CODEWITHSUNDEEP
cmemorysegmentation-fault
Alon Barenboim
Alon Barenboim

Reputation: 438

strtol()/atol() causeing segmentation fault

My problem is this:

#define MAX_LINE_LENGTH 100
#define MAX_RESOURCES_NUM 150
#define MAX_REQUESTS_NUM 150
#define MAX_REPAIRS_NUM 150
struct resource {
    long ID;
    char* name;
    long qty;
    sem_t freeResources;
};
struct request {
    long licensenum;
    long time;
    long repcount;
    long *repIDarr;
};
struct repair {
    long ID;
    char* name;
    long hours;
    long rescount;
    long *resIDarr;
};
struct resource* resarr[MAX_RESOURCES_NUM];
struct request* reqarr[MAX_REQUESTS_NUM];
struct repair* reparr[MAX_REPAIRS_NUM];
int timevar, threadcount;
//Func declarations
int main(int argc, char* argv[]){
    FILE *resfile, *reqfile, *repfile;
    char line[MAX_LINE_LENGTH], *refer;
    int i = 0;
    timevar = 0;
    threadcount=0;
    if(argc < 4) { 
        printf("Not enough arguments!");
        return 0;
    }
    if((resfile = fopen(argv[1], "r")) == NULL || (repfile = fopen(argv[2], "r")) == NULL || (reqfile = fopen(argv[3], "r")) == NULL){
        perror("Open file failed"); 
        return 0;
    }

    //Initialize resources
    while(fgets(line, 100, resfile) != NULL){
        char* token;
        refer = line;
        token = strsep(&refer, "\t");
        long ID = atoi(token);
        char* name = strsep(&refer, "\t");
        token = strsep(&refer, "\t");
        if(token = NULL){
            printf("X\n");
            return 0;
        }
        long qty = atol(token);
    }
    fclose(resfile);
    fclose(reqfile);
    fclose(repfile);
    return 0;
}

This is only part of the code, that is enough to reproduce the problem with.

I have a txt file called resources.txt:

13  car lift    8
17  front alignment 2
03  headlights adjust   2
10  oil drain   4
23  computerized check  2
35  pneumatic drive 4
40  ceiling winch   2
99  John Smith  1
29  air compressor  1
66  flats tub   1
88  paint gun   1

I separate any line with strsep() by TAB.

The problem is that I get a Segmentation fault as soon as I run the program.

When I delete this line:

long qty = atol(token);

at the end of main, I don't get the error.

I can't find what causes it.

The strangest thing is that when I run the program on my personal computer it runs fine (on CentOS) but when I run it in my college computers (with the same CentOS) it shows me the error.

Any ideas?

Upvotes: 0

Views: 511

Answers (2)

Alon Barenboim
Alon Barenboim

Reputation: 438

So I pigured out what caused the problem. I found out that fgets didn't return EOF when I thought it would, because it catches '\n' before EOF, so I ended up handling a string '\n' like it was a true line of information what caused strtol get a NULL pointer, which caused a Segmentation Fault error. The fix was relatively easy; I had to prevent from this while loop:

while(fgets(line, 100, resfile) != NULL){
    char* token;
    refer = line;
    token = strsep(&refer, "\t");
    long ID = atoi(token);
    char* name = strsep(&refer, "\t");
    token = strsep(&refer, "\t");
    if(token = NULL){
        printf("X\n");
        return 0;
    }
    long qty = atol(token);
}

to iterate when 'line' points at a string that has only '\n', so I added a condition to the while loop:

while(fgets(line, 100, resfile) != NULL && strlen(line) > 1)

And now it's perfect.

Upvotes: 0

alk
alk

Reputation: 70931

  if(token = NULL){

does not what you expect.

Change it to be

  if (token == NULL) {

And to not step into this "stupid" ;) trap again, you might like to consider using "Yoda-Conditions" from now on, that is putting the "constant" to the left, like so:

  if (NULL == token) {

Because if you would have done

  if (NULL = token) {

the compiler would have complained massively.

And, BTW, if you'd have traced the code using a single-step debugger, you mostly likely would have notice this bug very soon.

Upvotes: 3

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