fclose() causing segmentation fault

Question

I have a tab-delimited text file that I am parsing. Its first column contains strings of the format chrX, where X denotes a set of strings, e.g., "1", "2", ..., "X", "Y".

These are each stored in a char* called chromosome, as the file is parsed.

The text file is sorted on the first column lexicographically, i.e., I will have a number of rows starting with "chr1", and then "chr2", etc.

At each "chrX" entry, I need to open another file that is associated with this entry:

FILE *merbaseIn;

// loop through rows...

if (chromosome == NULL)                                                                                                                                                   
    openSourceFile(&chromosome, fieldArray[i], &merbaseIn, GENPATHIN);                                                                                                      
else {                                                                                                                                                                    
    if (strcmp(chromosome, fieldArray[i]) != 0) { // new chromosome                                                                                                   
        fclose(merbaseIn); // close old chromosome FILE ptr                                                                                                                                                                                                                                    
        free(chromosome); // free old chromosome ptr                                                                                                                          
        openSourceFile(&chromosome, fieldArray[i], &merbaseIn, GENPATHIN); // set up new chromosome FILE ptr                                                                  
    }                                                                                                                                                                       
}  
// parse row

I have the function openSourceFile that is defined as follows:

void openSourceFile (char** chrome, const char* field, FILE** filePtr, const char *path) {
    char filename[100];                                                                                                                                                           
    *chrome = (char *) malloc ((size_t) strlen(field));
    if (*chrome == NULL) {                                                                                                                                                        
        fprintf(stderr, "ERROR: Cannot allocate memory for chromosome name!");                                                                                                      
        exit(EXIT_FAILURE);                                                                                                                                                         
    }                                                                                                                                                                             

    strcpy(*chrome, field);                                                                                                                                                       
    sprintf(filename,"%s%s.fa", path, field);                                                                                                                                     

    *filePtr = fopen(filename, "r");                                                                                                                                              
    if (*filePtr == NULL) {                                                                                                                                                       
        fprintf(stderr, "ERROR: Could not open fasta source file %s\n", filename);                                                                                                  
        exit(EXIT_FAILURE);                                                                                                                                                         
    }                                                                                                                                                                             
}      

The problem is that my application quits with a Segmentation Fault going from the first chromosome to the second (from chr1 to chr2) at the following line, where I close the first chromosome file that I opened:

fclose(merbaseIn);

I know I'm not passing fclose a NULL pointer, because up until the Segmentation Fault, I am reading data from this file. I can even wrap this in a conditional and I still get the Fault:

if (merbaseIn != NULL) {
    fclose(merbaseIn);
}

Further, I know openSourceFile works (at least for chr1, when setting up the first file handle of FILE*) because my application parses chr1 rows and reads data from the FILE* source file correctly.

What is it about this fclose call that is causing a Segmentation Fault to occur?

Answer 1

valgrind --db-attach=yes --leak-check=yes --tool=memcheck --num-callers=16 --leak-resolution=high ./yourprogram args

It's very likely the segfault is caused by memory corruption on the heap, not anything that's affecting locals. Valgrind will immediately show you the first wrong access you make.

Edit: The --db-attach option to valgrind has been deprecated since release 3.10.0 in 2014. The release notes state:

The built-in GDB server capabilities are superior and should be used
instead. Learn more here:

http://valgrind.org/docs/manual/manual-core-adv.html#manual-core-adv.gdbserver