Invoke zlib functions rather than calling gunzip in a subprocess.
Inspired by the debian nauty-zlib-blisstog patch.

--- a/dimacs2g.c	2022-11-15 22:15:46.000000000 -0700
+++ b/dimacs2g.c	2022-11-16 14:11:09.504416535 -0700
@@ -17,17 +17,10 @@
 
 /*************************************************************************/
 
+#include <zlib.h>
 #include "gtools.h" 
 
-#if HAVE_GUNZIP && HAVE_POPEN
-#if !POPEN_DEC
-extern FILE *popen(const char *command, const char *type);
-extern int pclose(FILE *stream);
-#endif
-#endif
-
-#define GUNZIP "gunzip -c"
-
+#define BUFSIZE 256
 #define MAXCOMMENT 200
 static char comment[MAXCOMMENT+3];
 static int commentlen;
@@ -37,40 +30,30 @@ typedef struct
    int v,w;
 } vpair;
 
-static int
-nextchar(FILE *f)
-{
-    char s[2];
-
-    if (fscanf(f,"%1s",s) != 1) return EOF;
-    else                        return s[0];
-}
-
 static boolean
-readdimacsgraph(FILE *f, sparsegraph *g)
+readdimacsgraph(gzFile f, sparsegraph *g)
 /* Reads a graph from Bliss format into a sparse graph */
 {
-    int n,c;
+    int n;
     unsigned long ne,j;
     int haven;
     int i,v,w;
-    int haveptn;
     DYNALLSTAT(vpair,elist,elist_sz);
+    char buffer[BUFSIZE];
 
+    memset(buffer, '\0', BUFSIZE);
     commentlen = 0;
     haven = 0;
     j = 0;
-    while ((c = nextchar(f)) >= 0)
+    while (gzgets(f, buffer, BUFSIZE) != NULL && strlen(buffer) < BUFSIZE - 1)
     {
-        switch (c)
+        switch (*buffer)
         {
         case 'c':
-            commentlen = 0;
-            while ((c = getc(f)) != '\n' && c != EOF)
-            {
-                if (commentlen < MAXCOMMENT)
-                    comment[commentlen++] = c;
-            }
+            commentlen = strlen(buffer);
+            if (commentlen > MAXCOMMENT)
+                commentlen = MAXCOMMENT;
+            memcpy(comment, &buffer[1], commentlen);
             comment[commentlen] = '\0';
             break;
 
@@ -80,7 +63,7 @@ readdimacsgraph(FILE *f, sparsegraph *g)
                 fprintf(stderr,"Duplicate p line\n");
                 exit(1);
             }
-            if (fscanf(f," edge %d %lu",&n,&ne) != 2)
+            if (sscanf(&buffer[1]," edge %d %lu",&n,&ne) != 2)
             {
                 fprintf(stderr,"Bad p line\n");
                 return FALSE;
@@ -95,7 +78,7 @@ readdimacsgraph(FILE *f, sparsegraph *g)
                 fprintf(stderr,"Missing p line\n");
                 return FALSE;
             }  
-            if (fscanf(f,"%d%d",&w,&v) != 2 || w < 1 || w > n)
+            if (sscanf(&buffer[1],"%d%d",&w,&v) != 2 || w < 1 || w > n)
             {
                 fprintf(stderr,"Bad n line\n");
                 return FALSE;
@@ -108,7 +91,7 @@ readdimacsgraph(FILE *f, sparsegraph *g)
                 fprintf(stderr,"Missing p line or too many e lines\n");
                 return FALSE;
             }
-            if (fscanf(f,"%d%d",&v,&w) != 2 || v < 1 || w < 1 || v > n || w > n)
+            if (sscanf(&buffer[1],"%d%d",&v,&w) != 2 || v < 1 || w < 1 || v > n || w > n)
             {
                 fprintf(stderr,"Bad e line\n");
                 return FALSE;
@@ -118,11 +101,22 @@ readdimacsgraph(FILE *f, sparsegraph *g)
             break;
 
         default:
-            fprintf(stderr,"Unknown line %c\n",c);
+            fprintf(stderr,"Unknown line %c\n",*buffer);
             return FALSE;
         }
     }
 
+    if (errno)
+    {
+        fputs("Corrupted data file\n", stderr);
+        return FALSE;
+    }
+    else if (strlen(buffer) == BUFSIZE - 1)
+    {
+        fputs("Corruped data line\n", stderr);
+        return FALSE;
+    }
+
     if (j != ne)
     {
         fprintf(stderr,"Wrong number of e lines\n");
@@ -159,13 +153,11 @@ readdimacsgraph(FILE *f, sparsegraph *g)
 int
 main(int argc, char *argv[])
 {
-    FILE *infile;
+    gzFile infile;
     int j,firstarg;
     SG_DECL(g);
-    size_t flen;
-    boolean nocomment,nofiles,nswitch,iszip,dreadnaut,badargs;
+    boolean nocomment,nofiles,nswitch,dreadnaut,badargs;
     long nmin,nmax;
-    char zcmd[550];
     char *arg,sw;
     char *prestring,*poststring;
 
@@ -212,35 +204,21 @@ main(int argc, char *argv[])
     for (j = firstarg; j < argc || nofiles; ++j)
     {
         if (nofiles)
-            infile = stdin;
-        else
         {
-            flen = strlen(argv[j]);
-            if (flen >= 3 && strcmp(argv[j]+flen-3,".gz") == 0)
+            infile = gzdopen(STDIN_FILENO,"r");
+            if (infile == NULL)
             {
-#if HAVE_GUNZIP && HAVE_POPEN
-                if (strlen(argv[j]) > 500) gt_abort(">E file name too long\n");
-                sprintf(zcmd,"%s \"%s\"",GUNZIP,argv[j]);
-                if ((infile = popen(zcmd,"r")) == NULL)
-                {
-                    fprintf(stderr,
-                       ">E dimacs2g: cannot open gunzip pipe for \"%s\"\n",
-                       argv[j]);
-                    gt_abort(NULL);
-                }
-                iszip = TRUE;
-#else
-                gt_abort(">E dimacs2g is not compiled with gunzip support\n");
-#endif
+                fputs(">E Can't open stdin\n",stderr);
+                gt_abort(NULL);
             }
-            else
+        }
+        else
+        {
+            infile = gzopen(argv[j],"r");
+            if (infile == NULL)
             {
-                if ((infile = fopen(argv[j],"r")) == NULL)
-                {
-                    fprintf(stderr,">E Can't open file %s\n",argv[j]);
-                    gt_abort(NULL);
-                }
-                iszip = FALSE;
+                fprintf(stderr,">E Can't open file %s\n",argv[j]);
+                gt_abort(NULL);
             }
         }
 
@@ -270,14 +248,8 @@ main(int argc, char *argv[])
 
         if (nofiles)
             nofiles = FALSE;
-        else if (iszip)
-        {
-#if HAVE_GUNZIP && HAVE_POPEN
-            pclose(infile);
-#endif
-        }
         else
-            fclose(infile);
+            gzclose(infile);
     }
 
     exit(0);