The memory limitation occurs if an application attempts to expand the size of its data segment (or heap space) by at least 256K bytes (cumulative) after it has allocated one or more shared memory segments which were attached at an address chosen by the system (i.e., in the invocation of the shmat(2) system call, the value of the shmaddr argument is zero).
In Figure 1, the address space of the application is depicted as it might appear immediately following the invocation of shmat(2). The arrow indicates the direction of future growth of the application's data segment.
This memory limitation is perceived by the application whether it attempts to expand its data segment directly using sbrk(2) or indirectly using malloc(3) or some interface supported by its run-time environment, e.g., an interpreted language. Malloc(3) and sbrk(2) will fail and set errno to ENOMEM (decimal 12).
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| | low address
| text segment |
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| data segment |
| || |
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| || |
----------\\||/--------------
| \\/ |
| shared memory segment |
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| stack |
| | high address
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Figure 1. Application Address Space after Invocation of shmat(2)
The ENOMEM error indicates that the available address space is not large enough to fulfill the requested growth of the data segment. In Figure 2, the address space of the application is depicted as it might appear when the ENOMEM error occurs. In the scenario described above, the error occurs because the expanded data segment would overlap the shared memory segment(s), which would be a violation of the virtual memory protection scheme. Even though there is still additional "free space" in the applications address space, the system cannot fragment the data segment, since some applications expect it to be contiguous.
¨ ------------------------------
| | low address
| text segment |
| |
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| |
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| |
| data segment |
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| shared memory segment |
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| stack |
| | high address
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Figure 2. Application Address Space When ENOMEM Occurs
The work-around for this problem is for the application to request a specific address as the second argument to shmat(2). Given a prudent choice for the value of this address, the application can arrange for the shared memory segment to be placed high enough in memory to avoid conflict with the growing heap. The value of this address may be determined by obtaining the current size of the data segment and then adding the maximum future data requirements of the process.
In the following code fragment, the programmer has determined that the maximum future data requirements for this application is 0x100000 (5 MB); when sbrk(2) is called with an argument of zero, it returns the current end of the data segment. The shared memory segment will be attached at an address which is at least 5 MB beyond the end of the data segment.
if (shmat(shmid, sbrk(0) + 0x100000, SHM_RND) == -1)
{
perror("shmat");
}
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