finding reoccuring string in I/O stream -C? - c

Question

I'm quite new to C. I'm trying to write a code that finds a string in a I/O stream, and I don't understand what I'm doing wrong. I know the error is probably in the large while loop (in the code below).
I want the function to return the location in bytes from the beginning of the stream and -1 if it fails for some reason. It just keeps returning -1 for any file I try it on.
long find_string(const char *str, const char *filename, long offset)
{
FILE *f = fopen(filename, "r");
if (!f){
return -1;
}
int s=0,c;
c = fgetc(f);
if(c == EOF){
return -1;
}
char *check = malloc(sizeof(char));
fseek(f, 0L, SEEK_END); // Sees and stores how long the file is
long sz = ftell(f);
fseek(f, 0L, SEEK_SET);
if(fseek(f, offset,SEEK_SET) != 0){ // finds the position of offset
return -1;
}
while(fgetc(f) != EOF){
c = fgetc(f);
if(c == str[0] && ftell(f) < sz){
check[0] = c;
offset = ftell(f);
}
s++;
for (unsigned int r=1; r < (strlen(str));r++){
c = fgetc(f);
if(c == str[s]){
check = realloc(check, sizeof(char)*s);
check[s] = c;
s++;
}
}
if(strcmp(check, str)==0){
free(check);
fclose(f);
break;
}
else{
check = realloc(check, sizeof(char));
offset = -1;
}
}
return offset;}
Any help is greatly appreciated

Answer 1

This would be much easier if you simply memory-mapped the entire file and ran a standard string searching algorithm on it.
For memory mapping, see: Linux - Memory Mapped File
For string searching code, see: strstr() for a string that is NOT null-terminated

Answer 2

Please check the lines with comment updated
long find_string(const char *str, const char *filename, long offset)
{
FILE *f = fopen(filename, "r");
if (!f){
return -1;
}
int s=0,c;
c = fgetc(f);
if(c == EOF){
return -1;
}
char *check = malloc(sizeof(char));
fseek(f, 0L, SEEK_END); // Sees and stores how long the file is
long sz = ftell(f);
fseek(f, 0L, SEEK_SET);
if(fseek(f, offset,SEEK_SET) != 0){ // finds the position of offset
return -1;
}
c = fgetc(f); // Updated
while(c != EOF){ // Updated
if(c == str[0] && ftell(f) < sz){
check[0] = c;
offset = ftell(f);
}
s++;
for (unsigned int r=1; r < (strlen(str));r++){
c = fgetc(f);
if(c == str[s]){
check = realloc(check, sizeof(char)*s);
check[s] = c;
s++;
}
}
if(strcmp(check, str)==0){
free(check);
fclose(f);
break;
}
else{
check = realloc(check, sizeof(char));
offset = -1;
}
c = fgetc(f); //Updated
}
return offset;}
since you are using fgetc at the condition and start of the look, you actually comparing the second char of file with first char of str. update and check.