C Language: popen() with fread()?

问题

I've been stuck on this for a few days and it's getting really frustrating.

I'm using popen() to call a command line process and get its output and store it in a C string. I was using fgets() but it seems that breaks after a new line, so I'm using fread(). The only problem is that the returned C string is sometimes messed up.

Here's my code:

const char *cmd = "date";//This the shell command
char buf[BUFSIZ];//Output of the command
FILE *ptr;
int c;

if ((ptr = popen(cmd, "r")) != NULL)
     while(fread(buf, sizeof(buf),1, ptr))
          while ((c = getchar()) != EOF)
               printf("output = %s", buf);

(void) pclose(ptr);

The final C string sometimes has weird characters in it that shouldn't be there, or sometimes no string is even available. Can anybody please help? ):

Edit: Here is what I was doing when using fgets() The Shell command can be anything that outputs text though. Not just "date."

if ((ptr = popen(cmd, "r")) != NULL) while (fgets(buf, BUFSIZ, ptr) != NULL) printf("output = %s", buf); (void) pclose(ptr);

回答1:

Well, fgets is the right way to do it.

FILE *ptr;

if (NULL == (ptr = popen(cmd, "r"))) {
    /* ... */
}

while(fgets(buf, sizeof(buf), ptr) != NULL) {
    /* There is stuff in 'buf' */
}

I think the reason fgets wasn't working for you is that you were doing something wrong.

Now, here's why I think you are running into trouble with your current code:

You are not checking how much fread actually returned
You are reading with getchar and discarding stuff
You don't have a NUL terminator in the buffer

Get this right and it will all be better: fread might legally read less than you told it to.

回答2:

The output from date doesn't include the '\0' (NUL) character you need to properly terminate the string. Keep track of the number of characters read and put in the NUL yourself.

Though really, you should be using fgets, getline or similar text-oriented functions to read from a program such as date. getline is especially easy (and safe since it does some memory management for you):

FILE *fp = popen("date", "r");
char *ln = NULL;
size_t len = 0;

while (getline(&ln, &len, fp) != -1)
    fputs(ln, stdout);
free(ln);
pclose(fp);

回答3:

fread doesn't insert a NUL terminator after what it reads. You need to check the return value to know how much it read, and only print that much. If you read with fread, you typically want to write the data with fwrite, something on this order:

long bytes;
while ((bytes=fread(buf, sizeof(buf), 1, ptr))>0)
    fwrite(buf, bytes, 1, stdout);

回答4:

Below is the correct way to use fread for process output with popen:

const char *cmd = "date";
char buf[BUFSIZ];
FILE *ptr;

if ((ptr = popen(cmd, "r")) != NULL) {
    /* Read one byte at a time, up to BUFSIZ - 1 bytes, the last byte will be used for null termination. */
    size_t byte_count = fread(buf, 1, BUFSIZ - 1, ptr);
    /* Apply null termination so that the read bytes can be treated as a string. */
    buf[byte_count] = 0;
    printf("%s\n", buf);
}

(void) pclose(ptr);

As you can see, the primary problem is to correctly deal with null termination. The two size parameter of fread is also important, you have to let it read character by character. Note that in the case of popen, fread will only return 0 if the process has exited without giving any output. It will not return 0 if it takes a long time for the process to print anything.

If the output is larger than BUFSIZ, you can wrap fread with a while loop.

来源：https://stackoverflow.com/questions/6699992/c-language-popen-with-fread

标签

popen

fread