I'm very new to C, and can't seem to figure out what's wrong with the following code.
int main() {
char filen[] = "file.txt";
FILE *file = fopen ( filen, "r" );
if ( file != NULL )
{
char line [ 128 ];
while ( fgets ( line, sizeof line, file ) != NULL ) /* read a line */
{
int i;
char *result;
for(i=0; i< NUM;i++)
{
char *rep;
rep = (char *) malloc (sizeof(mychars[i][0]));
strcpy(rep, mychars[i][0]);
char *with;
with = (char *) malloc (sizeof(mychars[i][1]));
strcpy(with, cgichars[i][1]);
result = (char *) malloc (sizeof(char) * 128);
result = str_replace(line, rep, with);
}
fputs(result, stdout);
}
}
fclose ( file );
return 0;
}
Valgrind is giving me this error:
==4266== Invalid read of size 1
==4266== at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==4266== by 0x5118A8D: fputs (iofputs.c:37)
==4266== by 0x400A0F: main (repl.c:35)
==4266== Address 0x0 is not stack'd, malloc'd or (recently) free'd
repl.c corresponds to the line beginning with fputs toward the end of this code.
Also, mychars is a two dimensional array that looks like this:
char *mychars[NUM][2] = {
"a", "97",
"b", "98",
....
Can someone please tell me how to fix this? Also, any pointers on how I should improve my current code (especially with malloc) would be much appreciated.
Edit: Code for str_replace
char *str_replace(char *str, char *orig, char *rep) {
char buffer[4096];
char *p;
if(!(p = strstr(str, orig)))
return NULL;
strncpy(buffer, str, p-str);
buffer[p-str] = '\0';
sprintf(buffer+(p-str), "%s%s", rep, p+strlen(orig));
return buffer;
}
EDIT 2 New Code for str_replace, and main
For testing purposes, I've replaced my str_replace method with the one found here:
What is the function to replace string in C?
And my main is changed slightly:
int main() {
static const char filen[] = "file.txt";
FILE *file = fopen ( filen, "r" );
if ( file != NULL )
{
char line [ 128 ];
while ( fgets ( line, sizeof line, file ) != NULL ) /* read a line */
{
int i;
char *result;
for(i=0; i< NUM;i++)
{
char *rep;
rep = (char *) malloc (sizeof(mychars[i][0]));
strcpy(rep, mychars[i][0]);
char *with;
with = (char *) malloc (sizeof(mychars[i][1]));
strcpy(with, mychars[i][1]);
result = str_replace(line, rep, with);
}
fputs(result, stdout);
}
}
fclose ( file );
return 0;
}
But I'm still getting
==6730== Invalid read of size 1
==6730== at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==6730== by 0x5118A8D: fputs (iofputs.c:37)
==6730== by 0x400995: main (repl.c:29)
==6730== Address 0x0 is not stack'd, malloc'd or (recently) free'd
Perhaps the most frustrating part of this is not knowing what these Invalid read errors are.
EDIT 3
I've updated the code in the center of the for loop as such:
int i;
char* result;
result = &line[0];
for(i=0; i< NUM_CGICHARS;i++)
{
char *rep;
rep = (char *) malloc (sizeof(char));
strcpy(rep, cgichars[i][1]);
char *with;
with = (char *) malloc (sizeof(char)*3);
strcpy(with, cgichars[i][0]);
result = str_replace(result, rep, with);
fputs(result, stdout);
free(rep);
free(with);
}
And now I'm starting to get output! However, after only two iterations, I get a segmentation fault, with valgrind giving me a whole bunch of this:
==9130== Invalid read of size 1
==9130== at 0x4C286D2: __GI_strlen (mc_replace_strmem.c:284)
==9130== by 0x5118A8D: fputs (iofputs.c:37)
==9130== by 0x4009DF: main (teststep1.c:27)
==9130== Address 0x0 is not stack'd, malloc'd or (recently) free'd
Best Answer
In these two lines
you first allocate space for
result
that you then loose immediately after by overwriting it with the return ofstr_replace
. That function probably returns0
, so yourfputs
fails.BTW, don't cast the return of
malloc
, in C this is superfluous and may hide the fact that you forgot to include the prototype.Edit: Your
str_replace
function is completely wrong in its memory handling. Never return the pointer to a local variable, the space isn't valid after you have left the function.