finding reoccuring string in I/O stream -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

Ouch. I don't know what your specific problem is but you have made this 3 times harder than it has to be. Consider rethinking the whole thing. — Duck, Apr 22 '14 at 12:24
In order to teach you how to fish, rather than returning -1 for all errors, you could return different values. That at least will tell you what section of the function is failing (and would also help us) — Peter M, Apr 22 '14 at 12:26
possible duplicate of [C: searching for a string in a file](http://stackoverflow.com/questions/2188914/c-searching-for-a-string-in-a-file) — John Zwinck, Apr 22 '14 at 12:30

score 0 · Answer 1 · edited May 23 '17 at 11:57

0

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

edited May 23 '17 at 11:57

Community

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answered Apr 22 '14 at 12:34

John Zwinck

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score 0 · Answer 2 · answered Apr 22 '14 at 12:37

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.

finding reoccuring string in I/O stream -C?

2 Answers2