take their input from stdin. What is the cannonical >way to get them to work on a string? Thanks! Yacc doesn't read the input directly, so there's no work there. My recollection is that lex uses two macros: GET() and UNGET() to obtain/pushback characters. If you take a look at the lex-generated C code you will spot them. What you need to do is supply your own version of these macros at teh top of your file. Jonathan
wollman@emily.uvm.edu (Garrett Wollman) (05/12/91)
In article <187@shasta.Stanford.EDU> shap@shasta.Stanford.EDU (shap) writes: >Yacc doesn't read the input directly, so there's no work there. My >recollection is that lex uses two macros: GET() and UNGET() to >obtain/pushback characters. If you take a look at the lex-generated C >code you will spot them. > >What you need to do is supply your own version of these macros at teh >top of your file. > >Jonathan Well, sort of. Flex, for one, does not allow the user to redefine GET and UNGET; with the way flex scanners work, that would mean an *extremely* serious performance hit. [In fact, the major performance feature of flex is the fact that it uses read() to read a block, rather than reading a line at a time like conventional lex does.] Thankfully, you can spot a flex scanner very easily in your code... #ifdef FLEX_SCANNER /* flex specific code here */ #else /* old slow lex specific code here */ #endif But, flex uses a macro to do this read()ing, so that, without too much hassle, you can write a string-scanner that works correctly under both flex and lex. Your users will thank you for it. In particular, by redefining the following macro (taken from a 2.1-beta skeleton): /* gets input and stuffs it into "buf". number of characters read, or YY_NULL, * is returned in "result". */ #define YY_INPUT(buf,result,max_size) \ if ( (result = read( fileno(yyin), buf, max_size )) < 0 ) \ YY_FATAL_ERROR( "read() in flex scanner failed" ); to something like this #define YY_INPUT(buf,result,max_size) \ { \ int len = strlen(my_string); \ if(!len) { \ result = 0; \ } else { \ strncpy(buf,my_string,result=min(len,max_size)); \ my_string += result; /* possible fencepost error? */ \ } \ } [there are probably some errors... in which case please remember that it's now 1:15 in the morning here.] -GAWollman Garrett A. Wollman - wollman@emily.uvm.edu Disclaimer: I'm not even sure this represents *my* opinion, never mind UVM's, EMBA's, EMBA-CF's, or indeed anyone else's.