unable to free string

When trying to execute this piece of code, the program craches when it reaches free(zin). When I remove the free(zin), everything works well, but I think it's better to free the allocated memory. Does anyone know what I'm doing wrong ?

Code:

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void readFile(){
        FILE *f;
        char *zin, *tag;

        if ( !(f = fopen("test.txt", "r")) ) return;

        zin = malloc(sizeof(char));
        while (fgets(zin, SKY_MAX_ZIN_SIZE, f) != NULL){
                tag = fileZoekTag (zin);
                fileProcessTag(tag);
        }
        free (zin);
        fclose(f);
}

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Code:

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zin = malloc(sizeof(char));

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Did you intend to allocate more than one byte? If SKY_MAX_ZIN_SIZE is bigger than 1, you are lying to fgets. So the loop will either write beyond memory you can write to, or the loop will never go anywhere.

That was the problem indeed. Thanks.